AFTER the heady days of increased base metal prices and frantic gold exploration of the late 1980s, mineral exploration in 1990 continued at a much reduced level. Besides the flat gold price, three factors have had a large impact on worldwide exploration, the ending of flow through funding in Canada, the introduction of the Australian gold tax and the squeezing of margins on the deep South African gold mines. Base metal exploration has not been encouraged by the recession in North America and Europe and the uncertainty surrounding events in the Gulf at year-end. Rationalization of exploration interests was much in evidence during the year as the lower gold price forced a re-assessment of the viability of smaller and more remote discoveries. Many junior exploration companies were forced to make radical write-offs of exploration expenditure on these prospects.
One note of hope was the opening of Eastern Europe and, less certainly, also the U.S.S.R. to commercial exploration. Although a considerable amount of discussion has taken place on the formation of joint ventures with state companies, few contracts have yet been signed.
Exploration in Canada has been badly affected by the ending of tax incentives and the lack of profitability of a number of the smaller new discoveries. Estimates by the Prospectors and Developers Association of Canada suggest that spending will be around $C610 million for 1990 with a projected 30% cut in 1991.
In eastern Canada most exploration spending is concentrated on the major producing areas in Ontario and Quebec. Agnico-Eagle Mines reported discovering a probable extension to its Eagle deposit in Quebec. The extension which is 850 m west of the Eagle shaft was tested by 6,000 m of drilling at year-end. On the Douay West property, 13 km northwest of Joutel, Inco, Vior and Cambior have further encouragement from a gold zone in silicified pyritic iron-rich sediments. Intercepts include 9.3 m of 23.7 g/t Au, 1.5 m of 3 g/t Au, 2.1 m of 7.2 g/t Au and 1.5 m of 2.7 g/t Au.
At the Louvicourt massive sulphide discovery of Aur Resources and Louvem Mines, near Val d'Or, infill drilling confirmed the high grade of the massive sulphide discovery. At year-end pilot holes for shaft sinking had been drilled and a drill programme to define the extent of the mineralized horizon outside the deposit commenced. Preliminary engineering studies were also undertaken on a resource of around 36 Mt of 3.1% Cu, 1.3% Zn, 18 g/t Ag and 0.8 g/t Au. The long running legal dispute over ownership of the deposit was settled to leave Aur Resources in control of the deposit while Louvem has 40%. Majors, Noranda, Teck and Cominco have substantial stakes in both junior companies.
Along the Porcupine-Destor structure east of Matheson in Ontario, Noranda and Newmont have mounted a substantial drill programme on the contiguous Lightning and Teddy Bear properties in a joint venture with Freewest Resources. Five rigs were active on the property during the year and assays range up to 58 m of 5.5 g/t at depths from 300 m to 700 m.
At its Eagle River prospect in northwestern Ontario, Hemlo Gold and Central Crude have cut further significant mineralization. Among the better intercepts from the 10-hole programme are 10.3 m of 11.6 g/t Au and 2 m of 10.6 g/t Au. The deposit will probably be brought into production by the purchase of the mill and surface facilities from the Magnacon mine, 20 km to the north. Initial reserves at Eagle River were put at 1.6 Mt averaging 7.8 g/t after 20% dilution. At Red Lake in Western Ontario, Inco Gold is undertaking an underground exploration to test the former Cochenour-Willans mine. A bulk sample has been taken which will be used as the basis for a feasibility study.
In Manitoba, Inco announced that it is to increase its exploration budget for 1991 to $C6 million in the Thompson nickel belt, including 40,000 m of drilling, following a decision to increase production at its existing facilities. At the Trout Lake massive sulphide deposit, Hudson Bay Mining and Smelting in partnership with Granges and Manitoba Mineral Resources has cut a new zone beneath its existing mine at depths of 560-940 m.
In the La Ronge belt in Saskatchewan feasibility studies continue on a number of small deposits. Windarra Minerals received a preliminary feasibility report on Preview Lake where it is in partnership with Cameco and Uranerz. Mineable reserves are put at 1.3 Mt of 8.3 g/t Au in the Contact Lake deposit and a further 3 Mt at the Bakos and Pap deposits. At Hanson Lake near Flin Flon, Cameco and Trimin Resources continue to evaluate their Hanson Lake joint venture. Geological reserves are put 9.8 Mt of 1.0% Cu, 5.8% Zn, 0.4% Pb, 0.5 g/t Au and 25 g/t Ag.
One of the highlights and lowlights of the year was the continuing exploration at Eskay Creek in British Columbia. During the year a new zone, the 21C Zone, was encountered at depth. Initial holes include 20 m of 18 g/t Au and 14 m of 10 g/t Au. Earlier, 140,000 m of drilling on the 21 zone had been completed by autumn and a decline had been sunk. Resources in the 21B zone were put at 1.5 Mt grading 49.2 g/t Au, 1,280 g/t Ag, 2.1% Pb and 5.1% Zn. As in so many of recent Canadian discoveries, litigation and stake building have rapidly taken over. Both Corona Corp. and Placer Dome have taken stakes in Stikine Resources and Prime Resources, the operating companies. At year-end the deposit was overstaked by Tagish Resources, who claimed that the original staking was null and void. Other operators in the area have reported encouraging drill intersections. On the SIB claims, about 5 km from Eskay Creek, American Fibre Corp. and Silver Butte Resources have cut 14.3 m grading 14.4 g/t Au and 1,060 g/t Ag within a graphitic mudstone.
Amongst the more advanced projects, Geddes Resources has now discovered a third, new copper zone at the Windy Craggy massive sulphide deposit where probable and possible reserves had already been upgraded to 210 Mt grading 1.59% Cu, 0.18 g/t Au, 3.62 g/t Ag and 0.09% Co at a 0.5% copper cut-off. At the Mount Milligan copper-gold porphyry, where control of the project formally passed to Placer Dome in the autumn, more than 168,000 m of drilling have been completed and an open-pittable reserve of 400 Mt grading 0.2% Cu and 0.48 g/t Au has now been delineated in two deposits within a sulphide system extending over an area of 10 [km.sup.2].
One of the most significant B.C. exploration developments during the year was the spectacular 50 m intersection grading 2.92% Cu, 1.58% Pb, 9.09% Zn, 3.38 g/t Au and 170 g/t Ag reported by Cominco and Redfern Resources at their Tulsequah Chief property.
Like Canada, most activity has concentrated around producing areas, particularly in Nevada, but generally in the western U.S. Homestake Mining is proceeding on an underground test of gold mineralization at more than 1.5 km depth. The company is planning to drift north from the Homestake mine to test the area, a distance of more than 5 km. This drift will also test the potential of the intervening ground at depth.
American Barrick continues to drill in the area around its Goldstrike deposit in Nevada. In the Purple Vein area, north of Betze, it has intersected higher grade mineralization than expected (12 g/t Au) and was proceeding to an initial economic assessment at year-end. Lac Minerals continues to intersect good grades on its Rosebud property in Nevada and is now moving to pre-feasibility study.
One of the more promising prospects in the western U.S. is the Crown Jewel project of Crown Resources and Battle Mountain on Buckhorn Mountain, Washington. Results from 90 holes on 15 m centres are encouraging, with the best holes cutting 21 m of 43.2 g/t Au and 53 m of 8.1 g/t Au. Permission was received to drill a further 150 step-out holes.
Chile was probably the most active exploration country with substantial interest in gold exploration, especially by majors in the developing Maricunga district. At the Refugio prospect Bema Gold Corp. claims to have defined a resource of around 100 Mt of 1.2 g/t Au. Niugini Mining investigated a low-grade deposit at San Cristobal, east of Antofogasta. Reserves were put at 10.1 Mt at 1.2 g/t Au, mainly suitable for heap leach operations. Not all activity is for gold or copper, and in southern Chile Outokumpu has signed an agreement with Empresa Minera de Aysen to explore a zinc deposit south of General Carrera Lake.
Battle Mountain has given the go-ahead for the development of the Kori Kollo mine in Bolivia, which is owned by its 51%-owned Bolivian affiliate, Inti Raymi. Proven and probable reserves are put at 4.1 Moz of gold and 26.8 Moz of silver.
In Central America, Greenstone Resources and Boliden have acquired the Santa Rosa project in Panama from Freeport-McMoRan. Freeport defined reserves of 11.3 Mt grading 2.1 g/t Au but much of the area is unexplored and there is considerable further potential.
The deep level gold mining industry in South Africa has been badly affected by continuing cost inflation which has eroded margins in a time of relatively flat or declining gold prices. This has led to the curtailment of new investment. However, drilling for new reserves has continued at a number of areas in the Witwatersrand Basin.
Problems with production costs at Western Areas have posed problems for the developing South Deep mine, which led to the prospect being floated as a separate company on the Johannesburg Stock Exchange. Although the overall resource is large at 116 Mt at 9 g/t Au, it is deep and is being developed from existing facilities at Western Areas. The rights issue will enable the company to re-imburse Western Areas for the development work conducted from the latter's 95 level.
Drilling continued in the Potchefstroom Gap area, especially by Anglo-American group companies. Their three main areas are the New Central Witwatersrand Areas joint venture, Stoompoor-fontein and adjacent to Vyfhoek. At Stoompoor-fontein, six rigs were following up a discovery hole, whereas drilling at Vyfhoek concentrates on testing the Bird and Carbon Leader Reefs.
One of the major prospects in the Orange Free State is the Sun and Oribi area held by Anglo-Vaal in a joint venture with Utah-BHP. The Sun area is the most advanced and drilling is aimed at delineating grade continuity. To June 1990, R205 million had been spent in the area. Reef depths vary from 2,700-4,400 m and a primary reef is present. The aim is to define an area capable of supporting a milling rate of 3 Mt/y with a single-lift shaft to the surface.
In the same general area, another Anglo-Vaal company, Loraine, has undertaken an exploratory drill programme immediately to the north of its present lease area. This area will be floated as a separate company, Target, and will be the subject of detailed drilling over the next two years.
Both Anglo-American (in the area around Ventersdorp) and Gencor have reported encouraging results outside the presently defined margins of the Witwatersrand Basin.
Platinum exploration was encouraged by the decision to go ahead with the Platreef mine at Potgietersrus and the steep rise in the price of rhodium. The increasing demand for rhodium has led to a re-evaluation of the potential of the UG2 Reef, which is relatively rhodium-rich. On the northern limb of the Bushveld complex, Gencor is examining the possibility of large, relatively low-grade PGE mines. One un-named prospect has reserves of more than 150 Mt of between 1 g/t and 2 g/t PGE, and the Volspruit prospect around 50 Mt of similar grade.
Mineral lease agreements have been concluded by Messina in respect of its Doornvlei and Voorspoed deposits in the Lebowa homeland. Reserves are put at 12.6 Mt grading 5.05 g/t PGE of Merensky Reef material and 18.9 Mt of 5.15 g/t on the UG2.
Exploration in Zimbabwe increased with some diversification from gold into diamonds, coal, platinum, and a feasibility study on uranium deposits in the north of the country. Three large projects continued to evaluate the PGE potential of the Great Dyke. Rio Tinto, Valley Exploration (Anglo-American) and Plateau Mining are accelerating their programme after encouraging scout diamond drill holes. The other large project is the Hartley prospect, initiated by Delta Gold. Resources are around 37 Mt of 4.7 g/t PGE. Delta has since signed an agreement with a BHP subsidiary for that company to undertake a revised feasibility study.
Ghana remains the focus for considerable activity. BHP-Utah is drilling on two prospects at Nsuta in the west of the country and at Dunkwa in the Ashanti area and reports favourable grades and widths.
The slump in exploration has hit hard in Australia. The Australian Drilling Industry Association estimated that 240-280 rigs were idle at mid-year, 60-70% of the total. Around 2 million metres were drilled compared with 4 million plus at recent exploration peaks. This has driven down reverse circulation drilling costs to $18/m in western Australia where the decline in spending has been most severe.
One project where reserves have increased substantially is the Telfer Mine of BHP Gold and Newmont; gold resources have increased to more than 7 Moz with fresh discoveries notably at West Dome. Here, around 35 Mt containing 1.75 Moz have been defined. A further 500,000 oz have been discovered at Big Tree, 25 km southeast of Main Dome.
Western Mining is developing an underground operation at the Redeemer deposit, near Leinster. Overall reserves are put at 5.4 Mt of 3.4 g/t Au of underground material and 1.4 Mt at 2.3 g/t suitable for open-pit mining.
