The matrix porosity and related properties of a leucocratic granite from the Krudum Massif, West Bohemia.
The nature and origin of granites has intrigued earth scientists for hundreds of years (Pitcher, 1997). In order to obtain detailed information about the characteristics of a specific granitic rock it is important to study its physical, geomechanical, and chemical properties. These properties may be greatly modified where the rock is fractured. These data, in addition to an understanding of the present geological setting of the granite, are essential when planning the construction of artificial underground structures such as tunnels, radioactive waste repositories, and storage facilities for oil or natural gas. Schild et al. (2001) studied a granite matrix using a range of petrophysical methods to determine its porosity, permeability, and P-wave velocity. Hamm et al. (2007) concluded that the hydraulic parameters of fractured rocks need to be assessed precisely in order to understand the relationship between the hydraulic conductivity and fracture properties.
A number of studies have shown that the ultrasonic pulse velocity test provides a useful and reliable non-destructive tool for assessing the structure of rock. The internal damage within a material may be seen from its mechanical characteristics as defined by the modulus of elasticity and compressive strength (Hassan et al., 1995) while structural irregularities may be detected by ultrasonic tests (Schild et al., 2001; Zinszner et al., 2002; Vasconcelos et al., 2008; Chaki et al., 2008; Novakova et al., 2011). The most popular and rapid method of pore structure analysis in a wide variety of porous materials is that of mercury porosimetry (Ritter and Drake, 1945). This technique enables the size and volume of mesopores and macropores to be measured in solid porous rocks. It is based on the property of mercury to behave as a non-wetting liquid in a variety of solid materials. The measurement of pore-size distribution is complicated by the fact that most porous materials are characterised by irregular pore shapes; this may be overcome if it is assumed that all pores have a common geometric shape (Felch and Shuck, 1971). A model for such pore surfaces was developed by Good and Mikhail (1981). In the model, pores are represented by cylinders. In reality, pore surfaces are very rough and contain void spaces that correspond to the entrance of branch pores.
[FIGURE 1 OMITTED]
Scanning Electron Microscopy (SEM) and stereological analysis form valuable tools with which to characterise microcracks in granites (Hommand Etienne and Houpert, 1989). The use of a high resolution microscope is comparatively inexpensive and may be undertaken on a considerable range of samples (Bogner et al., 2006). However, the imaging and analysis of single features (e.g. pores, cracks, microfissures) by electron microscopy is usually limited to very small areas (Helmuth et al., 1999). The strong relationship that exists between the rock microstructure and its physical properties was confirmed by David et al. (2000). Machek (2011) used SEM to determine cracks within granite rock and it was concluded that the majority of the granite matrix porosity could be attributed to grain-boundary and cleavage cracks. This microcracking depends upon the rock mineralogy, fabric, and microstructure (Akesson at al., 2004).
By combining various methods it is possible to accurately characterise the rock, especially its composition and microstructures. In this paper, the results of a number of laboratory methods applied to samples taken from a single core are described and compared. The presented data form a small part of the comprehensive research project, "Research of an influence of a granite matrix porosity over a radioactive waste geological disposal safety including methodology and measuring devices development". The main objectives of this paper are to outline a coherent approach to investigating granite matrix porosity, to compare the applied methods, and to consider the implications of recent studies of granite matrix.
2. GEOLOGICAL SETTING
The studied locality is situated near town Krasno, 20 km southwest of Karlovy Vary town in western Bohemia (Fig. 1). Borehole KZ-25 was drilled in 1974 during prospecting for a nearby feldspar quarry (Vysoky Kamen Quarry). The borehole was located in the eastern part of the Krudum Massif. This massif is a broadly triangular granite body covering 50 [km.sup.2]. The central part of the massif comprises a porphyritic two-mica granite (the Tfidomi Granite) while its western part is dominated by a two-mica granite (the Milife Granite). The southern and eastern margins of the massif comprise the youngest and the most fractionated topaz-albite granite (the Cista Granite) (Jarchovsky, 2006). Post-magmatic brittle deformation has given rise to a suite of mineralogically distinct veins (barren quartz, quartz-cassiterite, quartz-wolframite, quartz-arsenopyrite, quartz-fluorite, and quartz-hematite) (Dolnicek et al., 2012). The southeastern margin of the Krudum Massif is cut by the ENE-WSW trending Vysoky Kamen Fault. The western block uplifted along this fault and was then eroded (Jarchovsky, 2006).
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The KZ-25 borehole was drilled in an area of leucocratic granite with alkali feldspar syenite layers lying on the basal Li-F granite (Jarchovsky, 2006; Fig. 2). The leucocratic granite is medium to fine grained. In this granite, micas are rare and the Na-feldspar to K-feldspar ratio varies significantly (from 0.5 to 2.5). The granite is locally composed by alkali feldspar syenite, a rock with no micas and a low quartz content (Streckeisen, 1974) (Table 1, Table 2). Numerous small pegmatite bodies of veins of irregular shape and smudge-like pegmatite bodies were also found in the quarry (Rukavickova et. al, 2009). The location of the borehole has now been quarried for raw feldspar.
