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Field comparison of seismic sources for high resolution shallow seismic reflection profiling on the Ljubljana Moor (Central Slovenia).

INTRODUCTION

The seismic reflection method is based on reflection of artificially generated seismic waves from subsurface structures. Seismic waves reflect from boundaries within rocks and sediments where a significant contrast in seismic impedance exists. These may be beddings, faults, erosional discontinuities, etc. Since the 1980s and 1990s, the seismic reflection method has been progressively more used in shallow surveys too and has become known as high-resolution seismic reflection (HRS) method. Although the HRS method was derived from deep oil and gas seismic reflection method it is less routine and requires a site-specific approach, due to greater influence of near-surface conditions, the presence of a heterogeneous low-velocity layer, sensitivity to groundwater level and water saturation of near-surface sediments.

The capacity of shallow, HRS surveys has been well proven. It is highly useful in shallow surveys of unconsolidated, non-lithified, partially consolidated and lithified sediments (Wyatt et al., 1996; Buker et al., 1998; Bachrach et al., 1998; Bachrach and Nur, 1998; Steeples and Miller, 1998; Francese et al., 2007). The depth range of the method is between several meters up (ultra high-resolution seismic reflection) to several hundreds of meters with high vertical and lateral resolution (Pullan and Hunter, 1990; Steeples and Miller, 1990; Miller and Steeples, 1991; Feroci et al., 2000; Steeples, 2000). The method is highly successful in stand-alone applications (Wang et al., 2001; Dusar et al., 2001; Sugiyama et al., 2003; Campbell, 2009; Bruno et al., 2010; Green et al., 2010) and in conjunction with other methods such as Ground Penetrating Radar and geoelectrical methods (Abbott and Louie, 2001; Chow et al., 2001; Improta et al., 2010; Chwatal et al., 2005; Kato et al., 2005; Bexfield et al., 2006).

The site-specific approach for this method includes suitable data acquisition parameters and a proper seismic source and geophone choice (Knapp and Staples, 1986; Miller et al., 1994; Steeples and Miller, 1998; Steeples, 2000). Seismic source choice is the most important as various sources have different energy and frequency content, dominant frequencies, source wavelets and effectiveness under different geological and near-surface conditions. Other important factors in seismic source selection include: portability, cost of operation, site preparation requirements, source repeatability and time cycle between shots, environmental damage and safety requirements. Various seismic sources have been tested in different geological conditions. Miller et al. (1986) tested 15 different seismic sources, including a piezoelectric source, a sledgehammer, various seismic rifles and shotguns, various weight drop systems and various types and amounts of explosives. Similar tests by Miller et al. (1992) and Miller et al. (1994) included 13 and 12 different seismic sources, respectively. An extensive analysis of an in-hole shotgun source was described by Pullan and MacAulay (1987). General findings include a wide variability of seismic source energies, ranging over several orders of magnitude and changes in relative effectiveness of sources depending on the survey target depth, water saturation of surface material, depth of water table, compaction and a grain size of shallow subsurface material.

We tested two different seismic sources: an accelerated weight dropper system and a seismic shotgun. Our motivation was to determine the optimal seismic source--ground surface combination for various types of surfaces encountered on the Ljubljana Moor. Frequency spectra and signal-to-noise (S/N) ratios were analysed for a number of combinations of sources and surfaces as well as properties of source generated coherent noise.

GEOLOGICAL SETTING

Seismic sources were tested on the Ljubljana Moor (Figure 1), a Quaternary tectonic basin filled with up to 150 m of fluvial and lacustrine sediments. There are several motivations for geophysical investigations, since the area is interesting from different geological standpoints. As a part of the greater Ljubljana basin area it is an area of significant seismic hazard with prominent historical earthquake activity. Due to large thicknesses of unconsolidated Quaternary sediments pronounced seismic site effects and local seismic wave amplifications are expected in the area. Several regional (Dinaric, NW-SE trending) faults, including Zelimlje and Ortnek faults, cross the eastern part of Ljubljana Moor (Figure 1). Whereas these faults have pronounced surface traces north and south of the Moor, their exact locations and relationships within the basin are poorly constrained (Premru, 1983; Mencej, 1989; Bavec et al., 2009). Studies of the most damaging M=6.1 1895 Ljubljana earthquake macroseismic data suggest that the earthquake occurred along a Dinaric fault.

The Quaternary sediments of the Ljubljana Moor also contain important aquifer systems, providing fresh water to communities on the Moor and to Ljubljana, the capital of Slovenia. Aquifer structure is very complex, due to the complex basement structure, sedimentation and relationship between alluvial sediments of different rivers flowing into the basin (Breznik, 1975; Mencej, 1989). All these factors make HRS method a potentially powerful tool for the study of the structure of Quaternary sediment and pre-Quaternary basement morphology on the Ljubljana Moor.