CRA has announced its intention to develop a new underground gold mine at Peak, south of Cobar in New South Wales. Mining reserves are put at 3.9 Mt of 7.1 g/t Au, 0.8% Cu, 1.3% Pb, 1.5% Zn and 14 g/t Ag.
MIM Holdings announced the discovery of a new gold discovery, 110 km southeast of its Mount Isa mine in Queensland. The Tick Hill deposit is currently under investigation with five intersections of more than 15 m of 20 g/t Au. Also in the Mount Isa area, Placer Pacific has announced a significant gold-copper discovery, Trough Tank. In a joint venture with Shell Australia, Placer has an indicated resource of 13.6 Mt of 1.9% Cu and 1 g/t Au. To August 1990, 59 diamond and 66 percussion and reverse circulation holes had been drilled. Intercepts included 18 m of 3.3% Cu and 1.7 g/t Au and 28 m of 7.9% Cu and 2.5 g/t Au.
The renewed interest in base metals of the last few years has provided impetus in Northern Queensland and the Northern Territories. CRA has located a promising Zn-Pb-Ag at Lawn Hill, 250 km northeast of Mount Isa. The zone is 30-40m thick and averages 7% Zn, 1% Pb and 30 g/t Ag, including a 12 m thick zone averaging 13% Zn.
Perhaps the highest grade find reported during the year was the discovery of another rich gold vein at the Hishikari deposit of Sumitomo Metal Mining. The Sanjin vein varies in thickness from 10-40 m and averages 70 g/t Au. Reserves are put at 700,000 t containing 1.6 Moz, increasing mine reserves to 8 Moz at what is probably the highest grade operation of any size in the world at present.
The Mediterranean region has seen considerable exploration over the past few years with little tangible success. However, discoveries last year are very encouraging. In Turkey, exploration successes have been reported by Cominco and Euro Gold. In the northeast of the country, near the Black Sea coast, Cominco reports up to 25 g/t Au over 15 m on its Corak prospect. At its nearby Tak property values were up to 16.8 m at 8.6 g/t.
Cominco is reported to be spending $C2.5 million in Turkey, largely concentrating on the Black Sea area, where it is searching for polymetallic deposits as well as gold. Euro-Gold, which is a joint venture between ACM Gold and Metall Mining Corp., has located high-grade gold mineralization on its Dikili prospect in West Turkey. Drilling is difficult, but gold intersections include 11.2 m of 35 g/t, 12 m of 21.1 g/t, 12.5 m of 17.8 g/t and 8.5 m of 23.8 g/t. A second phase of drilling started during the year and included an intersection of 4 m of 42.5 g/t Au. ACM anticipates a final feasibility study in early 1991.
Exploration for zinc-lead deposits in the Irish midlands has been given a fillip by the positive results from the feasibility studies at the Galmoy deposit and the realization that previous exploration models may not explain all the mineral deposits. Further encouragement has come from the nearby Lisheen deposit, where a joint venture between Chevron and Ivernia West has defined 2 Mt of 12.5% Zn, 1.8% Pb and 34 g/t Ag. Corona Corp., Outokumpu and Minorco have all taken stakes in the junior companies. Cominco is exploring in a joint venture with Ennex, as is MIM with Navan Resources. A number of other companies are actively exploring along the Rathdowney trend containing the deposits.
Gold exploration has, however, suffered a setback following the Minister of Energy's decision to declare an exclusion halo around the Lecanvey deposit of Burmin, which lies on the slopes of Croagh Patrick mountain, the site of an annual religious pilgrimage. It seems likely that this dispute will eventually be resolved in court.
Eastern Europe and the U.S.S.R.
One of the major developments in 1990 was the opening of Eastern Europe to commercial mineral exploration and, in particular, joint venture agreements with para-statal companies. Although not much of substance has been achieved, initial discussions and reconnaissance have been undertaken by a number of companies. The problems of transforming mineral exploration and exploitation to a commercial basis are, however, formidable. In particular, the problem of widespread pollution caused during the last 40 years has rebounded against current exploration. As an example, if the large undeveloped Celina/Mokrsko gold deposit, south of Prague in Czechoslovakia, is to be mined, then chemicals other than cyanide must be used for gold extraction.
One of the more advanced projects is the agreement between the Finnish company Outokumpu and state Bashkirian kombinat in the southern Urals area to exploit two copper-zinc deposits. Reserves are put at 4 Mt at 5.5% Zn and 4.4% Cu and 7 Mt at 2.5% Cu and 1.8% Zn.
Greenland is suffering from a decline in mining and the government is trying to encourage further exploration by a series of incentives. Perhaps the most encouraging prospect is that of Platinova Resources in the Skaergaard Complex. The company, originally in a joint venture with Corona Corp., has intersected two distinct areas of mineralization at depths of up to 600 m. The precious metals are in the upper zone and in the form of discrete droplets of native gold with varying amounts of platinum and palladium, which can all be recovered very simply and cheaply using gravity separation and flotation. The lower zone has more complex and ill-understood mineralogy but is richer in palladium. Values from hole 90-11 are reported as 3.3 g/t Au, 1 g/t Pd and 0.08 g/t Pt.
Geophysical prospecting methods are continually being refined for use in exploration programmes, but increasingly as aids to the determination and understanding of the geological structure in areas of interest. Their indirect applicability can be illustrated with respect to gold which is the most popular target in current exploration programmes. It is not possible to get direct geophysical responses from gold because of the low grades, except of course where electromagnetic detection methods are used for shallow individual nuggets.
Doyle of Western Australia has provided a welcome review of how geophysical methods can be of value through the association of gold with particular host rocks, marker beds, or structures which are of unusual magnetization, density, electric polarization or conductivity. Useful markers may be magnetic dolerites, banded iron formations, magnetic shales, conductive and/or polarizable pyrites, or other sulphides detectable by IP, and resistive silicified zones.
The mapping of faults and shear zones with which gold may be associated can be aided by magnetic and electromagnetic surveys. Sometimes there is depletion of magnetite as in carbonatized zones which therefore give magnetic lows. Resistivity patterns may indicate altered rocks which may be mineralized and detected by VLFEM and MMR methods. Magnetic, gravity and seismic methods have been important in the Witwatersrand, magnetic, EM, and IP in the Abitibi greenstone belt of Canada, and in Australia. Although the methods are sometimes unreliable because of the low contrasts, the excellent account demonstrates the usefulness of geophysical surveys in gold exploration programmes.
In all exploration programmes, there are advantages in combining the data from different methods, especially now that this can be done so effortlessly by making use of the modern methods of digital image analysis. A good illustration is the way different types of images from gravity, magnetic and gamma-ray spectrometry from the Rae Province of the Canadian Shield were compared with each other and with geologic maps to evaluate their effectiveness for displaying the geologically relevant content of the data sets by John Broome of the Canadian Geological Survey (Geophysics 55, pp. 977-997).
Shading methods were useful for enhancing weak directional anomalies in the aeromagnetic data. Multidirectional, shaded-relief images produced by overlaying three coloured, shaded relief images were useful for the analyses of anomalies associated with structure. Vertical gravity derivative images, and vertical magnetic derivative images of the Shear Zone clarified structural trends. Composite images using three different geophysical parameters show correlations between magnetic, gravity and radiometric data which can be related to the geology. Subtle variations in uranium, thorium and potassium concentrations determined by gamma-ray spectrometry cause effective displays using ternary radio-element images.
Another interesting application of the use of digital image processing and integration of geophysical data sets is provided by Kwarteng and Chavez (Geophysics 55, pp. 965-976). They used the technique to develop exploration models from airborne electromagnetic (EM), magnetic and very low-frequency electromagnetics (VLF-EM) data collected over an area in northwest Arizona with uranium-mineralized breccia pipes. Apparent resistivity and overburden thickness were derived from the EM measurements using halfspace models. A number of standard digital image processing techniques were applied to the data. One of the best models incorporated apparent resistivity and total-field magnetics. The combination outlined 13 anomalous combinations. Two of these were known to be orebodies and most of the other anomalies corresponded to suspected pipes that are still being evaluated.
Potential Field Methods
Most existing techniques for potential field data enhancement and interpretation require data on a horizontal plane, although in ground surveys they are collected on an irregular surface. Pilkington and Urquhart (Geophysics 55, pp. 549-555) describe a method of reducing such data to a horizontal plane by computing an equivalent source distribution that models the observed field on a mirror image of the observation surface. This surface is then replaced by a horizontal plane and the effect of the equivalent sources is computed on the required horizontal level. This calculated field approximates the field reduced to a horizontal plane. The technique is illustrated with two-dimensional synthetic data examples in which the maximum errors occur in areas of steep topographic gradients and increased magnetic field intensity. The approach is also applied to a portion of a helicopter-borne aeromagnetic survey from the Gaspe region in Quebec, Canada, where the results are a horizontal shifting of anomaly maxima by up to 150 m and changes in anomaly amplitudes of up to 100 nT.
As gravity surveys become more widespread in mining applications, there is a growing need for high-precision gravity meters. The most popular modern instrument is the La Coste-Romberg, model D, equipped with electronic readout and which has a reading accuracy of 5[Mu] Gal. Casten and Haussman (Geophysical Prospecting 38, pp. 489-498) have described how they have improved the observation accuracy by the supplementary installation of electronic feedback. Repeat readings in the laboratory gave a standard deviation of 4[Mu] Gal in normal mode and 1 [Mu] Gal in feedback mode. They find that in the field, the accuracy of a single reading becomes more than 6[Mu] Gal in the normal mode and 3[Mu] Gal in the feedback mode respectively. The improvement is therefore not as good as expected so further developments, for example using electronic levels rather than the standard liquid ones, are indicated.
Terrain corrections in gravity surveys are of the greatest importance in mineral exploration surveys where the anomalies are small and are usually done in two parts. The first is the correction for topography close to the station using charts and the other is for more distant, say > 2 km, from the station. The second is normally calculated using a machine procedure which usually accesses a digital-terrain data set. Allen Cogbill (Geophysics 55, pp. 102-106) describes a new method for the calculation of the inner-zone terrain corrections for gravity stations in the U.S. using the 30 m Digital Elevation Models (DEMs) produced by the USGS National Cartographic Information Centre (NCIC). He notes, however, that the method is not restricted to the use of DEMs -- one's own digital elevations may be included. The procedures can be used to several kilometres from a station and will lead to greater overall accuracy of the total terrain correction at each station. His procedures are fast enough to be used on modern personal computers.
Another refinement to the methods of doing terrain corrections is described by Zhou, Zhong and Li (Geophysics 55, pp. 232-238). They point out that the current methods using rectangular and fan-shaped prisms with flat tops are not appropriate in areas of rugged topography. They have developed an elegant substitute using triangular elements which allows dipping surfaces. The method involves a simple formula using a surface rather than a volume integral for computing the corrections to a high accuracy.
An early requirement in the interpretation of gravity data is the separation of the gravity effect of the feature of interest (the signal) from interfering effects (the noise) of other geological features. In many geologic situations both signal and noise behave randomly. Pavlouski and Hansen (Geophysics 55, pp. 539-548) have introduced an elegant new method of anomaly separation based on frequency-domain Wiener filtering. It differs from previous Wiener filtering schemes in that it provides, through direct modelling of known geology (e.g. outcrop and borehole data), a more objective estimate of the signal power spectrum required for defining the transfer function of the filter.
The technique is illustrated both with synthetic and field data from the southern Paradox basin in Utah where there is significant overlap between the gravity effects of sources in the Precambrian basement and the gravity signal arising from the density contrast across the Mississippian-Pennsylvanian interface. The illustrations demonstrate the superiority of the method over conventional wavelength filtering methods. It clearly has wide applicability in mineral exploration surveys.
The algorithms for calculating the gravitational attraction of a two-dimensional polygon are among the most widely used in the interpretation of gravity data. They assume that the density within the polygon is uniform and this is often not the case. Guspi (Geoexploration 26, pp. 253-265) has extended the method to deal with inversion when the density contrast varies with depth. It is illustrated convincingly for a sedimentary basin but the principle could clearly be extended to other structures.
This issue of changing gravity contrast with depth is also addressed by Bhaskara Rao (Geophysics 55, pp. 226-231) who has been analysing the anomalies of sedimentary basins by an asymmetrical trapezoidal model with a quadratic density function. He derives the equation for the anomaly of the trapezoid and develops methods of interpretation using the Marquardt algorithms. The usefulness of this model is nicely demonstrated with the interpretation of data over the San Jacinto graben and the lower Gadovari Basin.