3. EXPERIMENTAL PROCEDURE
3.1. SAMPLING AND CHEMICAL ANALYSIS
The KZ-25 core is stored in the Material Documentation Depositories of the Czech Geological Survey. The required samples were taken from this core at depth levels of 18, 28, 38, 48, 58, 76, 88, 98, and 108 metres (Fig. 3), Figure 4 shows the polished samples prepared from these samples. The diameter of the original core is 150 mm while that required for most tests is 50 mm. Therefore, a new core with the required diameter was cut from the original. Samples with heights of 10, 20, 50, and 100 mm were then cut from the new core using a water-cooled diamante saw. In this paper, these samples are referred to as 'unsaturated'. A macroscopic petrological description of the samples was then conducted. To determine the chemical composition of the rocks within the KZ-25 borehole, chemical analyses of rocks were undertaken in the laboratory of Czech Geological Survey.
3.2. CONNECTED AND TOTAL POROSITY
The required samples (h = 50 mm; d = 50 mm) were cut from the original core and then oven dried for 24 hours at 105 [degrees]C. During heating and cooling, the temperature was changed gradually by 0.3 [degrees]C/min so as to prevent any cracking that may result from the development of a high temperature gradient inside the sample (Reuschle et al., 2006; Chaki et al., 2008). Following temperature equalisation, the specimens were water saturated under vacuum as proposed by ISRM (1977). The specimens were kept under vacuum for 24 hours before being submerged by water at atmospheric pressure for between 24 hours to several days to allow the saturation. The connected porosity ([n.sub.ISRM]) was obtained by weighing the saturated [m.sub.sat] and dry [m.sub.d] samples, respectively (Eq. 1),
[n.sub.ISRM] = [m.sub.sat]-[m.sub.d]/[m.sub.sat]-[m.sub.sat]' (1)
where [m.sub.sat]' is the weight of the saturated sample immersed in water. An alternative calculation of the connected porosity ([n.sub.hc]) was also undertaken to assess the influence of the measuring procedure: the value of msat measured after the hydraulic conductivity test was used in Eq. 1. The total porosity ([n.sub.T]), which includes isolated voids and microfractures, was calculated from Eq. 2,
[n.sub.T] = 1-[P.sub.d]/[G.sub.s] (2)
where specific gravity [G.sub.s] was determined through a standard test procedure in a pycnometer and dry density [p.sub.d] was calculated as a ratio of [m.sub.d] and volume V. The moisture (W) was calculated using Eq. 3, where [m.sub.sat] is saturated and [m.sub.d] dry sample, respectively.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
W = [m.sub.sat]-[m.sub.d]/[m.sub.d] (3)
3.3. MERCURY (HG) POROSIMETRY
The Pascal 140+240 (Thermo Scientific) mercury injection system was used to determine the connected porosity of the samples. Porosity and open pore size distribution of each sample were computed using the PASCAL (Pressurization with Automatic Speed-up by Continuous Adjustment Logic) method and the Washburn (1921) equation (Eq. 4):
pr =-2 [mu] cos[THETA] (4)
where p is pressure, r pore radius, [mu] surface tension, and [THETA] contact angle.
The apparatus uses pressure interval from 0.1 kPa to 200 MPa and from this it is possible to identify pores with widths ranging from 3.7 nm to 58 urn. The pressure applied depends on the size of the threshold controlling access to the void. The measurement of the injected mercury volume at different steps of increasing pressure gives the volume of pores for a range of thresholds.
3.4. HYDRAULIC CONDUCTIVITY
The saturated samples (h = 50 mm; d = 50 mm) were placed in the pressure cell. Figure 5 shows a sketch of a sample during the test. Both upper and lower surfaces were connected to pressure controllers manufactured by GDS Ltd. The cell pressure was kept constant during all tests, at [[sigma].sub.c] = 1000 kPa. The average effective stress inside the specimens corresponded to the average effective stress of the specimens in particular depths in situ. A constant pressure difference between upper and lower surfaces of [DELTA] = 50 kPa was kept during the test and the volume of water that passed through the sample was recorded.
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3.5. ULTRASONIC VELOCITIES
P-wave and S-wave velocities were measured vertically in the samples (h = 50 mm; d = 50 mm) using an apparatus that comprised two pairs of piezosensors, a precise impulse generator, and an oscilloscope. The two pairs of piezosensors, V103 and V153 (Olympus), were used as transmitter and receiver respectively. The resonance frequency of the sensors was 1 MHz. The contact between the sensors and sample was improved using ultrasonic couplant. The saturated and oven dried samples were measured in accordance with that proposed by ISRM (1978). Length to diameter ratio of 1 was used due to feasibility of hydraulic conductivity measurement. Dynamic Young's modulus [E.sub.d] (Eq. 5), dynamic shear modulus [G.sub.d] (Eq. 6), and Poisson's ratio v(Eq. 7) were calculated according to (Zisman, 1933).
[E.sub.d] = [rho][v.sub.S.sup.2](3[v.sup.2.sub.p]-4[v.sup.2.sub.S])/[v.sup.2.sub.p]-[v.sup.2.sub.s] (5)
[G.sub.d] = [rho][v.sup.2.sub.s] (6)
v = [v.sup.2.sub.p]-2[v.sup.2.sub.s]/2([v.sup.2.sub.p]-[v.sup.2.sub.s]) (7)
where [E.sub.d] is dynamic Young's modulus, [rho] is density of the studied sample, [v.sub.s] is S-wave velocity, [v.sub.p] is P-wave velocity, [G.sub.d] is dynamic shear modulus, v is Poisson's ratio.