SITE CHARACTERISTICS

On the surface of the Ljubljana Moor, the upper 10 to 50 cm thick layer is very heterogeneous, depending on the location. In marshlands and grasslands it is represented by hydromorphic grounds, other locations feature an anthropogenic layer, either a ploughed layer or a gravel layer. This layer is underlain by an up to 20 m thick succession of Holocene lacustrine sediments, composed of clay layers with different thicknesses, ranging from <10 cm to 15 m. The lacustrine clay is underlain by a succession of alternating middle and late Pleistocene lacustrine clays and alluvial sands and gravels down to the pre-Quaternary basement (Sercelj, 1965; Grmiscar and Ocepek, 1967). It is expected that the alternating clays, sands and gravels produce distinct seismic horizons, but their lateral continuity is questionable due to the heterogeneous sedimentation.

A very strong seismic horizon is expected at the pre-Quaternary basement, composed of Triassic limestone and dolomite or Permian and Carboniferous sandstones and conglomerates. The depth to the basement is variable. Previous geophysical investigations (vertical electrical sounding and seismic refraction) and borehole data have shown the depth of the pre-Quaternary basement in the investigated area to range between 100 and 180 m.

The groundwater level in the area is always high. Depending on the amount of precipitation the water table was at the depth between ~1.5 m and 0.1 m during the course of data acquisition.

The large majority of the site is characterised by low ambient noise, mostly generated by wind gusts and vegetation. Rarely there was some mechanical noise from nearby fields, but data acquisition was paused during periods of increased noise. In several localised areas, close to major roads and railway lines, noise from traffic is present and data acquisition was undertaken during low traffic density periods (close to noon or at night). In the small settled area (Kozuh), just west of the Ljubljana-Ig road, buildings acted as reflecting interfaces for source-generated, air-coupled waves.

FIELD PROCEDURES

EQUIPMENT

Data was acquired using two 24-channel ABEM Terraloc VI 18-bit seismographs in master-slave mode. Both were connected to a custom made 96-channel input / 48-channel output rollalong switch. Seismic lines consisted of 100 geophones (40 Hz), connected to 12-channel seismic cables. Two different seismic sources were used: a GISCO ESS100 accelerated weight dropper and a custom made seismic shotgun.

The GISCO ESS100 is a car-mounted electric seismic source (Figure 2). It is an accelerated weight dropper system using an elastomer band to spring load the 45.5 kg (100 lb) weight. The source was first used with a 1 kN/m elastomer band, which was later replaced with a ~4 kN/m rubber band. The source is mounted 40 cm above the ground. A 20 X 20 X 2 cm iron plate acts as interface between the weight and the ground. The elastic band maximum dilatation is about 1.6 m, giving an approximate source energy of 1.5 kJ with the 1 kN/m elastomer band and about 5 kJ with the 4 kN/m rubber band.

The custom made seismic shotgun consists of a 120 cm long metal body with a manual spring-loaded firing mechanism (Figure 2). The barrel consists of a reinforced firing chamber on the firing end of the shotgun, made to fit 12-gauge Cheddite Trap shells. The shells were custom made, using a 3cm long aluminium cylinder instead of birdshot and double the explosive charge (60 g) of ordinary 12-gauge shells. The shotgun is fitted with a 60 X 40 cm carbon fibre board acting as stabiliser and air wave suppressor. The firing depth is adjustable between 20 and 80 cm.

DATA ACQUISITION

Data was acquired with a spread of one hundred 40 Hz geophones spaced 2 m apart and 48 active channels. The geometry was on-end with a near offset of 2 m. Shot gathers made with the ESS100 weight dropper contain four stacked shots and the shot gathers made with the seismic shot gun contain a single shot. Shot depth was kept constant at 50 cm in pre-made shot holes. No additional water was applied. Seismic data was acquired with 100 [micro]is sampling and 16384 samples without analogue pre-filter.

All data was obtained on the Ljubljana Moor in the course of the acquisition of a 5.5-km long HRS line. The seismic line crossed varied terrain, including: well consolidated gravel roads, paved (asphalt) roads, grassland, marshes and ploughed fields. Different seismic sources were used depending on accessibility. The accelerated weight dropper was used in areas with good 4WD car access: gravel roads, grassland and ploughed fields. The seismic shotgun was used in areas with limited access, mostly marshlands. Ground surface types and seismic sources for each profile shown in Figure 1 are listed in Table 1.

Different surface types have different elastic properties, porosity and water saturation. These are directly reflected in seismic source energy absorption and seismic wave attenuation. Surface types found on this seismic line include:

Marshland--includes several different types of land surface, all with high water saturation: grassland with very shallow groundwater table (<0.5 m), roadside ditches (along gravel roads) and irrigation channels (used when water level was low).