One of the standard procedures in the analysis of magnetic anomalies is their reduction to the pole and a number of procedures have been described for doing this. Of course the reduction is of limited value at low latitudes and Kis (Geophysics 55, pp. 1141-1147) argues that reduction to the equator can be used at low latitudes where reduction of two-dimensional anomalies to the pole has instabilities. He illustrates the procedure by application to anomalies produced by two-dimensional and three-dimensional prisms.
The recently compiled gravity and magnetic anomaly maps of North America (Tanner and Hinze, Leading Edge 7, pp. 14-21) will stimulate research and exploration for many years to come. Arkami-Hamed and Urquhart (Geophysics 55, pp. 218-225) have now reduced the magnetic anomalies to the pole using a generalized technique which takes into account variations in the directions of the core field and magnetization of the crust. The reduced-to-the-pole anomalies show good correlations with a number of regional tectonic features, such as the Mid-Continental rift and the collision zones along plate boundaries, which are also apparent in the vertical gravity gradient map of North America. The correlation is not consistent of course as gravity and magnetic anomalies often arise from different sources.
Pederson, Rasmussen and Dyrelium (Geophysical Prospecting 38, pp. 795-804) describe how component maps may be constructed from aeromagnetic total field anomaly maps. This makes it possible to follow magnetic lineaments more closely than by looking at the total field anomaly. Their directional filtering technique is a valuable addition to those already available. Thus upward continuation is useful for regional-residual separation in contrast to directional filtering which amplifies structures in particular directions. Their technique is nicely demonstrated with data along E-W flight lines 200 m apart at an altitude of 30 m above ground over the Siljan impact structure. Their maps clearly show the trends of two generations of dyke injection between the granites and porphyrites, and the Palaeozoic structures.
The second derivative method of processing aeromagnetic data is a well-established procedure for enhancing the short wavelength content of magnetic fields. It is particularly useful for resolving and sharpening local anomalies. The contours on second derivative maps may coincide with important features such as lithological boundaries, faults and mineralized zones.
An impressive illustration of the value of the technique is provided by Church and Pettipas (CIM Bulletin 83, pp. 69) who have been looking at aeromagnetic data provided by local mining companies at the Silver Queen and Equity Silver mines from the Houston area of British Columbia. They find that the principal zones of alteration associated with mineralization and known from detailed mapping are further delineated and extended in a way not possible by a cursory examination of the original magnetic data.
Magnetic survey data are routinely interpreted by estimating source depths or locations and there are many algorithms for doing this. However, there is still a need for a fast method of processing a magnetic grid to derive trends and depth estimates in an automatic or semi-automatic manner. Reid, Allsop, Granser, Millet and Somerton (Geophysics 55, pp. 80-91) describe how this may be done using Euler's homogeneity relation using measured or calculated gradients. The data do not have to be pole-reduced so that remanence does not produce interference. Geological constraints are imposed by the use of a structural index.
Their model studies show that the method can be used to locate or outline confined sources, vertical pipes, dykes and contacts with remarkable accuracy. This promises to be an important new method and is convincingly illustrated using data from a structurally complex area in Britain.
The inversion of magnetic data to obtain basement topography is also examined by Pustisek (Geophysics 55, pp. 782-785), who describes a procedure based on the Schmidt-Lichtenstein theory of non-linear equations and which has great computational efficiency. It is only about 50% more time-consuming than the forward modelling algorithm.
The interpretation of magnetic anomalies in terms of dyke-like bodies is a popular procedure and may be done in the space or the frequency domain. The most applicable method in the frequency domain has been the Fourier transform and the reason for its widespread use is the development of the FFT algorithms for discrete data. Isaac Marobhe (Geoexploration 26, pp. 291-301) now examines the possibility of using the discrete Hartley transform in the determination of body parameters from total field magnetic anomalies caused by dyke-like bodies of infinite strike length. He finds that the ratio of the Hartley transform to the amplitude spectrum gives the function from which the centre, magnetization, angle and dip may be determined. The depth and width of the dyke are determined from the amplitude spectrum. The method is tested using theoretical anomalies and shows that most of the estimates of the parameters are in agreement with the parameters used to compute the anomalies. He finds that proper scaling of the transform is required to give good estimates of the depth and magnetization intensity.
Two-dimensional (profile) inversion techniques for magnetic anomalies are widely used in exploration geophysical as 3-D methods are restricted in their applicability because of their dependence on good initial values and the capabilities of existing computers. Wang and Hansen (Geophysics 55, pp. 1321-1326) have now developed a fully 3-D inversion algorithm for routine application to large data sets. It is a 3-D generalization of compudepth which was based on the fact that the Fourier transform of the horizontal derivative of the anomalous total field over a prismatic structure is a superposition of exponential functions. The new algorithm determines a series of coefficients from which the depths and locations of polyhedral vertices are calculated by solving complex polynomials. The vertices are used to build a 3-D geologic model. The technique was used to construct a remarkably realistic model of the Medicine Lake Volcano.
The advantage of vertical-gradient measurements over total magnetic field measurements are now well established so that the method is used routinely in Canada as an aid to mineral exploration. The interpretation, however, is still largely qualitative as an aid to geological mapping. Quantitative interpretation is usually only done on isolated anomalies. A great advance is the automated method for the interpretation of vertical-gradient anomalies described by Keating and Pilkington of the Canadian Geological Survey (Geophysics 55, pp. 336-343). They have developed a quantitative method for interpreting the magnetic vertical-gradient anomalies of dipping dykes and planar geological contacts. The method is based on a linearization of the equations describing these anomalies. They demonstrate the method with data from the Val d'Or mining area in Canada.
Electrical resistivity methods have been used with considerable success for many years in mineral exploration programmes despite the limitations in the methods of data analysis. One-dimensional automatic inversion using linear digital filters is the most widely used method. In two dimensions, the usual practice is to rely on pseudo sections from which it is quite difficult to get a good picture of resistivity distributions. The trial and error methods of forward modelling using finite elements or finite differences require a large computer and highly experienced geophysicists.
Shima (Geophysics 55, pp. 682-694), of the Oyo Geotechnical Institute in Japan, now proposes a new method using alpha centres in which potential and conductivity can be calculated very quickly and the distribution of conductivity can be obtained as a continuous function that closely corresponds to the actual ground and so is suitable for automatic analysis of resistivity structures. He demonstrates the effectiveness of the method over the prism and simple fault models; and to simulate measurements between boreholes and adits. They demonstrate the effectiveness and high resolution of the method. He uses the method to identify the form and distribution of lava in the ground and also to investigate the distribution of intrusive rock and alteration in a steep mountainous area.
In resistivity soundings, interpretations can really only be made in areas of flat topography underlain by horizontal homogeneous layers. Molano, Salamanca and van Overmeeren (Geophysical Prospecting 39, pp. 705-718) have developed the GEA-58, a geoelectrical instrument which can make continuous soundings along a profile with any colinear electrode configuration. They use the finite-difference and finite-element methods to model complex earth resistivity distributions in 2-D, in order to calculate apparent resistivity responses to any colinear current electrode distribution in terrains where the earth resistivities do not vary along the strike. In addition, a pseudo-depth section of apparent resistivities measured in the field with the GEA-58 is compared with the numerical solution of a real complex resistivity distribution along a cross-section. Their model results show excellent agreement with the corresponding analytical and experimental data.
The execution and interpretation of resistivity soundings is always challenging. The topography is never flat and the lines are never straight. Zohdy and Bisdorf of the USGS (Geophysics 55, pp. 956-964) overcame these problems very elegantly in their Schlumberger soundings to explore the geothermal potential of the Medicine Lake area of Northern California. They expanded the array along the winding roads and the measured resistivities were corrected using the geometric factor for the exact geometry instead of that for the linear array. This works provided (a) the distances from the current electrodes to the centre of the array are equal and (b) the proper geometric factor is used to calculate the apparent resistivity. They used the data to produce resistivity maps and cross-sections to identify geothermal targets. The procedures are exactly applicable to mineral targets.
The interpretation of dc resistivity data on a traverse across a 2-D structure is usually analysed by trial and error modelling which requires a lot of interpreter time. The inexpensive method of Pelton et al (Geophysics 43, pp. 788-803) cannot realistically fit complex models as it is based on a single 2-D prism with a rectangular cross-section. This is particularly relevant in one field where surveys are carried out in mountainous areas where topographic effects may produce virtual anomalies in the resistivity section.
Tong and Yang of Taiwan (Geophysics 55, pp. 354-361) have been looking at the problem of incorporating topography into 2-D resistivity inversion. Their method is based on the finite element method and their iterative inversion scheme is derived from the second-order Marquardt damped least-squares method. The algorithm has been tested on both synthetic and field resistivity data with topography incorporated explicitly into the inversion model. Their theoretical and field studies indicate that the technique is exceptionally efficient and is a great improvement on conventional interpretation schemes.
In 1966, C.H. Bristow described an electrical resistivity method of detecting subsurface cavities which has attracted a lot of attention. It is clearly of interest in the mining industry where structures are contemplated above abandoned mines. There has been some unresolved controversy about the limitations of the method which has now been thoroughly examined by Lowry and Shive of the University of Wyoming (Geophysics 55, pp. 514-520).
They use a slight variation of Bristow's technique on modelled data, and find that the size and location of subsurface cavities can be located with surprizing accuracy. However, in the presence of geological noise, the accuracy is reduced considerably. They conclude that in these situations, cylindrical cavities cannot be detected if the depth to the top is greater than the diameter and for spherical cavities, the limit is the radius. No more satisfactory method than Bristow's is suggested.
The magnetometric response (or resistivity) technique (MMR) is an electrical prospecting technique which energizes the earth with an applied dc current and measures the resultant magnetic field. It is not as widely used as the corresponding potential field methods simply because surface measurements cannot detect layering.
Veitch, Asten and van Leenwen (Geophysics 55, pp. 1605-1612) have therefore been considering the electrical and magnetometric fields in a layered earth when the electrodes are buried. This is timely because of the increasing interest in borehole geophysics. The method has advantages in that it reduces the masking effect of conducting overburden and the distorting effect of surface topography. It is of particular interest in determining the off-hole distance and direction to a conductor.
EM methods have been used extensively for a wide variety of investigations ranging from mineral exploration, engineering studies, geothermal and groundwater exploration, and permafrost mapping. More recently, EM sounding techniques have been applied to geological mapping. Sinha (Geophysics 55, pp. 1148-1157) has been using multifrequency and transient (TEM) sounding techniques at several locations in southern Ontario to assess their value in stratigraphic mapping of gently dipping sedimentary formations and to investigate the possibility of detecting folds, faults and grabens.
His investigation of a valley buried under resistive glacial till and dolomite at depths between 100 m and 200 m gave the depth and shape of the valley which agreed with the results of a high-resolution reflection survey. The TEM data were found to be less affected by shallow conductors and lateral inhomogeneities than the multi-frequency data and also had a greater depth of investigation.
The conventional mode of operation of a fixed loop EM prospecting system is to use a rectangular loop with the long side parallel to the expected strike. However, the resolution is poor when the conductors form closely-spaced groups. It has been argued that in this situation, having the loop across the conductors, the tx parallel configuration, is preferable. Duckworth and Cummins (Geophysical Prospecting 38) have carried out a physical scale comparative study of the two configurations.
Their results show that, over thick or dipping conductors, the coupling in both configuration is comparable but the tx-parallel configuration is shown to provide more consistent indications of the direction and magnitude of conductor dip.
Over wide conductors, separate conventional surveys are needed to define the opposite edges of the conductors, but with the tx-parallel only a single survey is needed. For parallel conductors, the tx-parallel does indeed provide better resolution. It was also superior in distinguishing between the response of sheets dipping in opposite directions, even with zero separation. This important confirmation should lead to more widespread use of the tx-parallel configuration as it is likely to result in the planning of more efficient drilling programmes.
Airborne electromagnetic methods have been in use for over 40 years and have been particularly successful in prospecting for sulphides in resistive host rocks, notably in Canada. However, inversion techniques have only been developed for half-space and one-layer models. Keating and Crossley of the Canadian Survey (Geophysics 55, pp. 705-711) show that by introducing some approximations to the rectangular thin-plate model, it is possible to apply inverse theory to the interpretation of time-domain EM data. The method is demonstrated using synthetic profile data and INPUT data obtained over the Lake Waconicki test area in Quebec. This is clearly a most important development. Remarkably, the program runs in less than 15 min on a 32-bit microcomputer (16 MHz, 80386PC).