The calibration of the piezosensors was undertaken using radiation through three steel cylinders of the same class and the lengths 30, 60, and 90 mm. The delay of the wave on the piezosensors was calculated using linear regression and subtracted from all measurements. For P-waves it was 0.273 (is, for S-waves 0.253 (is. The measurement error was specified by repeated radiography of randomly chosen samples. The error was 0.8 % for P-waves and 1.3 % for S-waves.
3.6. DEFORMABILITY IN UNIAXIAL COMPRESSION
This method determines the elastic parameters of rock. The cylindrical samples (50x100 mm) were loaded with uniaxial stress. During the experiment, both the transverse and longitudinal deformation of the samples was recorded by resistivity tensometers (20/120LY41 Hottinger Baldwin Messtechnik) attached to the surface of the sample. The loading was implemented in five cycles with a constant gradient of axial stress of 0.5 MPa x[s.sup.-1]. The loading maxims were 20, 30, 40, 50, and 60 % of compressive strength while the unloading minimum was set to 5 % of the compressive strength. Static Young's modulus and Poisson's ratio were calculated following the methods of Zavoral et al. (1987). Static Young's modulus E and Poisson's ratio v were determined from the hysteresis of the first and second loading loop (Eq. 8, 9).
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (8)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (9)
where [[epsilon].sub.ai] and [[epsilon].sub.di] are appropriate average values of longitudinal and transverse deformation respectively, [[sigma].sub.1.3] and [[sigma].sub.2.5] are maximal and minimal values of corresponding loading and unloading stresses. Finally, the uniaxial compressive strength was determined as proposed by ISRM (1979).
3.7. SCANNING ELECTRON MICROSCOPY
Nine polished symplex were studied using a scanning electron microscope (Quanta 450).The polished samples were made in orientation perpendicular to the borehole KZ-25. Following the fissure distribution plans, the fragments and polished samples were studied in their natural state and coated with gold. The secondary (SE) and backscattered (BSE) electron detectors were used for taking photographs of each sample. The SE mode is the most important because these electrons can be collected easily using a positively biased collector grid placed on one side of the specimen due to the low exit energy of a few electronvolts. Unlike SE, BSE move on straight trajectories and are not affected by electrostatic collection fields (Reimer, 1998).
Nine samples were taken from distinct depth levels. The morphology and structure of each of the pore networks were described using identical microscope settings: a detector with backscattered electrons, beam current 30.00 kV, working distance 10 mm, spot 8, and magnification 74x. The pore network was drawn manually from the highest applicable magnification in Corel Draw software. The number of pores and fractures were then counted. The final pictures were compiled by Quantum-GIS software and the relative directions of the fractures were computed in MapInfo software. In addition, the specific types of microfractures and pores were investigated using the scanning electron microscope and variable magnifications of up to 5000x. More than 100 detailed photographs of the samples were taken to document the microfractures.
4.1. SAMPLING AND CHEMICAL ANALYSIS
The most common rocks within the KZ-25 borehole are leucogranite and alkali feldspar syenite (Table 1). The studied granite has enhanced amounts of albite and K-feldspar but average amounts of quartz. This type of granite is characterised by small amount of dark minerals, especially micas in this case. Table 2 presents the chemical composition of the rocks within the borehole. The average volume of SiO2 is about 70 wt %. The leucogranite samples contain more than 70 wt % Si[O.sub.2], 12-14 wt % A[l.sub.2][O.sub.3], and 2-4 wt % [K.sub.2]O and [Na.sub.2]O.
4.2. CONNECTED AND TOTAL POROSITY
The connected porosity of the samples describes the pores and fractures that are accessible by water whereas the total porosity also includes isolated voids and microfractures. Therefore, the total porosity should be greater than the connected porosity. The connected porosities [n.sub.ISRM] and [n.sub.hc] were both calculated using same equation but with different weights of saturated sample depending on the method used. It is seen that the connected porosity [n.sub.hc] is greater than [n.sub.ISRM] in all samples. (Table 3). The connected porosity is somewhat depth dependent with the highest connected porosity in the sample from 28 m and the lowest in the sample from 108 m. The total porosity was found to be highest in the sample from 58 m and lowest in the sample from 76 m. There is no correlation between the connected and total porosities (Table 3).
4.3. MERCURY POROSIMETRY
The higher values recorded by Hg porosimetry reflect the fact that the measurements have been obtained using high pressure in addition to the comparatively high influence exerted by the open surface pores. The only exceptions come from the samples at 18 m and 38 m, in which the Hg porosimetry is lower by almost 30 %. The highest porosity is associated with the sample from 28 m (Table 5), in accordance with that found by other methods. In general, the pore size distribution demonstrated by mercury porosimetry is characteristically similar for all of the samples irrespective of depth (Fig. 6). This trend is, however, impaired by the increases in porosity seen in the samples from 58 and 88 m. The cumulative curves in a semilogarithmic scale show two clear breakpoints. The first occurs between 200 and 500 nm while the second occurs at about 7000 nm. Figure 6 also suggests that the measured mercury porosity is significantly influenced by pores of less than 500 nm. The contribution of these pores into the sample porosity ranges between 0.25 % and 1.71 %. In contrast, the contribution of pores of greater than 500 nm was found to be quite similar for all the samples (between 0.48 % and 0.69 %). Thus, in terms of variability, the influence of the smaller pores is significantly higher.