Grassland--is generally elastic, but in the investigated area ranged from very soft and elastic (in some areas each step produces a temporary depression) to compact and hard. The shallow subsurface was damp, but never water-saturated (groundwater table below ~0.5 m).

Gravel roads--were made of crushed limestone and dolomite gravel in thicknesses 50--200 cm and well consolidated. Only the accelerated weight dropper was used, as the seismic shotgun would require destructive shot holes preparation. Geophones were planted on the road edge deep into the gravel.

Ploughed fields--a single ploughed field was crossed. The surface was a very soft and porous gyttja up to a depth of about 0.5 m with the characteristic "polzarica" clay below.

Asphalt road--a short part of a gravel road was asphalted and allowed testing of this surface type as well. Geophones were planted on the road edge into gravel.

DATA ANALYSIS

Seismic data was processed and analysed with Parallel Geoscience Seismic Processing Workshop (SPW) software. All shot gathers were processed in the following way: first the average frequency spectrum of 40 shot gathers for the entire (2-96) m offset interval and 0-400 ms two-way traveltime (TWT) interval was determined for each seismic source-surface type combination. Then, analysis windows were defined by offset and TWT intervals for reflections and coherent noise types (Table 2). For each seismic source--surface type combination, 20 shot gathers were analysed and the average and median dominant frequencies were determined together with standard deviation for reflections and each coherent noise type (significant signals). Analysis results are summarised in Tables 4, 5 and 6.

To analyse the attenuation of seismic waves on different surface types, S/N ratio of 4 traces with largest offset (channels 45-48, offsets 90-96 m) was determined in the 50-250 ms TWT interval, where useful reflection signals were expected (Table 3).

All shot gathers displayed in this paper (Figures 3-12) show the following processing steps: raw shot gather on the upper left, the same shot gather with AGC (80 ms window) and spectral whitening (10-150 Hz) on the upper right and a frequency spectrum in the lower part of the figure with annotated dominant frequency of the shot gather (note that this is not the dominant frequency of reflections).

GENERAL SIGNAL-TO-NOISE RATIO ANALYSIS

Signal-to-noise (S/N) ratio analysis was performed for 7 different seismic source--surface type combinations (Table 3). All S/N ratios are in the same order of magnitude, implying similar energies for both seismic sources. The seismic shotgun produces a fairly similar S/N ratio for shots into saturated and unsaturated soil, although a significant improvement was observed in shots into saturated soil. The ESS100 accelerated weight dropper S/N ratio results show more variability, over an entire order of magnitude, from the lowest value of 2.8 [+ or -] 1.5, obtained on grassland surface to the highest value of 12.3 [+ or -] 11.1, obtained on paved road surface.

While S/N ratios are illustrative of seismic source energy and efficiency of source energy ground coupling and are a measure of data quality, other factors such as the presence of coherent source-generated noise, strong ground roll and guided waves as well as the frequency content and dominant frequencies also influence data quality.

ANALYSIS AND COMPARISON OF SHOT GATHERS

1. Seismic shotgun: shot into saturated vs. unsaturated soil

Shot gathers used in this comparison were obtained on profile Barje 1 (Figure 13) and in a long, straight roadside ditch due east of profile Barje 1. The surface on Barje 1 profile is characterised as grassland with variable soil water saturation level and compaction (Table 1), while the surface along the roadside ditch is characterised as marshland with water-saturated material. Groundwater level was between 10 cm below and 10 cm above ground surface, shot depth was constant at 50 cm. The results of this comparison are summarised in Table 4 and Figures 3 and 4.

A pronounced difference was observed in seismic shotgun shot gathers depending on whether the shot was made in saturated or unsaturated soil (Table 4, Figure 3 and Figure 4). Higher dominant frequencies were found for seismic reflections in saturated soil shot gathers (36.7 Hz in unsaturated vs. 41.2 Hz in saturated, a 12% difference), yielding an increase in resolution of seismic data (Figure 3). However, by far the largest and most important difference is the near-complete absence of ground roll in shot gathers made with shots in saturated soil evident in Figure 3.

2. Accelerated weight dropper vs. seismic shotgun on grassland surface

A direct comparison was made between the ESS100 accelerated weight dropper with 4 kN/m rubber band and seismic shotgun on a particularly soft grassland surface with damp (but unsaturated), highly porous subsurface material on profile Barje 1 western end (Figure 13). Shot gathers obtained with the seismic shotgun (Figure 5) show significantly clearer seismic horizons than shot gathers obtained with the accelerated weight dropper (Figure 6). Also, the depth range is better with the seismic shotgun: shot gathers obtained with the accelerated weight dropper show clear seismic horizons only down to the distinct pre-Quaternary basement at approximately 150 ms TWT, while shot gathers obtained with the seismic shotgun show clear seismic horizons down to at least 240 ms TWT (Figure 5). Frequency spectra (Figures 5 and 6) show that both sources have similar dominant frequencies, but the seismic shotgun has better high-frequency content. The peak around 80 Hz in the ESS100 frequency spectrum is attributed to the air-coupled wave. Only five test shots were made with the accelerated weight dropper, so no statistical analysis was made due to the small data sample. To directly compare the S/N ratio, five shot gathers obtained with the seismic shotgun closest to the shot gathers obtained with the ESS100 were analysed.