In areas of conductive overburden, abnormal decay curves that are depressed or change sign are found in coincident loop and central loop TEM soundings. This is generally accepted now as being due to polarizable material such as clay in the overburden. Hohmann and Newman (Geophysics 55, pp. 1098-1100) have been examining the responses of near-surface 3-D bodies, computed using the integral equation solution which Newman published five years ago (Geophysics 51, pp. 1608-1627).
Results show that the field over these surficial patches are more pronounced than those computed for polarizable overburden layer. The results also suggest that inductive IP could be used in exploration for shallow targets, provided the host rocks are fairly resistive.
Pellerin and Hohmann (Geophysics 55, pp. 1242-1250) have described a scheme to correct the MT static shift which occurs when shallow conductors are present. It is based on 1-D inversion of TEM sounding taken at the same location as the MT sounding site. From the estimated 1-D resistivity structure, an MT sounding curve is computed at frequencies of about 1 Hz and higher. The observed MT curves are then shifted to the position of the computed curve to eliminate the static shift. The scheme is accurate when the overlap region between the MT and TEM sounding is 1-D but helpful information is provided even in multidimensional environments.
The primary use of airborne electromagnetics is in the search for base metal deposits. The interpretation in terms of location, dip, depth and quality of geologic features, based on nomograms constructed from laboratory models, is usually problematic.
Bartel and Becker (Geophysics 55, pp. 1338-1346) have developed a new method based on spectral analysis for use with INPUT data. Because the amplitude spectrum varies exponentially with the target-receiver distance, the target depth is easily interpreted from the scope of a semilogarithmic plot of the spectrum. Conductance and dip can be evaluated directly from the observed field data. The method is useful for depth determination because it is unaffected by dip, size, conductance or profile position. The method has application in automatic interpretation of portions of AEM data profiles with large data sets over many conductors.
The very low frequency electromagnetic (VLF-EM) method is used widely in mineral exploration and engineering geophysical exploration to detect subsurface conductors. It uses plane wave fields generated by communication transmitters in the frequency range 15-25 kHz. It is based on the measurement of the variations of the induced vertical magnetic fields produced by lateral inhomogeneities relative to the horizontal primary magnetic fields. The method has been widely used as reconnaissance technique in the last 25 years because of its simplicity, low cost and ease of qualitative interpretation. There is, however, a need to develop quantitative interpretation schemes.
Ajit Sinha (Geoexploration 26, pp. 213-231) has been using the method extensively for the detection and delineation of weak conductors such as those formed by water and/or clay-filled fracture and shear zones in the Precambrian rocks of the Canadian Shields as part of the Canadian Fuel Waste Management Program. He set out to develop a method for quantitative interpretation of ground VLF-EM data over two-dimensional sheet-like conductors of low conductance, finite depth and depth extent, embedded in a resistive host rock of finite resistivity, using characteristic curves. Numerical modelling of the VLF response of such conductors revealed that the axes-ratio variations are much smaller than the corresponding tilt angle variations and much less sensitive to the depths of the conductors. This was confirmed by field data.
He therefore developed an interpretation scheme to determine the conductance, depth and depth extent which depends on the peak-to-peak tilt angle response and the horizontal separation between the peaks. To obtain the depth extent, a knowledge of the best rock resistivity and approximate depths are required. The method is well illustrated by examples at East Bull Lake and the Chalk river Ontario. It will clearly be welcomed and widely used by mineral explorers everywhere.
Instrumentation and software developments are leading to continual refinements to shallow reflection which will undoubtedly be increasingly applied to problems in the mining industry. For simplicity, optimum-window common-offset data acquisition procedures are frequently employed to collect near-surface, high-resolution, seismic reflection data. However, because of large incidence angles, interpretations cannot be accurately evaluated using zero-offset evaluations alone.
Thomas Wilson (Geophysics 5, pp. 394-401) has been simulating hammer seismic data obtained over coal-bearing Pennsylvanian rocks in Virginia. For synthetic shot records he used a minimum-phase wavelet estimated from the data and subsurface acoustic properties derived from full-wave form and other geophysical logs to simulate the offset seismic response of the near-surface rocks. The Zoeppritz equations were used to model amplitudes. He finds that the offset seismic response is significantly different from the zero-offset response for reflections arising from depths less than about two thirds of the offset distance. For greater depths, zero offset simulations adequately approximate the offset response.
A fascinating evaluation of the use of seismic tomography to image a small subsurface body with a velocity contrast is provided by Bishop and Styles (Geophysical Prospecting 38, pp. 169-188) who constructed a three-dimensional concrete model and described the acquisition and processing of seismic travel time data through the model. They present image reconstruction of the data sets, using an algebraic reconstruction technique incorporating prior knowledge, and compare them with the actual model. Their reconstructions demonstate the importance of obtaining accurate data. Relatively small errors can seriously degrade the reconstruction. Ray path effects are also of great importance. The methodology clearly has application in determining the geometry of ore bodies.
Ground penetrating radar has become a well established technique for very shallow investigations. Wensink, Greeuw, Hofman and van Deen (Geophysical Prospecting 38, pp. 805-830) have carried out some experiments aimed at improving quantitative interpretation, in which they measured under water the near-field E-patterns of a pulsed, horizontal dipole antenna in air. They conclude that far-field conditions have not fully developed at about 2.50 m, although this is about 15 wavelengths. The angular distribution of the radiation patterns of the TA becomes wider as the frequency decreases. As expected, the normalized underwater pulse-spectra shift to lower frequencies with increasing distance as the attenuation constant of water increases with frequency.
Watson, Kruse and Hummer-Miller (Geophysics 55, pp. 70-79) have acquired experimental thermal Infrared Multispectral Scanner (TIMS) aircraft data from the Carlin gold mine area of Nevada.
They have developed a simple model to extract spectral emissivities for mapping surface lithology and lateritization based on the physical properties of geological materials. They prepared emissivity-ratio images which allowed generalized lithological discrimination, identification of areas with high silica content, and the first reported detection of the carbonate secondary reststrahlen feature.
When these are taken together with thermal-inertia images, they permit identification of areas of subtle lithologic variation not shown on conventional geological maps. The spectra derived from the data compare favourably with modelled laboratory spectra and establish for the first time the link among theory, measured physical properties and colour-composited(?) TIMS images.
Their results indicate that TIMS data provide a valuable tool for detecting subtle alteration associated with mineralization, particularly in less accessible or less well-known areas.
Instrumentation and Services
THE development of new geophysical and geochemical techniques and equipment continued during 1990 at a somewhat decreased pace from former years in part reflecting generally reduced levels of mineral exploration in most parts of the world. There were, however, some notable highlights as follows. Several new sensitive magnetometers for aeromagnetic survey work are in an advanced stage of development in North America. The first triaxial aeromagnetic gradiometer system has been built in southern Africa. In Australia, considerable funding is going into the development of airborne electromagnetic survey systems. The Global Positioning System (GPS) is now being employed by most airborne survey contractors improving the efficiency of survey operations and making feasible preliminary field compilation of survey results.
A new company called CGI Controlled Geophysics Inc. based in Mississauga, Ontario, was formed in 1989 from the exploration geophysics and data processing service divisions of A-Cubed Inc. The company carries out the planning, execution, data processing and interpretation of a wide variety of ground and airborne geophysical and remote-sensing surveys.
In June 1990, the assets and on-going business of Crone Geophysics Ltd in Mississauga, Ontario, were sold to its employees and the company was renamed Crone Geophysics & Exploration Ltd. J. Duncan Crone remains the President of the new company which will continue manufacturing ground geophysical instrumentation and carrying out geophysical surveys and consulting work.
Dataplotting Services Inc. and Geotech Ltd have merged their operations at a single location in Markham, Ontario. A new company, Geocorp Inc., has been formed by Geotech and Dataplotting to market airborne geophysical surveys, namely helicopter electromagnetic (HEM), aeromagnetic, VLFEM and spectrometer, geophysical instruments (HEM systems, magnetometers and ground EM) as well as to provide data processing services. Geocorp has also opened an office in Thailand to service clients in South East Asia.
Global Earth Sciences Ltd. of Borehamwood, U.K., had an eventful fourth year of operations, completing airborne geophysical and remote sensing projects in Asia, Africa and Europe. The year has also seen an expansion of the company's range of services, utilizing the new exploration sensors of Prospex Ltd (U.K.) that operate in the visible spectrum.
Intera Information Technologies Corp. of Calgary acquired the assets of Aero Service of Houston, Texas, from Western Atlas in June 1990. In addition to geophysical surveys, Aero provides both remote sensing and GIS services. Aero joins Intera Kenting of Ottawa, Ontario, in the Intera group of companies but will continue to be based in Houston.
Sander Geophysics of Kanata, Ontario, has established a new division for the interpretation of airborne magnetometer and gravity data. This consolidates SGL's expertise in forward modelling, Werner deconvolution, derivative calculation, and in interpretation of these data into geologically significant end projects.
In 1990, Terraplus Inc. of Richmond Hill, Ontario, opened an office in Denver, Colorado, to provide geophysical equipment and service to the U.S. market. Terraplus U.S.A. will also market the various products of Gem Systems Inc., ABEM AB, Pylon Electronic Development, Geosoft Inc., Exploranium, and BRGM Instruments.
AIRBORNE GEOPHYSICAL SURVEYING
Aerodat Ltd of Mississauga, Ontario, has undertaken a number of airborne geophysical surveys overseas including contracts in South America, South East Asia, the U.K. and the U.S. Aerodat operations in Canada during the last three quarters of 1990 decreased in line with the general reduction in exploration activities.
In Sarawak, Aerodata Pty Ltd of Perth, Western Australia, completed the flying of a 45,000 line kilometre high resolution magnetic and radiometric (crystal volume 48 litres downward-looking and 8 litres upward-looking) survey for the Geological Survey of Malaysia. The survey was flown along N-S lines at a basic line spacing of 500 m and covered two onshore blocks, one centred over Kuching (14,000 [km.sup.2]), the other to the south of Sarikei (4,400 [km.sup.2]). All data processing is being undertaken on-site in Kuching with final maps and imagery prepared in Perth.
Dighem Surveys & Processing Inc. of Mississauga, Ontario, reported a moderate level of domestic and international helicopter geophysical survey activity in 1990. The cooperative sales agreement with Compagnie Generale de Geophysique (CGG) of Massy, France, resulted in a U.N. helicopter electromagnetic/magnetic survey in excess of 8,000 line km being carried out in Yemen. Survey results will be used to locate base metals, precious metals and ground water.
Dighen also reported a notable increase in helicopter-borne geophysical surveys in the Cordillera, especially in British Columbia and the western U.S., which offset a substantial decrease in survey demand in the Canadian Shield. Helicopter survey interest in B.C. is being stimulated by the precious metal discoveries in the Eskay Creek region.
Terraquest Ltd of Toronto, Ontario, reports a significant downturn in the aeromagnetic line kilometers flown by the company in Canada for mineral surveys.
Airborne Data Acquisition and Processing
After a long gestation period, the Global Navigation System (GPS) has now reached the stage where it can be utilized as the prime navigation technique along with Doppler or INS (and with flight path camera backup) in aeromagnetic surveys. GPS can provide aircraft position fixes with an accuracy of about 10 metres or so that is more than adequate for airborne geophysical surveying. GPS is having a number of beneficial effects to the practice of aeromagnetic surveying. Firstly, surveys can be carried out in any part of the world. Secondly, a much more even network of traverse and control lines can be flown because the pilot does not have to read maps continuously. He can simply utilize a left-right indicator mounted in the cockpit to guide him along the survey lines. Indeed, pilots are flying without the benefit of topographic maps; they only need to input the waypoints along the lines. Thirdly, the flight path recovery process can be automated and a minimum of point picking is required to verify the position of the flight lines. This will reduce the overall cost of aeromagnetic surveys especially for offshore surveys where no existing electronic navigation chain is in place.
