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Visualization of the El Berrocal granite: application to rock engineering
ELSEVIER
Engineering Geology 49 ( 1998 ) 185 194
Visualization of the E1 Berrocal granite: application to rock engineering M.J. White
QuantiSci, Melton Mowhray, UK
Abstract
This paper outlines the visualization of the El Berrocal granite using a computer-based geological modelling system, EarthVision, and discusses the application of this visualization to engineering aspects of waste disposal in crystalline rocks. The El Berrocal Project was an international study with the aim of understanding and modelling the migration processes which have controlled the distribution of naturally occurring radionuclides in a fractured granitic environment. EarthVision was used to provide three-dimensional geological models of the site structure and properties. Modelling of the site structure concentrated on the development of visualizations of the main discontinuities in the granite. These included a model of the main mineralized structures, a model of the regional fracture network, models of local fracture networks between borehole clusters and a visualization of the mineralogy of the fractures in individual boreholes. These fracture models were visualized with the boreholes and access gallery to the mine. In addition, the fracture network in the region of a large scale tracer test was visualized with the injection and extraction zones for the tracer test. Three-dimensional interpolations of the rock and fluid structure were undertaken. A model of the hydraulic conductivity illustrated large-scale variations in hydraulic conductivity and channelling effects in the tracer test zone. A model of the sulphate concentrations in the groundwater illustrated the interpolation of spatial data based on structural domains. The visualizations of the geology of the El Berrocal granite illustrate that, despite limitations, geological modelling can be a powerful and graphic tool in rock engineering. The use of computer visualizations can provide the three-dimensional structural framework for computations, can aid decision making during the construction phase of waste repositories and can be useful in understanding and analysing the results of numerical calculations. © 1998 Elsevier Science B.V. All rights reserved.
KeyworEv Geologic modelling; Visualization: Fractured rock: Waste disposal: Granitic rock
!. Introduction
The El Berrocal Project was an international study with the aim o f understanding and modelling the migration processes which have controlled the distribution o f naturally occurring radionuclides in a fractured granitic environment (Miller et al., 1994). The investigations used an a p p r o a c h 0013-7952,/98/$19.00 '.~'~1998 Elsevier Science B.V. All rights reserved. Pll: $ 0 0 1 3 - 7 9 5 2 ( 9 7 )00048-3
which fully integrated the structural, hydrogeological and geochemical features o f the site. As part o f the integration o f these investigations a geological modelling system, EarthVision ( D y n a m i c Graphics, 1992) was used to provide threedimensional geological models o f the site structure and properties (White and del Olmo, 1996; White et al., 1996). This paper outlines the visualization
186
11..I. HT~it<" l'Sixim,crm,,.. (i<,ol.:.,v 49 : lYY,S': l,~'5 104
of the E1 Berrocal granite using EarthVision and discusses the application of this visualization to engineering aspects of waste disposal in crystalline rocks.
2. The EarthVision modelling system
Geological modelling systems are computerbased mapping, analysis and visualization systems. They concentrate on the development of static three-dimensional block models which provide a descriptive representation of the surface and subsurface. In these models discontinuities (faults, fractures, joints, veins and dykes) and lithological contacts are represented by a regular xy grid. At each xv point in the two-dimensional grid, the elevation z is defined as a variable. The properties of the rock and the fluids which occupy the pore volume can also be represented by interpolation and extrapolation of known values. Property modelling is commonly undertaken onto a three-dimensional array of either points or blocks. In the points approach, the three-dimensional array is treated in a manner analogous to a two-dimensional grid and is assumed to be varying smoothly from point to point in the array. EarthVision uses this approach to threedimensional property modelling.