3. Accelerated weight dropper: different elastic bands and plates

The GISCO ESS100 accelerated weight dropper was used on gravel roads in two configurations: with the original 1 kN/m elastomer band and aluminium plate on profile Barje 5 (Figure 17) and with the 4 kN/m rubber band and iron plate on profile Barje 4 (Figure 16). Both configurations produced very good data.

The source configuration with 4 kN/m rubber band and iron plate produced a double-peaked frequency spectrum for seismic reflections with the dominant peak at 29.6 Hz and a slightly weaker secondary peak at 80.1 Hz (Table 5). The very small standard deviation indicates high data consistency. The configuration with 1 kN/m elastomer band and aluminium plate produced a single peak at 40.4 Hz and a somewhat higher standard deviation. Frequency spectra show that both configurations produce considerable high-frequency content. This is evident both in shot gathers (Figures 7 and 8), where clear seismic reflections are observed down to at least 300 ms TWT, well into the pre-Quaternary basement, and in final seismic profiles, as the two profiles obtained with these two source configurations are the finest (Figures 16 and 17). Ground roll as well as the air-coupled wave is very limited with both configurations.

4. Accelerated weight dropper: different surfaces

The GISCO ESS100 accelerated weight dropper with the 4 kN/m rubber band and iron plate was used on several different surfaces: gravel road on profile Barje 4 (Figure 16), paved road just east of profile Barje 4, grassland surfa?e on profile Barje 3 (Figure 15) and a ploughed field on profile Barje 2 (Figure 14). The results of comparison are summarised in Table 6. The highest dominant frequency of reflections is observed in data from the paved road surface, followed by the data obtained on grassland, ploughed field and gravel road surface. With the exception of paved road surface, all surfaces show very similar dominant frequency around 30 Hz with only very small variation.

Shot gathers obtained on ploughed field surface show a high proportion of low frequency, high amplitude signals and only small content of high frequencies (Figure 9). This is evident also in the frequency spectrum as a near-complete absence of frequencies above 100 Hz (Figure 9). Shot gathers obtained on grassland surface (Figures 10, 11a and 11b) show higher variability than any other, except for shot gathers obtained on paved road surface (Figure 12), which is reflected in high standard deviation of reflection dominant frequency (Table 6). In general, seismic data obtained on grassland lacks high frequency content (Figure 10).

The best data in terms of high frequency content, seismic horizon clarity and data consistency with this seismic source was obtained on gravel road surface. A double peak appears in the seismic reflection frequency spectrum with one peak at 29.6 Hz and another at 80.2 Hz. A similar double peak is also seen in refraction data (Table 6). Seismic reflections are seen also in shot gathers (Figure 8) down to at least 300 ms TWT.

EXAMPLES OF PROCESSED REFLECTION PROFILES:

Sample profiles obtained on different surface types with different seismic sources are shown in Figures 13-17. On profile Barje 1 (Figure 13), acquired with the 12-gauge seismic shotgun, discontinuous seismic reflections are visible in the eastern half of the profile (left) and more continuous reflections in the western half. The pre-Quaternary basement shows up as a distinct seismic horizon at 120 ms TWT at the eastern end of the profile, dipping to 130 ms TWT in at the western end. Profile Barje 2 (Figure 14) was acquired using the accelerated weight dropper with the 4 kN/m rubber band on a ploughed field. The pre-Quaternary basement boundary is visible as the distinct and strong seismic reflector at 155-165 ms TWT. Profile Barje 3 (Figure 15) was acquired on a stretch of very heterogeneous grassland surface, ranging from soft and water-saturated to hard and dry, using the accelerated weight dropper with the 4 kN/m rubber band. Big problems with data variability in terms of high frequency attenuation and source energy absorption were encountered, which are reflected in poor profile quality. The pre-Quaternary basement boundary is found at 165 ms TWT at the beginning of the profile and is lost around CMP 1120. Profile Barje 4 (Figure 16) was acquired using the accelerated weight dropper with the 4 kN/m rubber band on a gravel road. Data quality is very high with excellent visibility of seismic reflectors in the Quaternary sediments as well as for the pre-Quaternary basement at 140 ms TWT. Profile Barje 5 (Figure 17) was acquired using the accelerated weight dropper with the 1 kN/m rubber band on a gravel road surface. Again the data quality is very high with seismic reflectors well visible in the Quaternary sediments. Pre-Quaternary basement is very good mapped as a dipping seismic reflector at 155 ms TWT at the eastern end of the profile, rising to 140 ms TWT at the western end of the profile.