Geopak Systems of Toronto, Canada, has devised a graphical presentation and interpretation system that has sophisticated grid file imaging features and drawing capability. This PC-based system that has a real-time image and computer-aided design capability has therefore been given the acronym RTICAD. RTICAD gives the user the capability to carry out a complete geological interpretation directly on the computer screen. Output features then provide for hard copy on a broad range of colour and black and white devices, including the spectrum from page-size pin printers to the full-size colour electrostatic plotters.
MPH Consulting Ltd of Toronto, Ontario, offers aeromagnetic data enhancement services which include Euler deconvolution depth estimates. Werner deconvolution, derivative and numerical processing methods are also available to provide comprehensive lithologic and structural interpretations as well as to evaluate the data for specific target models.
As of September 1, 1990, Paterson, Grant & Watson Ltd (PGW) of Toronto, Ontario, has taken over the development, marketing and support of the workstation/mini/mainframe-based software from Geosoft Inc. The comprehensive library consists of programs for imaging, frequency and space-domain processing and the plotting of geoscientific data. PGW is also offering a cost-effective alternative to traditional tablet methods of digitizing contour maps or point data. Such maps are now scanned into a raster image and digitized on a workstation screen in one of four modes, grid, contour, flight line, or random.
Picodas Group Inc., of Richmond Hill, Ontario, is now supplying through Scintrex its PDAS-1000 hard-disk-based data acquisition systems with 386 processors in place of the slower 286 processor used in earlier versions of the instrument. Picodas is also manufacturing and marketing a navigational aid instrument, PNAV 2001, intended to improve real-time grid flying for geophysical survey aircraft. The unit interfaces to any navigation receiver which can output Lat/Long, X/Y or range information including GPS, Loran C and Range Range transponder systems.
The Robertson Group plc in U.K. completed the compilation of a new magnetic map of the Republic of Yemen in full colour, by standardizing and integrating 25 surveys of different specifications. A set of 34 1:250,000 geological maps of the country is to be produced from satellite imagery under the same contract. The project also involved the completion of processing of 110,000 km of radiometric/magnetic data from the former Yemen Arab Republic.
RMS Instruments in Mississauga, Ontario, has incorporated a new microcomputer module in its DAS8 and DGR33A data acquisition systems that has an approximately 50% improvement on throughput, but more significantly has a fast turn on capability enabling the system to be ready for data collecting quickly from power-up or reset. In addition, recording on a variety of media, i.e. hard disk as well as magnetic tape, is now available.
TerraSense Inc. of Sunnyvale, California, has released the first major upgrade of its TerraTools software package for compiling and interpreting potential field and gamma-ray spectrometer data. This versatile software is being used by geophysical survey companies in Australia for mineral and petroleum applications and agencies of the German Government for Antarctic research.
In 1990, Urquhart Dvorak Ltd completed the delivery of a comprehensive radiometrics processing software package funded by the International Atomic Energy Agency in Vienna. The package was installed in Teheran, Iran, and is currently used to process older data and produce maps for geological interpretation. Recent acquisition of a Calcomp 58000 electrostatic colour printer by Urquhart Dvorak Ltd significantly enhances the company capability to provide colour pixel imagery to clients.
Anglo American Corp. of South Africa Ltd recognized the need to conduct future mineral exploration by utilizing combined horizontal and vertical airborne magnetic gradiometry. Providing the necessary finance and guarantees, Anglo American signed agreements with Poseidon Geophysics (Pty) Ltd of Gaborone, Botswana, and with Aerial Surveys Bostwana (Pty) Ltd of Gaborone, Botswana, to develop such a gradiometer system. After a thorough evaluation of available compensating systems, Poseidon decided to build what must be the world's first four-sensor triaxial gradiometer. This system is installed in a Cessna Titan 404 aircraft, belonging to, and suitably modified by, Aerial Surveys of Botswana (Pty) Ltd (Fig. 1). Magnetically the aircraft is exceptionally clean and the RMS Instruments AADC compensator was utilized to obtain a very low figure of merit. Scintrex strapdown H-8 caesium sensors are mounted one on each wingtip and with two mounted 2 m apart vertically on a tail stinger of the aircraft in order to provide the total field, the lateral, vertical and longitudinal gradients of the total field every 0.2 secs. The airborne triaxial gradiometer system should be a powerful tool in searching for minerals and in southern Africa that would include diamondi-ferous kimberlites.
Gem Systems of Richmond Hill, Ontario, has developed an optically-pumped potassium magnetometer using sensors provided by the Vavilov Optical Institute in Leningrad. The GSMP20 potassium magnetometer utilizes a single narrow spectral line so there is virtually no heading error although there are 30 [degrees] polar and equatorial dead zones. The Larmor frequency ratio is approximately 7 Hz/nT, i.e. about twice that of caesium, permitting a high sensitivity although the relationship between frequency and total field deviates slightly from being exactly linear. However, the absolute accuracy is within 0.2 nT. The noise envelope of the GSMP20 potassium magnetometer is 1 pT for one reading per second reducing to 10 pT for 10 readings/sec and its range is 10,000 to 100,000 nT.
In contrast to caesium magnetometers, helium magnetometers have a significantly higher Larmor frequency ratio by a factor of about eight that consequently provides a much higher resolution at a greater field sampling rate. EG&G Geometrics of Sunnyvale, California, has completed development of its G-833 metastable helium magnetometer based on technology originated at the Ministry of Geology in China.
Airborne tests of the single-cell system have yielded RMS noise levels of 0.005 [Gamma]. The sensor uses a swept frequency technique, so the polar dead zone common to self-oscillating caesium vapour magnetometers is eliminated although there are still 30 [degrees] equatorial dead zones. This allows survey flights to be made in any direction without reorienting the sensor in field inclinations of greater than 50 [degrees]. The sensors can be hardmounted in small wingtip pods. The Geometrics G-833 helium magnetometer has been installed on the Dornier 28 survey aircraft of the Bundesanstalt fur Geowissenschaften und Rohstoffe (BGR) for use in the German Antarctic programme. Another G-833 has been installed in the Twin Otter aeromagnetic survey aircraft of the Australian Bureau of Mineral Resources in Canberra.
Intera Kenting has replaced its KDSS data acquisition system in their Cessna Titan 404 with the Picodas PDAs-1000 system and will update its other aircraft. Field data processing has also been facilitated by the almost universal use of differential GPS navigation by Intera Kenting. It was a very busy year for Intera Kenting in 1990 flying aeromagnetic surveys in four Canadian provinces. Overseas, two surveys in Zimbabwe were undertaken, the largest being a 145,000 line km aeromagnetic survey funded by the Canadian International Development Agency.
Questor Surveys is providing a new mapping service in which the known geology and interpreted aeromagnetic data are integrated to provide complete geological map coverage. The Geomag Image Processing technique enhances low-amplitude, high-frequency anomalies and suppresses the high-amplitude, low-frequency regional anomalies. The resultant imagery is plotted as a shadow relief having no directional bias that prominently displays all of the magnetic information, even subtle sub-gamma lineations on the flanks of major anomalies.
RMS Instruments has improved its Automatic Aeromagnetic Digital Compensator (AADC) so that it can compensate up to four magnetometers of any type and the associated gradient. The AADC hardware has been made 25% lighter by the incorporation of the power supply into the main chassis. Remote control of the AADC via an RS-232 port is now standard, as well as serial control and communications to the RMS' GR33A graphic recorder.
By the end of 1990, Scintrex Ltd of Concord, Ontario, had completed the development and prototype testing of its new high-sensitivity, caesium sensor, the CS-2, for airborne, base station and portable magnetic applications. The earlier Scintrex H-8 sensor has a proven mean time between failures of 2,500 h and a lamp life time greater than 9,000 h. The CS-2 is expected to surpass these high standards. The orientation range with respect to the optimum direction compared to the magnetic field vector has been increased about 5 [degrees] to [+ or -] 35 [degrees]. Within this range the heading error is no more than [+ or -] 0.1 nT and the instrumental noise is below 0.0001 nT RMS. This orientation specification, along with an on-board polarity switch, permits the worldwide use of the CS-2 in a strapped-down (non-orienting) mode with a minimal heading error. Compared with other magnetometers which use locked oscillator designs, the CS-2 never loses lock, and can follow steep magnetic gradients at high sampling rates. In mid-1990, the CS-2 caesium magnetometer was flight tested on the Convair 580 aircraft operated by the Canadian National Aeronautical Establishment and the Scintrex ground test facility. The production engineering phase will include similar testing of the initial production models before the Scintrex CS-2 caesium sensors are marketed in 1991.
Terraquest has reported on the efficacy of vector horizontal gradiometry in diamond exploration using two wing-tip mounted Scintrex caesium magnetometers. On one survey in west central Canada where the total field data indicated that the anomalies were located along the flight lines, the use of vector horizontal gradiometry identified isolated anomalies due to kimberlites as occurring between the flight lines. This has resulted in a significantly decreased ground follow-up time and the use of wider-spaced flight lines.
Aerodat has developed a five-frequency helicopter electromagnetic (HEM) system in which the coils are mounted in an 8 m Kevlar boom and are separated by 7 m for all components. Customer-specified coil separations are also available to suit client requirements. High resolution HEM data can now be acquired at 500, 4,175 and 33,000 Hz in the horizontal coplanar mode and at 930 and 4,600 Hz in the vertical coaxial mode using an enhanced analogue/digital receiver. The DCSP-99 receiver can process up to six frequencies of HEM data simultaneously.
Spectrem, a new wideband digital towed-bird AEM system, has completed its second year of production surveys. The system, owned by Anglo American Corp. Botswana (Services) (Pty) Ltd, is an extensive modification by Anglo American of the prototype Prospect I system built by A-Cubed Inc., of Toronto, Canada. The Spectrem system was designed with two main objectives, namely to detect massive sulphides at depth in conductive environments and to carry out accurate electromagnetic sounding to depths in excess of 300 m below surface. The results obtained to date confirm that these objectives have been met. The fully digital system, which is mounted in a DC-3 aircraft, is capable of operating in either the time or frequency domains. In time domain operation STEP response delay times from 0.047 to 6.7 millisec are covered. The RMS dipole moment of the transmitter is 120,000 A[m.sup.2] and is currently being upgraded to 300,000 A[m.sup.2]. A high drag bird for fixed transmitter-receiver operation is utilized that allows accurate EM sounding and results in reduced noise levels.
CSIRO in Sydney and the Perth-based World Geoscience Corp. Ltd have been awarded a two-year grant under the Australian Generic Technology scheme to develop a light, cost-effective airborne EM system for determining ground salinity and for mineral exploration applications.
In cooperation with the German BGR agency, Dighem has developed a computer program for displaying resistivity sections in colour. Using the airborne data, resistivity contrasts caused by variously altered volcanic and sedimentary rocks can be easily recognized.
Geotech is now offering, for delivery in 1991, a new digital helicopter electromagnetic system, having single transmitter coil/electronics that is capable of generating a wide range of frequencies. Single transmitter and receiver coil pairs in each plane of the system used for all frequencies has reduced weight substantially. Digital signal processing throughout has allowed the introduction of self calibration, nulling and phasing. This state-of-the-art instrumentation is compatible with all existing ancillary systems.
Geoterrex reports that the two new fixed-wing Geotem systems introduced into the Canadian/Australian marketplace in 1989 have been extremely busy. Airborne EM surveys have now been conducted using the new Geotem systems in Canada, Botswana, Niger, Burkina Faso and Australia.
The airborne digital transient EM system, Questem, of Questor Surveys/World Geoscience is now operating in two survey aircraft -- a Skyvan in Canada and a Trislander in Australia. The sophisticated real-time signal-processing software achieves noise levels that hitherto have been unattainable from analogue systems, enabling Questem to fly successfully in areas affected by powerline, VLF and spheric noise.
Development of the GR-820 airborne gamma-ray spectrometer by Exploranium Ltd of Bolton, Ontario, was completed in early 1990. The automatic gain stabilization on potassium has been considerably enhanced to reject interference from localized uranium and thorium sources which could distort the accumulating spectrum used for gain analysis. The GR-820 uses a large, high contrast, very wide angle graphics display to show clearly the complete individual crystal or combined pack spectra. The display also has a Chart Recorder format to display the basic four window data. The Exploranium GR-256 portable spectrometer has now been modified to allow use with up to 512 [in.sup.3] of detector for small airborne surveys. The system automatically controls gain drift and provides four window data on the RS-232 port for data acquisition systems. The GR-256 will permit the acquisition of reasonable quality radiometric data in airborne surveys where it is not the prime data being collected.