[FIGURE 6 OMITTED]
4.4. HYDRAULIC CONDUCTIVITY AND DRY DENSITY
The hydraulic conductivity corresponds to the depth within the borehole (Fig. 7: middle). The highest value of 1.65x[10.sup.-10] m.[s.sup.-1] was measured in the sample from 18 m while the lowest value of 7.80x[10.sup.-13] m.[s.sup.-1] was in the sample from 108 m (Table 3). The dry density shows the opposite patterns in top 50 metres, as these values increase with depth. In the deeper part, there is no clear trend (Fig. 7: right). The lowest value of 2510 kg.[m.sup.-3] was calculated in the sample from 18 m while the greatest value of 2600 kg.[m.sup.-3] was calculated in sample from 108 m (Table 3). Any way there is an increasing trend between 18 m and 48 m and an opposite trend between 48 m and 98 m.
[FIGURE 7 OMITTED]
4.5. ULTRASONIC VELOCITIES
The results of the ultrasonic measurements of P-waves and S-waves for unsaturated, saturated, and dried samples are shown in Figure 8. P-wave velocities vary from 4173 m.[s.sup.-1] to 5446 m.[s.sup.-1] in the unsaturated samples, from 5308 m.[s.sup.-1] to 5910 m.[s.sup.-1] in the saturated samples, and from 4031 m.[s.sup.-1] to 5361 m.[s.sup.-1] in the dry samples (Table 6). The lowest velocities were measured in samples from 18 m and 28 m while the highest velocities were measured in the samples from 98 m and 108 m. The velocities broadly increase with depth (Fig. 8). S-wave velocities vary from 2569 m.[s.sup.-1] to 3475 m.[s.sup.-1] in the unsaturated samples, from 2870 m.[s.sup.-1] to 3635 m.[s.sup.-1] in the saturated samples, and from 2557 m.[s.sup.-1] to 3368 m.[s.sup.-1] in the dry samples (Table 6). The lowest velocities were again measured in sample from 18 m while the highest velocities were measured in the samples from 98 m and 108 m (although, in the saturated samples, this was in the sample at 76 m). Nonetheless, the velocities again broadly increase with depth (Fig. 8).
All moduli were calculated using Equations 4 and 5. Dynamic Young's modulus, dynamic shear modulus, and Poisson's ratio were calculated for the unsaturated samples, saturated samples, and dry samples. There are significant differences between moduli of the saturated samples compared to those of the unsaturated and dry samples (Table 7). Dynamic Young's modulus for the unsaturated samples varies from 39.58 GPa to 72.54 GPa, for the saturated samples it varies from 54.52 GPa to 79.84 GPa, and for the dry samples it varies from 38.19 GPa to 69.23 GPa (Table 5). The lowest values were calculated in the sample from 18 m while the highest come in the samples from 76 m or 108 m. The highest dynamic Young's modulus of saturated samples was calculated at 76 m. The values of the moduli increase with depth in the studied borehole (Fig. 9).
[FIGURE 8 OMITTED]
Dynamic shear modulus for the unsaturated samples varies from 16.56 GPa to 31.39 Pa, for the saturated samples it varies from 20.67 GPa to 33.88 GPa, and from the dry samples it varies from 16.41 GPa to 29.48 GPa (Table 5). Poisson's ratio for the unsaturated samples varies from 0.11 to 0.20, for the saturated samples it varies from 0.10 to 0.32, and for the dry samples it varies from 0.11 to 0.18 (Table 5). The lowest values were calculated in the samples from 28 m and 58 m while the highest values were from 98 m or 108 m. Figure 9 shows the progression of all moduli and Poisson's ratio within borehole.
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4.6. DEFORMABILITYIN UNIAXIAL COMPRESSION
Samples from 48, 58, 98, and 108 m were chosen in order to test their deformability. Due to lack of other samples it was not possible to perform the other tests. The static Young's modulus, Poisson's ratio, uniaxial compressive strength and moisture were calculated by these tests. Static Young's modulus generally increases with depth and varies from 49.2 GPa to 59.9 GPa. Poisson's ratio varies from 0.24 to 0.19 and decreases between 98 and 108 m. The highest compressive strength was measured in the sample from 98 m while the lowest was measured in the sample from 108 m (Table 6).
4.7. SCANNING ELECTRON MICROSCOPY
Figure 10 presents the SEM images from all the sampled depth levels. The three most common minerals are visible: quartz (dark grey, high relief); albite (dark grey, low relief); and K-feldspar (light grey). The images reveal the distribution of pores and microfractures in the granite. It appears that the majority of the pores and fractures occur in both albite and K-feldspar. The manually marked pore networks on the images (Fig. 11) documents, that pores and fractures are variously distributed within the rocks. A numer of fractures attain lengths of more than 1 mm (Figs. 10 a, b, c, f, h, i) and there is a network of interconnecting microfractures. The pore distribution does not appear to be depth dependent and may vary markedly within a given sample. The greatest numbers of pores occur in the sample from 38 m while the greatest numbers of fractures occur in the sample from 58 m. The least numbers of pores and fractures occur in the sample from 18 m (Table 7). The grain boundaries and minerals have been distinguished (Fig. 11) while the fractures and pores distributed in the minerals have been counted (Table 8). The highest grain boundary pore ratio occurs in the sample from 48 m.