No profile sections are shown for shotgun seismic source in saturated soil and for ESS100 accelerated weight dropper on paved road since the sections obtained with these source--surface combinations are too short.

DISCUSSION OF RESULTS

Seismic shotgun fired in water-saturated vs. unsaturated soil

The seismic shotgun produces significantly less coherent noise when the shot is located in water-saturated soil. Reduction in noise includes a significant reduction of air-coupled wave and a near-complete absence of ground roll. This is important, as ground roll represents the most important noise component in seismic reflection data. Also, the 12 % observed increase in peak frequency is favourable for better resolution (Table 4). A significant (42 %) increase in S/N ratio is observed in shots into saturated soil compared to shots into unsaturated soil with S/N ratio 5.8 [+ or -] 2.0 for shots into saturated soil and 4.1 [+ or -] 2.3 for shots into unsaturated soil (Table 3).

Pullan and Hunter (1987) obtained similar results in testing the 12-gauge "Buffalo gun", a similar source to the seismic shotgun we used. Shots made with the "Buffalo gun" in damp or water-saturated sediments produced significantly better results, especially in the high frequency range and reduced air-coupled wave compared to shots made into dry sediments.

Seismic shotgun vs. ESS100 accelerated weight dropper on soft grassland surface

The side-by-side comparison of the ESS100 system and shotgun on a particularly soft grassland surfare (logistically suitable for both seismic sources) showed that seismic shotgun produces a significantly better S/N ratio and improved appearance of seismic horizons. The ESS100 suffered from strong air-coupled wave and lower S/N ratio. A similar result was obtained by Miller et al. (1994). The data obtained with the seismic shotgun produced a significantly improved S/N ratio (14.1 [+ or -] 5.1) compared to the accelerated weight dropper (4.3 [+ or -] 1.1), a ratio of 3.3x. Therefore on Barje 1 profile the shotgun was selected as the seismic source. Both S/N ratios are high, which is attributed to strong seismic reflectors at this location. Note that neither value is in Table 3 due to the small data sample. The overall S/N ratio of shot gathers obtained on Barje 1 profile with the seismic shotgun is lower (Table 3). No statistical analysis of dominant frequencies of significant signals was made due to the small data set.

ESS100 accelerated weight dropper with 4 kN/m band on different surfaces

Highly significant differences were observed in ESS100 data quality on different surface types, permitting their characterization. The S/N ratio analysis showed a systematic decrease in S/N ratio from solid asphalt surface to gravel road surface and grassland surface (Table 3). This is not unexpected as the asphalt surface is the hardest and dampens the source impact very little, however, it is somewhat surprising that the S/N ratio is over 3x higher than on gravel roads. This effect is attributed solely to the surface at the source location, as the surfaces into which the geophones were planted were identical.

Data obtained with the accelerated weight dropper on asphalt also shows a large standard deviation, which reflects operational difficulties of using a (spring-loaded) weight dropper system on a very hard asphalt surface. Weight impact at a right angle produced rebound of variable size. This reduced the S/N ratio in the data. Rebound was nearly or completely absint when the impact occurred at a slight angle (2-4[degrees] from vertical).

The S/N ratio in data obtained on gravel roads (3.7 [+ or -] 1.3) and ploughed field (2.8 [+ or -] 1.5) is similar, although (as expected) it is higher on gravel roads. This surface produces some impact dampening as evidenced by 3-8 cm deep depressions formed by four consecutive impacts on the iron plate. The fairly small standard deviation in S/N ratio is also reflected in high uniformity of data. This is highly desirable, as constant processing can be applied over large segments of seismic profiles. Data obtained on the ploughed field suffers from strong high-frequency attenuation. This effect is obvious both in peak frequency of seismic reflections and in S/N ratio on far-offset traces. Both the peak frequency and the S/N ratio are the lowest of all surface types. High attenuation is also shown by the lowest peak frequencies for coherent noise of all surface types. This is not surprising as the surface is soft and highly compressible. Four consecutive impacts generated pits about 20-25 cm deep, in some places as much as 40-50 cm (these locations were considered unsuitable for data acquisition).

Data obtained on grassland shows much less uniformity, both in peak frequencies of seismic reflections and in S/N ratio. This is due to the heterogeneity of grassland surfaces and also probable near-subsurface heterogeneity. The S/N ratio of 6.1 [+ or -] 3.0 is higher than for gravel road and ploughed field surfaces, however, the large standard deviation is indicative of high variations in shot gather quality. Indeed, grassland surfaces show high variations in stiffness and water saturation--in transient areas between cultivated grass fields and marshland the surface is highly elastic and soft. The seismic source produces significant rebound as well as 5-15 cm deep pits after four consecutive impacts.