During 1990 Global Earth Sciences flew a number of multi-spectra scanner surveys for environmental monitoring, exploration and research projects. The annual three-phase program for the U.K. Natural Environment Research Council was completed well ahead of schedule, including the transcription of high-density tapes to computer-compatible tapes prior to enhancement and interpretation.
Three new services have been added to the processing and interpretation of remotely sensed imagery by Hunting Technical Services Ltd of Hemel Hempstead, U.K. A method of combining exploration data using the weights of evidence technique developed by the Geological Survey of Canada has been implemented on the company's image processing and GIS equipment. The method allows the measurement of the spatial correlation between known mineral occurrences and exploration data, the comparison of the strength of these correlations and the production of prospectivity maps based on the weighted combination of the data. It is particularly useful for comparing the effectiveness of exploration datasets, in predicting the distribution of mineralization, and for concentrating attention on those metallogenic characteristics of value to exploration.
A new information extraction technique based on principal components analysis of multispectral imagery is also being offered by Hunting in association with Geological Consultants (Ireland) Ltd. The method can be applied to both satellite and aircraft imagery and has proven effective at locating and mapping alteration zones in arid and semi-arid terrain. During development of the technique, several important western U.S. gold discoveries were highlighted on historical thematic mapper (TM) and airborne TM imagery acquired before discovery or disturbance of these deposits.
GROUND SURVEY TECHNIQUES
Geosoft Inc. of Toronto, Ontario, has improved its CPOLT colour plotting program to allow the quality of colour and shaded maps to approach the resolution of photographic work. Working together with major instrument manufacturers, Geosoft has also made available data conversion programs for reformatting instrument dumps to Geosoft input format. Modelling programs, including Geosoft's Magmod3 and Gramod3 for potential field data inversion, Interpex's Emix MM Plus and Resix IP2D for horizontal loop EM and IP resistivity modelling, and Northwest Geophysical's GM-SYS for forward modelling of gravity and magnetic profiles, have been customized for input and output in Geosoft formats. This gives users of the Geosoft 2-D mapping system the ability to process, filter, model and present their field data all within the same software working environment. Other new developments include the upgrading of Geosoft utilities to perform X and Y cartographic projections, shaded profile maps and volumetric grid calculations.
ABEM AB of Bromma, Sweden, has introduced the Wellmac digital well logging system, that uses up to seven different probes simultaneously to depths of 1,500 m. Each stainless steel probe has its own built-in alarm for leakage detection and its own microprocessor for immediate data transfer to the 18 kg weatherproof control unit, where the data can be stored on hard and/or floppy disk drives via an IBM-AT compatible computer. The suite of probes available include resistivity (SPR, normal, lateral), gamma-ray, density, magnetic susceptibility, temperature and will also include fluid resistivity, self potential, neutron, caliper, flowmeter and hole deviation. ABEM AB has also developed an ore delineation logger that uses a multi-parameter probe combination as for the Wellmac (see above). However, this is a single-operator system that has been designed mainly for production blastholes. Data are collected through a Geomac III field computer and are then transferred to another computer for ore limit and grade determinations.
IFG Corp. of Brampton, Ontario, reports the development of a new downhole combination orientation/vector magnetometer tool. This tool provides borehole dip to 0.1 [degree] accuracy and azimuth to 1 [degree] in holes which may dip up to 80 [degrees] from the vertical. The fluxgate sensors provide a three-component magnetic readout to 0.1 [Gamma].
Reflex Instruments of Sweden has incorporated a verticality probe option to its Maxibor borehole dip and direction indicator that measures the deviation of boreholes less than 2 [degrees] from vertical. The new Maxibor system now takes readings every 5 sec. To facilitate the wedging of drillholes, the stored data can be transferred directly into a computer for an immediate display of the results without further processing.
Software which provides tomographic imagery has been developed and is now available from Urtec Instruments of Markham, Toronto, for the Chinese RWL radio wave borehole logging system. This capability executed on standard PCs provides ray path attentuation parameters which outline cavities, fault zones and ore zones between drillholes. Major testing and introduction in Canada is planned for 1991.
In 1990, Apex Parametrics Ltd of Uxbridge, Ontario, completed the development of the MaxMin I-10 ground EM system and the MaxMin Computer. The improved instrument operates on 10 octavely-spaced frequencies ranging from 110 Hz to 56,320 Hz. Three switch selectable sets of 11 coil separations each are built-in, ranging from 12.5 to 400 m, 10 to 320 m, or 50 to 1,600 ft. The MaxMin I-10 coil configurations include maximum-coupled horizontal and vertical coplanar coils as well as two minimum-coupled perpendicular coil configurations. The expanded frequency and coil separation ranges provide for an increased number of applications, from mineral and groundwater exploration to applications such as pollution monitoring involving layered earth soundings.
The newly developed MaxMin Computer (MMC) displays and records the measured in-phase and the out-of-phase components and also computes, displays and records apparent ground conductivity values ranging from 0.1 to 3,276 millisiemens/m. Additionally, standard deviation is displayed and recorded for the measured component values, thus indicating the reading accuracy. The Apex Parametrics MMC comes with coil-control and correction programs for rough terrain surveys, as well as with data transfer and formatting programs, that perform best fit computations for the apparent ground conductivity values from the available multi-frequency data. John Betz Ltd of Etobicoke, Ontario, has developed a software algorithm for computing an approximate bedrock topographic picture from two- or three-frequency MaxMin EM data.
Improvements are ongoing with the Crone PEM receiver, mainly to the software. The graphic display of the recorded data in the form of profiles or decay curves facilitates the infield recognition of anomalies. With the support of Noranda Exploration, Crone has set up an electromagnetic scale model to simulate various subsurface conductor shapes and orientations, conductivities and depths, as well as multiple conductors. Both surface and borehole PEM surveys can be modelled with this facility.
Geo Instruments Pty Ltd of Ramsgate, Australia, has successfully introduced the Mark 3 version of Sirotem, the transient electromagnetic system. The graphical representation of the apparent resistivity versus time curve on the LCD display on the new main console greatly speeds up the ability to check the data quality in the field and also enables rapid understanding of the results on site. As the Sirotem console now has three receiver channel inputs, this has given impetus to the development of a three-component downhole receiver probe.
Geoprobe Ltd has developed a non-grounded four-channel EM instrument called EMR-101 for shallow conductivity and/or resistivity measurements at four depth-levels down to a maximum depth of 6 m. The instrument consists of four transmitters and four receivers housed on a rigid boom (see Fig. 2), separated by 1, 2, 3 and 4 m. The four transmitter-receiver combinations measure the ground resistivity at four optimum depths permitting the depth of an anomalous target to be determined. The four-channels of EMR-101 permit measurements of vertical gradient resistivity facilitating the search for small targets. EMR-101 may be used with only one-channel or up to four-channels in a traverse. The digital read-out display shows the following four measured parameters simultaneously: (1) Conductivity in mS (milli mhos/m); (2) resistivity in ohm-m; (3) quadrature component in ppm; and (4) in-phase component in ppm.
Lamontagne Geophysics Ltd, now of Kingston, Ontario, has continued to develop the UTEM inductive source resistivity (ISR) method. The main advantages of the UTEM ISR method over conventional resistivity techniques is that the source is a horizontally oriented, large loop, inductive source. This transmitter arrangement produces a primary electric field that is largely unaffected by horizontal structure, including overburden. The ample signal strength enables the survey to proceed rapidly using a small crew with considerable cost savings over the conventional DC resistivity methods. Conventionally, the UTEM ISR technique has relied on a grounded electrode dipole as the sensor at the receiver. Despite the high input impedance of the UTEM3 receiver (10 M[Omega]), surveying in winter is more difficult due to the inherent contact problems of frozen ground and snow cover. However, capacitive electrodes developed for the Lamontagne UTEM system may be employed to collect ISR data in both winter and summer. Thus the advantages of good winter access to mining properties in northern latitudes will open up a much wider variety of terrain to cost-effective resistivity mapping using the ISR method and UTEM's capacitive electrodes.
From June to September, 1990, Scintrex and Rudgeofizika of Leningrad, U.S.S.R., collaborated in testing novel geoelectrochemical methods developed in the U.S.S.R. in the Canadian Precambrian Shield. Of a family of five geoelectrochemical methods, three were selected for the 1990 test programme. These methods, named MPF, TMGM and CHIM, are used to detect the occurrence of ore deposits. The remaining two methods, to be tested in 1991, assist in the determination of tonnage and grade of mineral deposits using very few drillholes. The MPF technique relies on the fact that soil organics naturally trap mobile ions from ore deposits. Samples of the appropriate soil horizon are taken, extractions of the metal-organics of interest are made and the result is normalized for carbon content.
The TMGM method has been based on the scavenging of mobile ions by iron and manganese oxides in mineral soil. In the CHIM method an applied electric field attracts mobile metallic ions to collector electrodes (Fig. 3). The solutions in these electrodes are then analyzed for metals of interest. All three methods have detected gold, lead-zinc, copper-nickel and copper deposits under a wide range of conditions in the U.S.S.R. Test surveys were carried out over a range of deposit types, sizes, geometries, depths and over-burden conditions and included gold, copper, copper-nickel and lead-zinc. The surficial glacial cover ranged from lacustrine clays and silts to esker sands from 2-60 m in thickness. Encouraged by the results, Scintrex and Rudgeofizika are forming a joint company to provide geoelectrochemical services internationally.
Androtex of Mississauga, Ontario, has developed a new two-channel five-frequency IP phase receiver, model FDR-2, that incorporates automatic sensitivity adjustment and self-calibration. The FDR-2 receiver contains a 1,200 station memory and the data output is compatible with the Geosoft IP software. Androtex has also completed a new portable time and frequency domain, and phase angle IP transmitter for use in remote areas. The 3.5 kg STX-300 offers a wide output voltage range and stabilized output current using internal batteries.
BRGM Instruments of Orleans, France, augmented its IP line with the VIP 3000 3 kW time and frequency domain IP transmitter. The sealed VIP 3000 can provide 3000 V and regulated 5 A outputs that are displayed on an LCD, programmable time domain and frequency domain cycles from DC to 4 Hz, as well as remote control capability. The 16 kg microprocessor-controlled transmitter may be powered from any 45-450 Hz motor generator providing outputs for eight dipoles in complex array set-ups. Used with the BRGM Elrec 6 receiver, the VIP 3000 provides a fully automated multidipole IP prospecting system.
Scintrex has introduced a new time-domain IP receiver, the IPR-12, that can measure up to eight dipoles simultaneously with 18 parameters being measured at each dipole (Fig. 4). The built-in, solid-state memory records all information associated with a reading, including header information, measured and calculated data. Header information includes time, location, electrode array, random notes and other information that the operator may wish to enter. The 7 kg IPR-12 stacks the information for each cycle and calculates a running average for Vp, SP and each of 14 IP transient windows. The calculated data include an adjustable summed chargeability window, statistical error, Cole-Cole M and Tau, resistivity K factors and apparent resistivity and are monitored on a 16-line display screen.
Zonge Engineering and Research Organization Inc. of Tucson, Arizona, manufactures a number of transmitters that can be utilized for time and frequency domain IP, resistivity, complex resistivity, EM and controlled-source audio-frequency magnetotellurics (CSAMT) surveys. The GGT-series transmitters operate over the frequency range DC to 8 kHz and are powered by 120 Vac 400 Hz motor generators. They will all drive wire loops or grounded dipoles. The 30 kg GGT-3 sealed transmitter is backpack portable providing 3 kVA with a maximum current output of 10 A. The 51 kg medium-power GGT-10 transmitter can provide outputs of up to 30 A at 1,000 V. The 93 kg high power GGT-30 transmitter can provide 45 A of current at 1,000 V. Zonge offers two multichannel digital receivers to operate with the foregoing transmitters in the various applications. The more versatile multi-purpose GDP-16 receiver operates over the frequency range 0.001 Hz to 8 kHz using one to eight field expandable channels. It is software-controlled using C language and has 896 kilobytes of RAM data storage. The TIP-16 receiver is a less sophisticated compact portable receiver operating over one to six channels that may be utilized for time and frequency domain induced polarization and complex resistivity surveys. The Zonge TIP-16 operates over the frequency range dc to 32 Hz and parameter selection is effected using menu-driven software.