3. EarthVision modelling of the structure of El Berrocal
Visualization of the structure of the E1 Berrocal pluton concentrated on the following aspects of the investigations. 3.1. Visuali-alion o/lhe m#wra/ized SIFltcllo'd,~' ~Fi:4. 1: This model was used to illustrate the relationship of mineralized veins and dykes to the boreholes and the access gallery. Different amounts of data were available tk~r the modelling of these mineralized features. The mineralized quartz vein (Fig. 1 ) had been mapped at the surface, in the access gallery and in boreholes 13, 15 and 16. The Tarica Dyke ( Fig. 1 ) had been mapped at the surface and in borehole 11. The barite vein ( Fig. 1 ) had been
mapped at the surface only. For each of the mineralized structures two-dimensional grids were created lk)r both the top and the base from the input data. Thus, the features were represented with a finite thickness. Due lo the differences in the input data. different interpolation algorithms were used to generate the two-dimensional grids. The mineralized quartz vein grids used a minimum tension algorithm (Briggs, 1974), the Tarica Dyke used a combination of two planar grids and the grid for the barite dyke was created by planar extrapolation of surface data. 3.2.
I%ualization qf the re viomd/i'acture network
This model used data from surlace and regional (remote sensing) mapping which were correlated with borehole measurements (ENRESA, 1996a). Single grids represented the fracture planes, not only as planar surfaces, but also as curved surfaces. EH echelon fractures were realistically represented m this model (Fig. 2). This model was also used to illustrate the relationship of the structure to the boreholes and the access gallery. 3.3. l,Tsualizations of the geomet D" qlfi'acture Hclworks around borehole clusters (F~,,. 3) During the Berrocal Project, an acoustic televiewer (TLV) probe was used to investigate the geometry of fractures in three main borehole clusters (ENRESA, 1996a). The distance between boreholes in these clusters is approximately 20 m. These clusters are: I l) boreholes 11 and 12: {2) boreholes 2, 13, 15 and the access gallery: (3) boreholes 14, 16 and 18. The TLV probe measures the interval transit time of an acoustic wave through the rock. Thus, fractures and fracture zones are identified by their increased travel times. The data measured by the TLV probe were separated into three orders depending on this transit time. The fractures with the largest travel times (and, by assumption, the largest aperture) were compared between boreholes ( ENRESA, 1996a). Correlated fractures were considered to be those which could be projected to
M.J. White / Engineering Geology 49 (1998) 185-194
187
Fig. 1. Geological model of E1 Berrocal showing the geometry of the main veins and dykes in the granite host rock and their relationship to the boreholes and access gallery. Numbers shown are those of the boreholes.
within 5 m of a fracture of the same order in another borehole. A similar correlation was performed between the boreholes and the surface. Fig. 3 illustrates the geometry of the correlated fractures in the region of boreholes 14, 16 and 18.
3.4. Visualizations o f the fracture network in the tracer test zone (Fig. 4) The TLV data were used for a more specific task in the region of boreholes 2, 13 and 15, i.e.
188
,14. J. Illfite
l~Stt~meerm,. , Geolo,,..v 49 (1998) 185
194
Fig. 2. Model of the regional fracture network at El Berrocal. The data have been derived from remote sensing ( Landsat-MSS, spot and aerial photographs) and surface mapping, and have been tied to relevant borehole information.
enhancing the understanding of the hydrogeology of a tracer test. The tracer test was undertaken by injecting a marker fluid in boreholes 13 and 15 and extracting in borehole 2 ( E N R E S A , 1996b). The fracture network model for this borehole cluater was visualized with the boreholes and the injection and extraction zones. This illustrated how these zones were connected by the major fractures {Fig. 4). It was noted that the injection zone which illustrated the greatest response in the extraction zone (the upper zone in borehole 13) was not connected to the extraction zone in borehole 2 by any major fractures. This supported groundwater modelling observations, which indicated that the
groundwater flow did not occur only in the fractures with the largest apertures.