The dominant frequency of seismic reflections (Table 6) was the highest on paved road (60.0 Hz), followed by the grassland (32.2 Hz), gravel road (29.6 Hz, with secondary peak at 80.2 Hz) and ploughed field (28.9 Hz). The high dominant frequency in paved road shot gathers is expected due to low attenuation of signal. Dominant frequencies on gravel road, grassland and ploughed field surfaces are very similar, all close to 30 Hz. Due to the large data variability on paved road surface and otherwise very similar dominant frequencies on other surfaces, the S/N ratio is the deciding factor in surface type selection with this seismic source.

ESS100 accelerated weight dropper with 1 kN/m vs. 4 kN/m band

The 1 kN/m elastomer band and aluminium plate combination yields a higher S/N ratio (9.4 [+ or -] 4.4) than the 4 kN/m rubber band and iron plate (3.7 [+ or -] 1.3). The significance of this high difference is, however, questionable since the profile section on which the first combination was used is 1 km distant from the location where the second combination was used. The first location is characterized by significantly lower ambient noise (less vegetation) and appears to have a slightly higher impedance contrast at seismic horizons. For the target depth range of 200-300 m, both combinations produce sufficient S/N ratio for high data quality (Figures 16 and 17).

Influence of changing geological conditions

Some variability in seismic horizon clarity can be a result of changing geological structure and impedance contrast. This may be seen in profiles Barje 4 and Barje 5, both acquired using the accelerated weight dropper on gravel roads, but with different elastic bands. The pre-Quaternary basement boundary is well visible on both profiles, however, seismic reflectors in Quaternary sediments are slightly more apparent in Barje 4 than in Barje 5 (the same processing workflow was applied for both profiles). However, as the Quaternary sediments of the entire investigated area are the result of rather uniform sedimentation (lacustrine sedimentation over a large area in the upper part and distal large alluvial fan deposition in the lower area), large differences in reflectivity are not expected. Thus we attribute most of the data variability to the surface type and seismic source selection.

Improving resolution and S/N ratio

In some cases, due to logistical considerations, source availability or other factors, less than optimal source-surface combination choices must be made. Data quality can be improved during data acquisition by multiple shot stacking (for ESS100) and use of analog filters. Further improvement to resolution and S/N ratio can be made by removing random noise with frequency filtering and source-generated coherent noise by f-k filtering. Additional resolution improvement can be made with the use of deconvolution algorithms. With careful acquisition planning and consideration of source and surface specifics, useful data can be obtained on the majority of possible source-surface combinations.

CONCLUSIONS

Two different seismic sources were used on the Ljubljana Moor in the course of acquisition of a 5.5 km long HRS profile. The sources were evaluated for suitability on different surface types encountered on the Moor. Eight different source-surface condition combinations were tested. The two sources were directly compared only on (soft) grassland, but this is the only surface type on which both sources could likely be used.

Both sources were found suitable for high-resolution shallow seismic reflection profiling in given environment (Figures 13-17). Field comparison showed that careful selection of the seismic source is advisable, depending on the ground surface conditions. However, other considerations also play a role, such as land use permits and cost considerations. The seismic shotgun requires comparatively expensive shells and an operator with gun permit. Final shotgun operational cost per shot is about 5x higher than for the ESS100 accelerated weight dropper.

The seismic shotgun was found to be the most suitable for marshlands due to its simpler logistics, as well as the possibility of placing the shots into water-saturated soil. Thus, the seismic shotgun is the most favourable source for marshlands with shallow underground water table or high water saturation of soil. The near-complete absence of ground roll and significantly reduced air wave, as well as high source energy provide high S/N ratio for shots into saturated soil, compared to shots into unsaturated soil.

The accelerated weight dropper system was superior for gravel and paved road surfaces. Grassland results indicate the accelerated weight dropper to be generally superior, however, the seismic shotgun produced better results on particularly soft grassland surface. Thus, when used on this surface type, a careful side-by-side comparison of both sources is advisable.

The accelerated weight dropper system was used with two different bands. It was found to produce higher S/N ratio when used with the 1 kN/m elastomer band and aluminum plate than with the 4 kN/m rubber band and iron plate. The replacement 4 kN/m rubber band was used due to better availability after a series of breakages of the original 1 kN/m elastomer bands. The significance of this result requires further analysis. Both configurations produced good and consistent data.

Responses of four different surface types to the accelerated weight dropper seismic source were analysed. Asphalt surface produced the highest S/N ratio of all surfaces as well as a high frequency response. On asphalt, however, rebound problems caused very high variability in shot gather quality, making the data heterogeneous and less favourable for processing. Only in optimal shot conditions, the asphalt surface produces the best seismic data.