Illustrative views of the distribution of the pores and fractures within the borehole are presented in Figure 12 and 13. The relative direction of the various fractures was computed in MapInfo software. The rose diagrams were displayed in Tectonics_FP software. Figure 14 presents rose diagrams of the microfractures at different depths. It should be noted that these are not actual directions. There are two distinct orthogonal fracture sets in the granite samples from borehole KZ-25. Two sets of faults have also been found in granites in NE part of the Bohemian Massif (Novakova, 2008). The narrowest fracture with a width of 0.49 [micro]m was found in the sample from 28 m while the widest fractured with a width of 74.54 um was found in the sample from 48 m (Table 7). Measuring the length of the fractures is problematic due to the small visible area seen by the microscope. Nevertheless, the shortest fracture was measured in the sample from 28 m while the longest was in the sample from 108 m (Table 7).
There has been much recent attention directed towards the properties of granite matrix. A number of papers have used local granites to investigate microfracturing, porosity, and hydraulic conductivity (e.g. David et al., 2000; Begonha and Sequeira Braga, 2002; Mori et al., 2003; Lion et al., 2005, Novakova et al., 2011). Unless it forms part of a comprehensive project into, for example, the feasibility of a nuclear waste repository, these papers usually focus on a specific parameter which is studied by a single method or a set of closely related methods.
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[FIGURE 11 OMITTED]
In this study, the porosity has been obtained using three different methods. A comparison of these measured porosities shows that average value of [n.sub.ISRM] is 1.08 % while average [n.sub.hc] is 1.21 % (Table 5). This suggests that some air bubbles remained inside the samples even after a long period of saturation under vacuum. During the hydraulic conductivity test, the remaining air was expelled and therefore [n.sub.hc] gives greater values than [n.sub.ISRM]. Even higher values were obtained from mercury porosimetry with a mean average of 1.43 %. These [n.sub.Hg] values may be explained by the size of the samples because more of the originally isolated microcracks are accessible from the surface in smaller samples. It is possible that mercury porosimetry overestimates the porosity as the results may be affected by the surface roughness of the samples (Onishi and Shimizu, 2005). The estimation of the total porosity [n.sub.T] gives a significantly higher value than that of open porosity, with an average of 2.49 %. It may be that significant part of the total porosity occurs within an interconnected network determined after hydraulic conductivity test. Despite these differences, the obtained porosities follow the same trends irrespective of the actual method applied. The connected porosity should depend on the rock type, in particular its chemical or mineralogical composition and fabric. The sample from the depth of 26.4 m contains 78 wt % Si[O.sub.2], which is the highest recorded content within the borehole (Table 2).
[FIGURE 12 OMITTED]
The higher porosity values obtained by the mercury porosimetry in comparison to the standard methods may result from the use of high pressure during the measurements. However, given the size of the samples and the generally low porosity of the studied granite, the influence of open surface pores also ought to be considered. The mercury porosimetry revealed somewhat atypical characteristics in the pore size distribution in all of the studied samples. The existence of three distinct groups of pores is indicated by breakpoints on the curves. The real size of these pores and fractures, supposed to be as wide as 7000 nm, was also found by the SEM. It may be that these features in fact represent open surface pores induced during sampling and preparatory work. Furthermore, the porosity measured by the mercury porosimetry corresponds far more closely to those obtained by the other methods after excluding pores over 7000 nm. It is only possible to speculate as to whether the remaining two groups represent genuine categories of fractures and pores.
A completely different perspective on matrix porosity is provided by the SEM. The manual tracing of pores and fractures is somewhat subjective. It does, nevertheless, provide useful spatial and structural information (Novakova et al., 2010). In order undertake further geological analyses, it is necessary to know the orientation of the samples. This is simply not known for the analysed core. During the original survey no borehole logging methods were applied that could have been used to orientate the core later. Furthermore, due to quarrying, the location of the KZ-25 borehole does not exist anymore. Despite this problem, the SEM images show the density and structure of both pores and microfractures with up to three significant microfracture directions identified. The information regarding the connectivity between fractures provided by the SEM is important for future modelling. The process of manual tracing is more precise than the automated drawings generated by special programs although it is clearly time consuming (Machek, 2011).
[FIGURE 13 OMITTED]
Mori et al. (2003) defined four types of granite matrix pores: grain boundary pores, sheet silicate pores, solution pores, and crack pores. These types have all been identified in the SEM images from borehole KZ-25. However, after careful consideration, for the purpose of investigating the granite matrix we prefer to define the structures as either pores or microfractures. The former are defined as three dimensional cavities within the matrix while the latter are broadly planar gaps. This division allows an integrated approach based directly on pore shape.
There is no clear linear depth dependence in relation to the number or length of pores and microfractures (Fig. 12 and Fig. 13). The narrowest fractures mapped from the SEM images were estimated to have widths of around 500 nm. Therefore, only the major microfractures were depicted. In addition, this corresponds to a breakpoint indicated by the mercury porosimetry (Fig. 6). The pores mapped from the SEM images represent the smallest category shown by the mercury porosimetry. Nevertheless, a significant proportion of the microfractures may still have been neglected. It is also necessary to note that the dimensions of most of the microfractures measured from the SEM images are apparent, as these microfractures occur at various angles to the image plane.