Bartington Instruments Ltd of Oxford, U.K., manufactures a magnetic susceptibility system with a measurement range of 1-9,999 x [10.sup.-6] cgs units. The MS-2 console has a pushbutton control that gives digital readings in about 1 sec on a large LCD that are also output through an RS232 port. The MS-2 has a range of coil sensors that produce an alternating one oersted magnetic field at the sample. The change in frequency due to the change in inductance of the measuring coil is measured. The MS2B laboratory sensor can accept 1 in cubes and cores and can operate at two frequencies. The MS2D field sensor is used for in situ volume susceptibility measurements in the top 2 m subsurface.
Gem Systems has introduced a new option to the 0.1 nT GSM-19 Overhauser magnetometer/gradiometer series (Fig. 5) allowing data to be recorded every 0.5 sec as the operator walks along the survey line. Grid co-ordinates can be automatically assigned to pre-designated data points. The sensor is back-pack mounted for hands-free operation. The GSM-19 has undergone various software improvements, such as:
increased cycling time of 5 or 10 readings/sec;
decimal station spacing (i.e. 12.5 for in-between
60 Hz or 50 Hz powerline filters; and
standardizing the memory capacity to 128 Kb.
A |hip-chain' option automatically assigns a grid co-ordinate at a pre-designated distance interval to readings measured by the |walking option' of the GSM-19. Diurnal data from other magnetometers can be transferred to the GSM-19 to make diurnal corrections.
Geoscan Research of Bradford, U.K., is manufacturing three microprocessor-controlled fluxgate gradiometers having 0.5 m separation of the sensors. The FM9 basic version has a 5 nT resolution and a range of [+ or -] 20,000 nT. The FM18 has a 0.5 nT data storage resolution and a 4,000 reading internal memory. The FM36 has a 0.05 nT data storage resolution and a 16,000 reading internal memory (Fig. 6). Both the FM18 and FM36 keep track of the survey position giving both an audible and visual indication on the LCD display.
ABEM AB distributes a borehole radar system called Ramac for cross-hole reflection and tomography surveys that operates over the frequency range 10-80 MHz to a depth of 300 m. An IBM-AT computer with a math coprocessor operates the system. The data are recorded on a 20 Mbyte hard disk and a 5.25 in floppy disk. A directional antenna is used in single-hole reflection surveys to obtain direction data, such as the dip and strike of fracture planes and their angle of intersection to the borehole. The antenna incorporates an electrical rotation technique that enables information to be stored in three independent images. ABEM AB has made available a number of software programs, such as CROSSHOLE, RADAR-PLOT and TOMOCG to interpret reflection data.
Sensors and Software Inc. of Mississauga, Ontario, introduced an upgraded version of their ground penetrating radar system. The pulse EKKO IV radar is a PC-based digital system that includes 12.5 and 25 MHz antennae to augment the 50, 100 and 200 MHz capabilities of the Mark III system. The pulse EKKO IV system may be utilized to map bedrock depth, water table, soil stratigraphy, placer and heavy mineral beach deposits as well as the detection of tunnelling hazards.
The Exploranium GR-256 portable spectrometer has a new optional CPU unit that controls the recording times using an accurate quartz clock. Its main application is for gamma-ray base station recorders for precise control of ground and airborne surveys. Geosoft have recently completed a user interface for Exploranium GR-256 data for contouring/profiling and data integration.
In 1990, ABEM AB introduced the microprocessor-controlled Terrameter SAS 300C resistivity system, which retains all the advantages of the earlier SAS 300B model as well as incorporating a serial input/output port to interface with a computer such as the Geomac III handheld field computer. The signal current is switched from positive to negative in a regular pattern, thus eliminating electrode polarization. The transmitted signal plus SP and ground noise are measured at the stainless steel electrodes when the eddy currents, the IP effect and the EM cable transients have decayed to low levels. The signal averaging system takes consecutive readings automatically, and provides a continuous calculation and display of the running average until the preset number of cycles has been completed. The Multimac multi-electrode cable system complements the SAS 300C and is based on the Schlumberger array.
Oyo Corp. of Tokyo, Japan, is manufacturing an intelligent data-acquisition resistivity system suitable for tomography, resistivity mapping and 2-D profiling. The system consists of the McOhm-21 main unit, the microprocessor-controlled Geo-electric Node unit that switches surface electrode arrays or the Geo-electric Scanner that switches the electrodes in a borehole array. The 28 kg McOhm-21 can be programmed to measure three potential as well as injected current waveforms by using three arrays with a maximum of 250 electrodes each while monitoring the waveforms on a 7-in colour CRT. The data can be downloaded to a 3.5 in floppy diskette for archiving. Furthermore, the inversion for simple 2-D profiling for the data measured by 30 electrodes in a pole-pole array may be printed out using the internal thermal printer.
Tesla-10 have now added a Geonics EM-31 conductivity-meter to complement their vehicle-borne mobile magnetometer system that is being used to map saline areas and diabase dykes in agricultural areas of Western Australia. The diabase dykes form sub-surface walls against which the groundwater table may be dammed. The Tesla-10 conductivity/magnetic profiling system is called Geoliner and consists of three pairs of heavy-duty bicycle wheels on an articulated wooden and aluminium chassis. The geophysical instrumentation is mounted on the chassis and is towed by the vehicle (see Fig. 7) with simultaneous readings being taken every 5 m.
ABEM has available a software processing package called VESNA for shallow reflection surveys using the ABEM Terraloc Mark III seismograph. The menu-driven software runs on IBM XT, AT and 386 personal computers. The versatile program functions include spectrum analysis, filter design, common depth point sorting, normal move out corrections and various graphics for seismic data presentation.
Oyo Geospace is manufacturing a 24-channel signal enhancement seismograph for use in mining and engineering applications. The microprocessor-controlled McSeis-160M has a built-in CRT display, a high-quality thermal printer, and a 3.5-in-high density floppy disk drive to transfer the data to a personal computer for interpretation. A keypad allows the operator to quickly select acquisition parameters. Up to four parameter sets can be stored in the non-volatile memory for recall. The McSeis-160M incorporates a precision function generator to evaluate the performance of the seismic channels and to support extensive system diagnostics.
Oyo Corp. has also developed a 24-channel seismograph, the McSeis 16000, that is equipped with 16 bit A/D converters providing a maximum 100 [Mu]sec sampling for shallow seismic reflection and refraction data acquisition and data processing for on-site analysis. The system (Fig. 8) consists of two electronic units, namely the Model-1118A control unit for controlling the data acquisition and data processing and the Model 1231A display unit with a 7-in monochrome CRT monitor for displaying the interactive operation software menu, seismic data and interpretation results. There is also a thermal printer and 3.5 in floppy disk incorporated to permit the generation of on-site hard-copy.
GOLD has been the major source of exploration activity in 1990-91, but even this was at a lower level than in the past few years. There has been a work interest in marine placer deposits and a full-scale mining operation is under way in the Arctic off Alaska.
Prospecting and ore reserve drilling is done both in the winter on the pack ice and in summer from a modified Landing Craft Tank (LCT). Weather is a major controlling factor and both drilling seasons last approximately four months each, on the ice from December-April and at sea from July-October. Western Gold Exploration and Mining have adapted a Becker diesel top hammer drill to operate on a special broad-based skid in winter. This spreads the 41 t rig load over a 82 [m.sup.2] area which can safely work on only 0.75 m thickness of ice. With very little modification the whole rig package can be lifted by crane on to the drill-ship in about four hours.
The drill pipe used is dual-walled 140 mm o.d. x 83 m i.d. and, in sea depths up to 35 m deep, this can penetrate to 50 m below the sea bed. With a high-pressure booster compression, holes down to 70 m have been achieved.
After trials of many different bit designs the most satisfactory is an eight-track bit with a crowd-out profile. This pushes excess sediment away from the central bore and reduces the frequency of blocking. Bits with a crowd-in profile are satisfactory in the more granular sediments but tend to push too much sediment into the inner barrel and thus cause frequent blockages.
The drill ship uses a four anchor system laid out at 45 [degrees] and this gives possible coverage of 25 ha. In theory, 16 holes can be drilled from one anchor placing when drilling on 100 m centres. Moving on the anchors is done by controlled winching and takes less than 10 minutes to move from one hole to the next. During four seasonal programmes over the past two years they have averaged ten holes per day. The Becker drill can also operate in a purely rotary mode to produce cores for detailed geological evaluation.
Reverse circulation (RC) drilling in conjunction with Down-the-Hole hammers (DTH) combines the advantage of DTH penetration speed with rotary chip-sample efficiency. Several companies are marketing RC/DTH hammers and dual-wall rods, but the Sampler system from Entec Industries of Bangor, N. Ireland, seems to be one of the earliest market leaders.
The high-pressure air is split so that one flow operates the 2,000 blows/min hollow piston whilst the second air flow by-passes the piston and is jetted upwards behind the bit face. This causes a powerful suction which collects all the cuttings from the bit face and passes them directly up through the hollow piston and into the dual-wall drill pipe. Some other RC systems pass the cuttings up the outside of the hammer before they are guided via a special cross-over sub into the inner drill tube. This can produce significant mineral losses due to smearing against the borehole walls.
Entec Industries reports that in tests on a gold mining operation in western U.S., the Samplex hammer gave over 98% by volume cutting recovery and that just over 15% of the sample was 12 mm or larger. This enables the geologist to make a more detailed description of the rock types and helps produce more accurate borehole logs.
Bul Roc (UK) has developed a similar type of RC/DTH hammer and reports that penetration rates of over 12 m/h were fast enough to overcome ground-water entry and gave dry bulk samples in a hard limestone in N.E. England. Undoubtedly the use of a 21 bar (300 psi) compression in holes to 80 m or less contributed to the restriction in water flow. Water flowed into the hole during rod changes but if this water was blown clear before rock penetration was resumed, then dry bulk samples were obtained.
Halifax Tool Co. (Halco) has recently introduced its new RC/DTH hammer which has a stainless steel central sample tube to reduce corrosion problems. This tube can be replaced without dismantling the whole hammer, which is often a major problem on the drill site. Halco gives great importance to the ease of maintaining and servicing its hammer on site. The Halco dual-wall drill pipe is unique in that it is in integral lengths, has no separate central couplings and uses no |O' rings. Halco claims that both inner and outer tubes transmit torque.
Core Drill (UK) Ltd of Warwick answered this challenge by rapidly designing and making a double-tube swivel core-barrel and bit which cuts a 300 mm-dia. core x 365 mm (o.d.). The strata were sufficiently soft to use a special TC core bit and the double core springs effectively retained the marl and siltstone cores. One of the few problems has been in handling the full 1.5 m core-barrel and in safely transporting cores which weigh 182 kg/m.
A new Nirex drilling project to 1,600 m has been undertaken at Sellafield, Cumbria, England, plus a slightly shallower hole at Dounreay in Scotland. This is a joint contract (KSW) between Kenting Drilling, which supplies and operates the drill rig unit, Soil Mechanics Ltd, which provides geotechnical support and testing, and Gewerkschaft Walter Aktiengesellschafte (GWA) of Germany, which has supplied its special heavy duty wire-live core barrels and rods.
GWA developed this wire-line system for drilling long -- up to 1,800 m -- horizontal holes in German coal mines about 10-12 years ago. There are three sizes of 140 mm, 114 mm and 89 mm, but one critical feature is that they are a nesting series of barrels and rods. If the 140 mm barrel can no longer be advanced, the inner barrel is retrieved and the 114 mm barrel and rods run through the 140 mm string and the 89 mm follows through the 114 mm barrel and rods. Being relatively thick-walled, the rods are more rigid than most wire-line strings and with the correct location of stabilizers, accurately controlled directional boreholes can be drilled.
Being active and entrepreneurial, Core Drill (UK) Ltd has supplied several impregnated core bits made in Canada by DIMATEC Inc. of Winnipeg for this NIREX project. These bits of 159 x 95 mm had an impregnated depth of 12 mm and one bit has drilled over 283 mm in 214 hours and still has useful meterage left.
From the same supplier, Core Drill (UK) supplied many impregnated core bits to a mineral drilling contractor on a 12,000 m project in North Wales. The anticipated life of bits was 60 m at a bit cost of around 5/m [pounds]. By modifying and improving the bit specification, meterage was increased to an average of 150 m with bit costs down to 1.50/m [pounds].