The gravel road surface produced high-quality seismic data with the accelerated weight dropper system. The S/N ratio is, however, lower than on asphalt surface, but due to consistent results, as shown by the low standard deviation, and high dominant frequency of reflections, this source--surface combination produced the best results.

Ploughed field surface is poorly suited for high resolution seismic reflection due to high absorption and attenuation of signal. Both the seismic shotgun and accelerated weight dropper produced comparatively poor results. Seismic shotgun produced strong air wave due to the explosion blast escaping through the porous ground material. Also ground roll was very strong with this source. The accelerated weight dropper was problematic due to the plate sinking into the ground with each consecutive impact, dampening it and reducing source energy. It is thus highly recommended to avoid such surfaces if possible.

Grasslands proved a similarly unsuitable terrain for both seismic sources. The seismic shotgun was problematic due to strong ground roll and air wave. Data obtained with the ESS100 source showed large variations. The surface shows much local variation in stiffness and attenuation of seismic waves. Generally the best seismic sources were found to be: the accelerated weight dropper (regardless of the elastic band and plate used) on hard gravel road surfaces and the seismic shotgun fired into saturated soil. All provided consistently high-quality data. The accelerated weight dropper has poor performance on softer ploughed fields and grasslands as well as on asphalt surface. The seismic shotgun has less consistent performance when used in unsaturated soil. It is thus concluded that for seismic reflection profiling in the Ljubljana Moor hardened gravel roads and/or marshlands with high water saturation should be chosen.

ACKNOWLEDGEMENTS

The authors are thankful to Robert Stopar, Ivan Majhen and Sandi Kmeti? of Geoinzeniring d.o.o. for their extensive field support. Ivan Majhen also developed and manufactured the seismic shotgun. Thanks are also due to Jernej Jez and Blaz Milani? of Geological Survey of Slovenia for their extensive field support. This work was done in the scope of research programme Regional Geology (P1-0011 (C)), project Seismotectonic model of the Ljubljana basin (L1-2383) and PhD work (Slovenian Research Agency grant No. 1000-07-310075) at the Geological survey of Slovenia.

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Jure ATANACKOV (1), * and Andrej GOSAR (2)

(1) Geological Survey of Slovenia, Dimiceva 14, 1000 Ljubljana, Slovenia

(2) University of Ljubljana, Faculty of Natural Sciences and Engineering and Slovenian Environment Agency, Seismology and geology office, Dunajska 47, 1000 Ljubljana, Slovenia

* Corresponding author's e-mail: jure.atanackov@geo-zs.si

(Received August 2012, accepted January 2013)


Table 1 List of seismic profiles (shown in Figure 1),
corresponding ground surfaces and used seismic sources.

Profile   Ground surface         Seismic source

Barje 1   grassland (eastern     12-gauge seismic
          half unsaturated,      shotgun (ESS100
          damp, compact soil,    accelerated
          western half drier     weight dropper
          and more porous        also tested)
          soil)

Barje 2   ploughed field         ESS100 accelerated
                                 weight dropper

Barje 3   grassland (highly      ESS100 accelerated
          variable water         weight dropper
          saturation and
          level of compaction,
          from highly
          water-saturated
          to dry, soft and
          elastic to hard and
          compact)

Barje 4   gravel road            ESS100 accelerated
                                 weight dropper

Barje 5   gravel road            ESS100 accelerated
                                 weight dropper

Table 2 Analysis windows for significant
signals (reflections and coherent noise).

Signal          Offset        TWT
               interval     interval
                 (m)          (ms)

Reflections     24-96        50-150
Refractions     24-40        30-50
Ground roll      2-24       150-400
Air-coupled   where most   where most
wave          pronounced   pronounced
Guided        where most   where most
waves         pronounced   pronounced

Table 3 S/N ratios for different combinations of
seismic sources and surfaces.

Seismic source and          S/N ratio
surface

Shotgun + saturated      5.8 [+ or -] 2.0
  soil (marshland)
Shotgun + unsaturated    4.1 [+ or -] 2.3
  soil (grassland)
ESS100 accelerated       9.4 [+ or -] 4.4
  weight dropper + 1
  kN/m elastomer band
  + aluminium plate
  on hardened gravel
  road surface
ESS100 accelerated      12.3 [+ or -] 11.1
  weight dropper + 4
  kN/m rubber band +
  iron plate on
  hardened paved
  (asphalt) road
  surface
ESS100 accelerated       3.7 [+ or -] 1.3
  weight dropper + 4
  kN/m rubber band +
  iron plate on
  hardened gravel
  road surface
ESS100 accelerated       6.1 [+ or -] 3.0
  weight dropper + 4
  kN/m rubber band +
  iron plate on
  grassland surface
ESS100 accelerated       2.8 [+ or -] 1.5
  weight dropper + 4
  kN/m rubber band +
  iron plate on
  ploughed field
  surface

Table 4 Significant signal dominant frequencies for seismic
data obtained with the seismic shotgun used in saturated and
unsaturated soil.