The highest density of microfractures was found in K-feldspar (between 2.42 and 6.23 microfractures per square millimetre) while the lowest was found in albite (between 0.16 and 0.96). Quartz grains bear between 0.78 and 2.93 microfractures per square millimetre (Table 8). The 28 m sample also demonstrates that the length and connectivity of the microfractures is important. A relatively low number of well connected long fractures contribute significantly to the hydraulic conductivity. Mori et al. (2003) stated the most important contributor to granite matrix porosity were tiny fractures that followed grain boundaries, called grain boundary pores. The grain boundary pores form up to 40 % of all microfractures in the matrix. Moreover, the grain boundary pores, where found, were observed to be both long and well connected in the KZ-25 granite (Fig. 11).
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[FIGURE 15 OMITTED]
[FIGURE 16 OMITTED]
There is an ambiguous relationship between porosity and depth that has been observed in all the results obtained by the various porosity methods. In contrast, the hydraulic conductivity is clearly depth dependent. The porosity and hydraulic conductivity trends are not always followed down the vertical profile and distinct local anomalies can be seen (see depth intervals 18-28 m; 48-58 m and 88-98 m in Fig. 7). This demonstrates that there is no precise correlation between these two parameters. It may be that the hydraulic conductivity can be influenced by a small proportion of the microfractures as these create preferential drainage pathways inside the sample. Porosity, on the other hand, is a volumetric characteristic and it is influenced by all the microcracks in the measured sample regardless of their contribution to the hydraulic properties. A logarithmic relationship was found to exist between porosity and hydraulic conductivity (Fig. 15). The presented porosity and hydraulic conductivity measurement methods are rather time consuming. It would be useful to have an easy but reliable method that provides rapid results relating to either granite matrix porosity or hydraulic conductivity on large quantity of samples. It is easy to measure ultrasonic velocities in both the field and laboratory. Therefore, it is promising that it may be possible to correlate ultrasonic velocities and porosity (Fig. 16) or hydraulic conductivity (Fig. 17; Najser et al., 2011).
In borehole KZ-25, the velocity of both the primary and secondary waves decrease with an increase in open porosity. A change of 1.6 % (absolute value) in the open porosity is proportionally equivalent to a change of 1.2 km.[s.sup.-1] in the primary wave velocity of the dried samples. Begonha and Sequeira Braga (2002) found a logarithmic correlation between P-waves and open porosity in a weathered granite with porosities of up to 10 %. The ultrasonic velocities decrease with increasing hydraulic conductivity. The observed relationship is logarithmic. It is in accordance with the observed logarithmic relationship between porosities and hydraulic conductivity.
[FIGURE 17 OMITTED]
The laboratory measurements of ultrasonic velocities on dry, saturated, and unsaturated samples gave similar results. The velocities were seen to be very similar in the dry and unsaturated samples whereas [v.sub.p] were greater in the saturated samples. This comparison shows that dried samples are the most appropriate for ultrasonic velocity measurement. Similar results of the dry and unsaturated samples indicate that the drying process may be omitted eventually for general ultrasonic measurements.
The ultrasonic velocities of both the primary and secondary waves initially increase with depth. However, the measurements of ultrasonic velocities in the samples below about 50 m provided rather uniform results. This may reflect subsurface weathering and therefore the results suggest the KZ-25 granite is weathered to depths of around 50 m. A similar conclusion can be drawn from the density profile presented in Figure 7.
The samples all, with one exception, represent leucogranite (the sample at 18 m is an alkali feldspar syenite). The different mineralogical composition of that sample explains its slightly different behaviour. The majority of the studied parameters correspond with depth (e.g. high hydraulic conductivity, low density, slow ultrasonic velocities, and low values of moduli). The porosity of the sample at 18 m is, however, rather low when compared to the following near surface leucogranite samples and this low porosity was confirmed by the SEM images. It is possible that this reflects a greater amount of interconnected microfractures within the alkali feldspar.
The high porosity samples of the leucogranite come from horizons with slightly higher amounts of Si[O.sub.2] and lower amounts of A[l.sub.2][O.sub.3]. The change in the mineralogical composition of the leucogranite again represents a possible explanation.
The various porosimetry methods provided a range of values that followed similar trends. From the recorded data, it is noted that a logarithmic relationship exists between open porosity and hydraulic conductivity. The porosities, ultrasonic velocities, hydraulic conductivities, and bulk densities all show slight depth dependence. The mercury porosimetry showed the typical pore distribution in the studied granite. The mercury porosimetry suggested that some pores are wider than 7000 nm as they were also found by the SEM. The SEM imagery provides a unique view into the granite matrix and the applied resolution made it possible to map microfractures over 0.5 [micro]m. For greater detail, a yet finer resolution is essential. The ultrasonic velocity measurements offer a promising method for indirect porosity and hydraulic conductivity estimation. The most reliable results appear to come from the oven dried samples but the unsaturated samples may be suitable for a rapid general estimation. These data enable us to construct new hypotheses that will now form the basis of the next phase of research into granite matrix porosity.
The project was funded by the Ministry of Industry and Trade of the Czech Republic (Project Number: FR-TI1/367). For their participation, we are grateful to our colleagues at the Czech Geological Survey, Progeo Ltd., and NIR Rez Corp. We would like to thank Dr. M. Rene for his useful advice and Dr. K. Breiter for the geological description of the core. Thanks are also due to Dr. J. Schweistilova for the SEM, A. Jandeckova for the mercury porosimetry analyses, and Z. Fiala for the uniaxial compression tests. Dr. M.D. Rowberry provided a critical review of the English. We also thank to three anonymous reviewers for improvement the manuscript.