Synthetic diamonds are now the norm for many projects and, when coupled with good bit design and improved manufacturing techniques, they are giving remarkable cost reductions. Core Drill (UK) Ltd reports costs down to less than $1/m with its 66 mm non-coring bit set with Stratapax PCD insert, when drilling micro-bores for oil on the island of Gotland off the coast of Sweden. Bits have drilled over 1,200 m at an average of 7 m/h, a 300 m well can now be drilled in 50 h, including moving in and setting up the whole drill unit.
There are relatively few equipment manufacturers who are involved in both mineral and oil/gas drilling. Oreco Diamond Products of Rhyl, North Wales, is one of the few U.K. groups with a market in both fields. Its experience in developing PCD bits for North Sea exploration and production drilling has recently been used to develop a new 114 mm non-coring bit for use in opencast coal prospecting.
Such bits have achieved over 4,800 m at an average penetration of more than twice that of rock bits, with appreciably lower torque and less weight on bit than are needed for conventional rock bits. These lower power demands are very much kinder to the drill string, the rig and its engine and show appreciable savings in fuel.
One aspect of marine oil drilling which has not yet penetrated into mineral drilling is the use of sectional core barrels which can now be built up into quite remarkable lengths. Oreco reports a core run of 141 m achieved in only 17.5 h of rotation with 100% core recovery. To accept the much greater stresses induced in such long core barrels, Oreco has had to thicken up the outer barrel and modify the thread forms to accept the greater torque. It has also had to provide a special internal flow-tube so that any pressure kick can be handled whilst breaking down the barrel and to develop an internal stabilizer with sealed bearings to centralize the inner barrel when drilling inclined and horizontal holes.
One interesting feature of this barrel is that the bearing shaft can now be adjusted to provide the correct inner-barrel stand-off distance by a threaded spacing joint. This does away with the antique method of placing shims in the barrel-head to modify the inner assembly length. Yet this is a common design feature that the better makes of mineral core barrel have used for many years. It really is time that these two sections of the drilling industry tried to work together.
Oreco uses a special polycrystalline diamond (PCD) insert called a CLAM cutter, which has a slightly convex face and a waved or ribbed surface. The convex face gives a greater cutting area, is much stronger on impact and this reduces edge wear and chipping. The waved, variable thickness diamond face absorbs stress better and prolongs cutter life. The inserts have a built-in side and back rake which cuts core cleanly and with less torque and vibration.
On a more modest scale, Christensen Mining Products, Salt Lake City, U.S., reports successful trials on its new |GOLD' impregnated core bits which are designed for use in the ultra-hard and fine-grained non-abrasive rocks such as cherts, quartz and hard siliceous dolomites. Penetration rates are increased by 15-20% and are more constant because this new matrix permits a steady exposure of fresh diamonds and the bit is thus self-sharpening.
One excellent feature of all Christensen wire-line barrels is their unique Quad latch system and matching ratchet-type overshot which the driller can activate from the surface. The Quad latches give the inner barrel assembly a much greater contact area against the locking coupling and on the overshot; also the open design allows free passage for mud or best circulation materials. The overshot has the unique ratchet release mechanism which the driller can pre-set and then activate by pulling sharply on the wire-line cable; there is never any need to leave the overshot and cable attached to a jammed inner-barrel assembly.
Christensen now markets an adaptor kit so that its Quad latch head can be fitted to any of the popular makes of wire-line barrels. All that is needed are the Quad latches, locking coupling, a new bearing and the ratchet overshot assembly. The new latches are mounted on a pivot head so that the inner barrel can easily be laid down whilst still attached to the overshot.
Late in 1990, Diamant Boart Craelius (UK) Ltd stopped the manufacture of diamond bits at its factory in Daventry, U.K. This was part of a group rationalization process whereby only two factories produce the same product; diamond bit production is now limited to Sweden and Belgium.
This left the U.K. with only two domestic diamond bit manufacturers -- J. K. Smit and Oreco Diamond Products. This number was nearly reduced to only one because Delepena Ltd, who bought J. K. Smit two years ago, decided to concentrate on the engineering range of diamond products and wanted to dispose of the bit manufacturing section. After months of negotiation, Core Drill (UK) Ltd purchased the drill bit section of J.K. Smit in March 1991. It will continue to operate from the factory in Colwyn Bay, North Wales, under the name of J. K. Smit (Diamond Drilling) Ltd. This was a brave move to keep the staff and plant in production and give the U.K. a second domestic manufacturer.
One loss to U.K. expertise has been the recent closure of Encore Drilling Co. Ltd as a drilling contractor. Although relatively small, this was a company with an excellent reputation for high-class and innovative mineral exploration contracting with the latest equipment and technology. The current dearth of mineral projects and unrealistic pricing of site investigation drilling contracts have finally persuaded Mr Richard Braithwaite to close the drilling operations.
Fortunately, his knowledge and expertise will not be lost to the industry because he will continue to offer his borehole surveying contract service using the new Reflex Maxibor system for which he is also sales agent. In addition to surveying and plotting out any borehole's track, he will also provide a deviation service to correct alignment or produce new deviation by the use of the Devico directional drilling core-barrel.
The Reflex Maxibor was recently used to survey the final 100 m which separated the French and English tunnel end-faces in the service tunnel link-up under the Channel. This highly accurate survey enabled the British tunnel-boring machine to be steered so that an exact alignment of the two tunnels took place after 37 km of tunnelling.
The Maxibor measures the relative displacement of two reflector rings mounted 3 m apart in a steel tube. A miniaturized video camera records their movements as the instruments bend to follow the line of a borehole. These movements are then recorded against the liquid level sensor which provides a constant vertical reading.
The Maxibor can store up to 4,000 readings in its memory which, when retrieved, can be downloaded into a hand-held computer which displays exact locations at every 3 m along the borehole. When coupled to a small plotter, it can print out the borehole path in plan and elevation and also three aspects of deviation. Within three minutes of withdrawing the survey tool from the hole, the driller and geologist can be looking at up to five print-outs of the borehole direction.
In addition to an accuracy of one in 1,000 and the absence of any film to be developed and read, the instrument is totally immune to effects from magnetism in the rocks or to induced electrical fields. This was well demonstrated on a recent project in Norway when a sewer pipeline had to be laid over a length of 380 m and passed under an office building, a railway line, a road and an electric sub-station. The initial hole was drilled using the Devico steerable core barrel and it needed 61 deviations by this barrel to keep the hole on course. This drilling took 31 days and was mainly in hard schist and gneiss. The 56 mm hole was later reamed to 250 mm by a DTH hammer and then to 600 mm by a raise borer. The initial borehole accuracy was critical because the final tunnel passes only 1 m beneath a major railway line.
The Devico steerable core barrel has two eccentric bearings which can displace the inner barrel and its attached core bit to give a maximum deviation of up to 1.5 [degrees] in 1 m. This can be so sharp a deviation that it is difficult to get conventional coring equipment around such a sharp bend. When the barrel orientation has been checked by the Maxibor survey the outer barrel is locked against the borehole wall by a rubber packer which is inflated by the drill fluid. This special packer has ducts running through it for the drill-flush fluid to pass back down the borehole. When the outer barrel is locked in place the inner barrel is drilled forward for 250 mm, the packer is deflated and the outer barrel advanced and the whole process repeated until the 1.5 m inner barrel is full of core. If sufficient deviation has been created, a conventional core barrel can then be re-introduced to continue the borehole.
The Devico special barrel cuts a 56 mm hole and produces a 29 mm core which will often be quite noticeably bent, giving visual proof of the deviation achieved. Larger size barrels of B (59 mm) and N sizes (76 mm) will soon be available.
Another reported case of an accurately steered borehole is from the Elura mine in New South Wales, Australia, which needed a raise bored shaft between the 400-800 m levels. The initial problem was to drill an H size (93 mm) vertical hole between these two levels and to keep this hole within a 300 mm-dia. circle over the 400 m. Two early attempts were abandoned at less than 60 m because of excess deviation.
These initial surveys were made with a single shot survey tool which was just not accurate enough in a vertical hole. To improve accuracy a Welnav survey tool made by Wellbore Navigation Inc. of Tusten, California, was used. This tool was cable-operated and could give a surface read-out at every 6 m survey station. To increase accuracy the non-magnetic rod housing the tool was rotated through increments of 45 [degrees] to give an eight-cluster shot on every survey station. When any deviation was recorded, a 92.8 mm side-tracker bit on a Navi-Drill down-hole motor was lowered into the hole and carefully orientated.
Because the Navi-Drill's outer housing does not rotate, its orientation remains constant. The deviation was produced by a spring-loaded deflection shoe placed close behind the bit. This shoe exerts a controlled pressure against the borehole wall and pushes the bit in the opposite direction.
The 70 mm Navi-Drill was powered by the flow of 200 l/min of drilling mud at 350 psi, which turned the bit at 800 rpm. After each Navi-Drill run the hole was re-surveyed and, if back on line, it was then reamed out to 93 mm and the stabilized H-size core barrel run until another deviation was needed. None of the hole corrections was for more than 0.5 [degree] misalignment. The hole was opened up by a raise borer to form a 6 m shaft with a cable-guided cage to carry men and materials.
The North-East Division of British Coal originally developed its counter-flush system for drilling geological planning boreholes; this entails running a wire-line string of rods without a core-barrel passing through a high-pressure stuffing box grouted into the mouth of the borehole. The drill-flush water is pumped down the outside of the rods and core is flushed back and continuously discharged through the open drill rod. Using a JKS Boyles B-20 Super Hydramatic machine, British Coal has recently completed a 598 m horizontal borehole under the North Sea at Wearmouth Colliery. This rig has the normal automatic sequencing of chuck and rod clamp so that rods are fed in and retracted very rapidly and torqued up and broken apart under controlled power. This speeds up the whole drilling cycle and reduces the manual requirements enormously. British Coal hopes to develop this system to drill a regular series of 76 mm holes up to 1,000 m long.
JKS Boyles has recently introduced its new Super 300 drill rig which is now rated to 490 m in AQ or 245 m in NQ wire-line. The 70 hp engine drives a JKS Brahma hydraulic pump and this power pack also contains the new simplified control panel. The rig has a 5.18 m mast which folds to 2.44 m for transportation. The rig can have an hydraulic rod clamp in addition to the standard hydraulic chuck.
PHOTO : Fig. 1: Triaxial aeromagnetic gradiometer system mounted on a Cessna Titan 404 survey aircraft; Poseidon Geophysics, Gabarone, Botswana.
PHOTO : Fig. 2: EMR-101 conductivity/resistivity meter for shallow ground measurements; Geoprobe Ltd, Mississauga, Ontario.
PHOTO : Fig. 3: A collector electrode used in CHIM geoelectrochemical tests by Scintrex and Rudgeofizika in Canada during the summer of 1990; Scintrex Ltd, Concord, Ontario.
PHOTO : Fig. 4: IPR-12 time-domain induced polarization receiver; Scintrex Ltd, Concord, Ontario.
PHOTO : Fig. 5: Gem Systems GSM-19 Overhauser magnetometer/gradiometer being demonstrated by Terraplus personnel at the Kirkland Lake geophysical field school; Haileybury School of Mines, Haileybury, Ontario.
PHOTO : Fig. 6: FM-36 fluxgate gradiometer; Geoscan Research Ltd, Bradford, U.K.
PHOTO : Fig. 7: Vehicle-towed Geoliner conductivity/magnetic mapping system; Tesla-10 Pty Ltd, Perth, Western Australia.
PHOTO : Fig. 8: McSeis 16000 24-channel reflection and refraction seismograph for shallow seismic surveys; Oyo Corp., Tokyo, Japan.
PHOTO : JKS-Boyles B-30H rig with jack-up platform and trailer combination undergoing trials before dispatch to Tibet.
C. J. Moon, B.Sc., Ph.D. and M. Aftab Khan, B.Sc., Ph.D., F.G.S., F.R.A.S., Department of Geology, University of Leicester, U.K. Peter Hood, B.Sc., M.A., Ph.D., P. Eng., Mineral Resources Division, Department of Energy, Mines and Resources, Geological Survey of Canada. R. O. Morris, M.A., M.I.M.M., F.G.S., Independent Consultant.
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|Title Annotation:||includes related articles on instrumentation, services and exploration drilling; review of international mineral exploration activities in 1990|
|Author:||Moon, C.J.; Khan, M. Aftab; Hood, Peter; Morris, R.O.|
|Date:||Jan 1, 1991|
|Next Article:||Mineral industry software.|