Seismic shotgun + unsaturated soil (grassland)

              Reflections   Refractions   Air-coupled
                 (Hz)          (Hz)        wave (Hz)

Average          36.7          87.8          36.9
Median           35.0          87.6          39.0
Standard         12.7          37.1           4.9
  deviation

Seismic shotgun + saturated soil (marshland)

Average          41.2          57.7          43.5
Median           41.5          56.1          44.0
Standard          7.6          25.4           5.6
  deviation

Seismic shotgun + unsaturated soil (grassland)

              Ground   Guided
               roll    waves
               (Hz)     (Hz)

Average        35.4     None
Median         35.9     None
Standard        4.5     None
  deviation

Seismic shotgun + saturated soil (marshland)

Average        None     None
Median         None     None
Standard       None     none
  deviation

Table 5 Significant signal dominant frequencies for seismic
data obtained with the ESS100 accelerated weight dropper with
1 kN/m elastomer and 4 kN/m rubber band. Double peaks appear
for reflection and refraction signals with the 4 kN/m rubber
band. The expression 29.7/80.14 (@ -0,02 dB at 67 %) means a
secondary peak which appears at 80.14 Hz with -0.02 dB strength
compared to the primary peak and is present in 67 % of shot
gathers.

ESS100: 4 kN/m rubber band + gravel road surface + iron plate

              Reflections   Refractions     Air-
                 (Hz)          (Hz)        coupled
                                          wave (Hz)

Average        29.6/80.1    88.3/151.2      None
              (@-0.02 dB    (0.0 dB at
               at 67 %)        50 %)
Median          29.1/78     78.0/155.5      None
Standard        3.7/4.8      8.8/10.0       None
  deviation

ESS100: 1 kN/m elastomer band + gravel road surface + aluminium
plate

Average          40.4          81.6         None
Median           36.7          78.8         None
Standard         11.1          25.3         None
  deviation

ESS100: 4 kN/m rubber band + gravel road surface + iron plate

               Ground     Guided
              roll (Hz)   waves
                           (Hz)
Average         32.2       None

Median          31.7       None
Standard         4.9       None
  deviation

ESS100: 1 kN/m elastomer band + gravel road surface + aluminium
plate

Average         24.5       None
Median          24.2       None
Standard         4.2       None
  deviation

Table 6 Significant signal dominant frequencies for seismic data
obtained with the GISCO ESS100 accelerated weight dropper with 4
kN/m rubber band on various surfaces--grassland, gravel road and
ploughed field (and a short, 60 m long section of asphalt road).

ESS100: 4 kN/m rubber band + hardened gravel road surface + iron
plate

              Reflections   Refractions   Air-coupled
                 (Hz)          (Hz)        wave (Hz)

Average        29.6/80.2    88.3/151.2       None
                             (@0.0 dB
                              at 50%)
Median          29.1/78     78.0/155.5       None
Standard        3.7/4.8      8.8/10.0        None
  deviation

ESS100: 4 kN/m rubber band + paved (asphalt) road surface + iron
plate

Average          60.0          71.2          None
Median           67.8          68.8          None
Standard         11.6          13.0          None
  deviation

ESS100: 4 kN/m rubber band + grassland surface + iron plate

Average          32.2          43.1          60.5
Median           33.0          39.2          39.5
Standard          4.8          15.7          42.9
  deviation

ESS100: 4 kN/m rubber band + ploughed field surface + iron plate

Average          28.9          39.4          222.5
Median           29.1          39.3          224.5
Standard          1.6           0.3          51.5
  deviation

ESS100: 4 kN/m rubber band + hardened gravel road surface + iron
plate

              Ground   Guided
               roll    waves
               (Hz)     (Hz)

Average        32.2     None

Median         31.7     None
Standard        4.9     None
  deviation

ESS100: 4 kN/m rubber band + paved (asphalt) road surface + iron
plate

Average        36.0     None
Median         34.3     None
Standard        3.0     None
  deviation

ESS100: 4 kN/m rubber band + grassland surface + iron plate

Average        14.8     None
Median         14.8     None
Standard        3.3     None
  deviation

ESS100: 4 kN/m rubber band + ploughed field surface + iron plate

Average        11.6     None
Median         10.2     None
Standard        3.3     None
  deviation
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Author:Atanackov, Jure; Gosar, Andrej
Publication:Acta Geodynamica et Geromaterialia
Date:Jan 1, 2013
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