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Lucie NOVAKOVA (1) *, Karel SOSNA (2), Milan BROZ (1), Jan NAJSER (2) and Petr NOVAK (3)
(1) Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, v.v.i., VHolesovickAch 41, 182 09 Prague, Czech Republic (2) ARCADIS Geotechnika Inc., GeologickA 4, 152 00, Prague 5, Czech Republic (3) ISATech Ltd., Osadni 26, 170 00, Prague 7, Czech Republic * Corresponding author's e-mail: firstname.lastname@example.org
(Received May 2012, accepted September 2012)
Table 1 Description of the rocks within borehole KZ- 25 (Pacal and Pavlu, 1979). Depth [m] Rock 0.0-1.5 Clay 1.5-7.0 Eluvium of the leucogranite 7.0-016.85 Weathered leucogranite 16.85-22.7 Alkali feldspar syenite 22.7-26.3 Alkali feldspar syenite with quartz 26.3--31.8 Leucogranite 31.8-32.8 Alkali feldspar syenite 32.8-37.5 Leucogranite 37.5-38.5 Alkali feldspar syenite 38.5-43.7 Leucogranite 43.7-44.7 Pegmatite 44.7-94.7 Leucogranite 94.7-95.15 Alkali feldspar syenite 95.15-100.4 Leucogranite 100.4-101.7 Alkali feldspar syenite 101.7-106.6 Leucogranite 106.6-107.4 Alkali feldspar syenite 107.4-119.5 Leucogranite 119.5-123.0 Granite with micas 123.00-126.0 Aplite 126.00-205.0 Li-topaz granite Table 2 The chemical composition of the rocks within the KZ-25 borehole (Hanak et al., 2010). K-alkali feldspar Rock Type syenite leucogranite leucogranite Depth [m] 18.8m 26.4m 36m Si[O.sub.2] 62.80 78.12 75.90 Ti[O.sub.2] 0.03 0.02 0.04 A[l.sub.2][O.sub.3] 19.84 12.43 13.64 [Fe.sub.2][O.sub.3] 0.04 0.04 0.04 FeO 0.16 0.16 0.20 MgO 0.05 0.07 0.06 MnO 0.03 0.03 0.03 CaO 0.40 0.33 0.38 [Li.sub.2]O 0.06 0.04 0.05 [Na.sub.2]O 3.90 4.16 4.23 [K.sub.2]O 10.06 2.54 4.01 [P.sub.2][O.sub.5] 0.67 0.31 0.38 F 0.33 0.23 0.23 Ign. loss 0.80 0.78 0.50 [H.sub.2]O(-) 0.12 0.12 0.05 F(ekv) -0.14 -0.10 -0.10 Total [%] 99.28 99.38 99.74 alkali feldspar Rock Type leucogranite leucogranite syenite Depth [m] 58.7m 75.5m 101.2m Si[O.sub.2] 77.92 74.48 69.54 Ti[O.sub.2] 0.03 0.04 0.03 A[l.sub.2][O.sub.3] 12.34 14.05 17.06 [Fe.sub.2][O.sub.3] 0.07 0.17 0.07 FeO 0.20 0.22 0.11 MgO 0.08 0.05 0.04 MnO 0.04 0.06 0.04 CaO 0.42 0.37 0.43 [Li.sub.2]O 0.05 0.04 0.03 [Na.sub.2]O 4.52 4.43 5.79 [K.sub.2]O 2.50 4.25 5.18 [P.sub.2][O.sub.5] 0.33 0.41 0.51 F 0.18 0.20 0.17 Ign. loss 0.57 0.44 0.54 [H.sub.2]O(-) 0.07 <0.05 0.10 F(ekv) -0.08 -0.09 -0.07 Total [%] 99.31 99.23 99.64 Table 3 The connected porosities ([n.sub.IRSM], [n.sub.hc]), mercury porosimetry ([n.sub.Hg]), total porosity ([n.sub.T]), hydraulic conductivity, and dry density. Depth [n.sub.IRSM] [n.sub.hc] [n.sub.Hg] [n.sub.T] [m] [%] [%] [%] [%] 18 1.61 1.77 1.04 2.26 28 2.00 2.10 2.24 3.16 38 1.89 2.08 1.90 2.61 48 0.86 0.95 1.27 2.12 58 1.34 1.59 1.74 3.24 76 0.55 0.65 1.05 1.96 88 0.59 0.67 1.94 2.63 98 0.45 0.56 0.92 2.33 108 0.40 0.54 0.73 2.06 Hydraulic Dry Depth conductivity density [m] [m.[s.sup.-1]] [kg.[m.sup.-3]] 18 1.65x[10.sup.-10] 2510 28 1.40x[10.sup.-10] 2540 38 1.30x[10.sup.-11] 2560 48 1.15x[10.sup.-11] 2585 58 9.52x[10.sup.-12] 2568 76 1.01x[10.sup.-12] 2564 88 2.33x[10.sup.-12] 2563 98 3.16x[10.sup.-12] 2552 108 7.80x[10.sup.-13] 2600 Table 4 The velocities of P-waves (Vp) and S-waves (Vs) of unsaturated, saturated, and dry samples. Depth Vp uns Vs uns Vp sat [m] [m.[s.sup.-1]] [m.[s.sup.-1]] [m.[s.sup.-1]] 18 4173 2569 5564 28 4251 2809 5308 38 4577 2957 5357 48 5151 3254 5763 58 4830 3163 5415 76 5267 3293 5810 88 5441 3393 5786 98 5446 3325 5910 108 5441 3475 5830 Depth Vs sat Vp dry Vs dry [m] [m.[s.sup.-1]] [m.[s.sup.-1]] [m.[s.sup.-1]] 18 2870 4031 2557 28 3022 4148 2641 38 3157 4582 2900 48 3345 4899 3093 58 3621 4867 3228 76 3635 5225 3267 88 3511 5194 3237 98 3517 5329 3348 108 3485 5361 3368 Table 5 Young's modulus (Edyn), shear modululus (Gdyn) and Poisson's ratio (v) of unsaturated, saturated, and dry samples. Depth Edyn Gdyn v dyn Edyn Gdyn [m] unsat. unsat. unsat. sat. sat. [GPa] [GPa] [-] [GPa] [GPa] 18 39.58 16.56 0.19 54.52 20.67 28 44.59 20.05 0.11 58.45 23.19 38 51.12 22.38 0.14 62.97 25.51 48 63.93 27.37 0.17 72.07 28.93 58 57.78 25.69 0.12 73.77 33.67 76 65.55 27.80 0.18 79.84 33.88 88 69.73 29.50 0.18 76.37 31.60 98 67.87 28.22 0.20 77.38 31.56 108 72.54 31.39 0.16 77.16 31.57 Depth v dyn Edyn Gdyn v dyn [m] sat. dry dry dry [-] [GPa] [GPa] [-] 18 0.32 38.19 16.41 0.16 28 0.26 41.06 17.71 0.16 38 0.23 50.20 21.53 0.17 48 0.25 57.80 24.73 0.17 58 0.10 59.25 26.75 0.11 76 0.18 64.52 27.36 0.18 88 0.21 63.51 26.85 0.18 98 0.23 67.15 28.60 0.17 108 0.22 69.23 29.48 0.17 Table 6 The deformation parameters. Static Young's Poisson's Uniaxial Depth modulus ratio compressive Moisture [m] [GPa] [-] strength [%] [MPa] 48 49.2 0.24 118 0.15 58 50.5 0.24 142 0.19 98 59.9 0.23 161 0.12 108 56.6 0.19 105 0.13 Table 7 Number of fractures and pores and their physical characteristics observed in polished samples from borehole KZ-25. The The shortest longest Depth Number of Number of Total fracture fracture [m] fractures pores [[micro]m] [[micro]m] 18 30 602 632 78.80 448.20 28 78 703 781 3.05 605.40 38 128 1252 1380 10.20 461.90 48 90 901 991 20.62 976.40 58 109 1538 1647 3.85 668.00 76 58 1061 1119 3.07 1795.00 88 121 1437 1558 4.72 668.80 98 88 864 952 19.25 717.40 108 65 911 976 55.67 1847.00 The The Total narrowest widest length of Depth fracture fracture fractures [m] [[micro]m] [[micro]m] [mm] 18 1.17 10.07 12.10 28 0.49 19.69 29.04 38 0.75 62.30 26.62 48 1.01 74.54 16.94 58 0.58 54.88 17.77 76 1.27 45.68 11.64 88 0.52 46.04 21.33 98 0.93 45.37 16.26 108 9.64 72.48 12.26 Table 8 Number of fractures and pores in the minerals. GBP-Grain boundary pores, fract.-fractures. Albite Depth [m] pores /[mm.sup.2] fract. /[mm.sup.2] area 18 155 10.5 5 0.34 0.59 28 223 36.5 6 0.98 0.25 38 499 74.0 5 0.74 0.27 48 238 19.9 11 0.92 0.48 58 587 63.0 6 0.64 0.37 76 491 51.3 5 0.52 0.38 88 397 65.3 1 0.16 0.24 98 172 21.7 6 0.76 0.32 108 433 40.2 10 0.93 0.43 K-feldspar Depth [m] pores /[mm.sup.2] fract. /[mm.sup.2] area 18 447 43.4 25 2.42 0.41 28 365 27.8 37 2.82 0.52 38 589 47.2 70 5.62 0.50 48 619 53.0 66 5.65 0.47 58 909 68.2 83 6.23 0.53 76 420 46.5 38 4.20 0.36 88 678 61.8 68 6.20 0.44 98 509 52.7 54 5.59 0.39 108 375 44.0 41 4.81 0.34 Quartz GBP Depth [m] pores /[mm.sup.2] fract. /[mm.sup.2] area ratio 18 0 -- 0 -- 0.00 0.00 28 115 19.9 7 1.22 0.23 2.28 38 165 28.4 17 2.93 0.23 2.15 48 44 32.5 2 1.49 0.05 8.68 58 42 18.0 2 0.85 0.09 5.69 76 150 23.4 5 0.78 0.26 1.41 88 362 45.5 21 2.64 0.32 1.38 98 183 24.7 14 1.89 0.30 1.31 108 103 18.0 6 1.05 0.23 1.49
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|Author:||Novakova, Lucie; Sosna, Karel; Broz, Milan; Najser, Jan; Novak, Petr|
|Publication:||Acta Geodynamica et Geromaterialia|
|Date:||Oct 1, 2012|
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