3.5. Visualization of the Mineralogy otthe Fractures (F& 5) The fracture network models described above were combined with analyses of the mineralogy of the fractures ENRESA, 1996a. During these analyses, the fractures were classified with respect to the infilling mineralogy. Sericite- and smectite-filled IYactures: sericite, smectite and kaolinite fractures: clay and carbonate fi'actures ( 1 0 50% carbonate):
M..L White / Engineering Geology 49 (1998) 185-194
189
4. Modelling of rock properties In addition to visualizing the geometry and properties of fractures, three-dimensional modelling of the rock and fluid properties was undertaken. Two aspects of the rock-water system were investigated: firstly, the hydraulic conductivity (k); and secondly, the hydrochemistry. A model of the variation in log k in the tracer test zone is illustrated in Fig. 6. This model was constructed by interpolation of 162 borehole k measurements (ENRESA, 1996c) using a minimum tension algorithm (Briggs, 1974). In Fig. 6, areas of the model with values of log k less than - 1 0 have been removed. This allows visualization of the variability of the log k model. Large variations in log k and channelling features are present as would be expected in a low-permeability granite. However, there are areas, away from the boreholes, where there is significant smoothing of the data. Therefore, the primary use of this model is to illustrate variation in k close to the boreholes. A model of the sulphate concentrations in the groundwater is illustrated in Fig. 7. This is based on 65 borehole measurements (ENRESA, 1996d) which have been interpolated in three-dimensional space with respect to the geological structure. That is, the borehole data have been interpolated separately north of the uranium vein, within the vein and to the south of the vein. This process is automated in EarthVision. The three-dimensional model demonstrates that the highest levels of sulphate occur within the main mineralized vein and supports a hypothesis that the source of sulphate in the groundwaters of E1 Berrocal is pyrite within the vein.
Fig. 3. Visualization of the correlated fractures in the vicinity of boreholes 14, 16 and 18.
and a fracture set with greater than 50% carbonate were recognized. Each fracture set was colourcoded with respect to the dominant mineralogy as illustrated in Fig. 5. This provided a visualization of the spatial variation in fracture mineralogy.
5. Application of geological modelling and visualization tools to engineering geology A primary application of geological modelling in site characterization studies is the provision of geological representations of the site to computational software tools (White et al., 1996). These can be used to accurately define the geological structure and rock mass properties of the area analysed. However, there are several limitations to
190
M.J. I~V/lile ' Engineering Geoh~g)' 49 / 1998) 185 194
Fig. 4. Visualization o1" lhe c o r r e l a t e d IYacturcs m 111c Iracer lest z o n e between b o r e h o l e s 2. 13 a n d 15.
the structural and the rock and groundwater property models illustrated in this paper which may limit their suitability as pre-processors for rock engineering calculations and must be taken into
account when drawing conclusions from the results. The fracture network visualizations are deterministic models derived from TLV probe data. As
M.J. White / Engineering Geology 49 (1998) 185 194
191
Fig. 5. Visualizationof the mineralogyof fractures in borehole 17 at El Berrocal:(a) sericite-and smectite-filledfractures; (b) sericite-, smectite- and kaolinite-filledfractures; (c) clay and crabonate fractures (10 50% carbonate); and (d) fractures with > 50% carbonate.
such, they do not represent statistically defined networks in which the components of fracture length, orientation and truncation are adequately represented. They are, in fact, simple extrapolations from the fractures recorded in boreholes. These deterministic fracture networks also suffer from sampling errors associated with the orientation of the boreholes and uncertainty in the features detected by the TLV probe. In certain situations, however, deterministic fracture models are used as the framework for three-dimensional calculations (e.g. Brrgesson and Hernelind, 1995) The three-dimensional models of the rock and groundwater properties are also subject to limitations. They are derived from relatively simple distance-weighted algorithms which do not purport to represent the true mathematical variation in the measured borehole data. However, the models are useful in illustrating the gross variation in the modelled properties over the scale investigated. The sulphate model in particular illustrates the influence
of geological structure on the hydrochemistry (Fig. 7). This is, in part, due to the capabilities of the software to interpolate data in relation to the structure. The sulphate model is relatively simple but there is no limit to the number of domains which can be constructed for property interpolation. Despite limitations, visualization of mapped fractures can be an important part of the engineering component of developing waste repositories. At some stage in developing a repository, engineers have to make up their mind if they should place repository tunnels at certain locations in the rock mass. These decisions will certainly be based on borehole information, with structural information combined with predictions of rock mass response to thermal and mechanical loading and groundwater flow. Geological modelling and visualization can provide a useful aid in these decisions. Analysis tools, such as volumetric calculations, allow repository plans to be readily validated with respect to the geological structure.
192
M.d. White, En,~ineerin,g (;eoloKL' q9 (lg9~;) 185 194
Fig. 6+ Three-dimensional model of the hydraulic conductivity in the tracer test zone. Areas of the model where log hydraulic conductivity is less than - - l 0 have been removed. This model illustrates gross variations in hydraulic conductivity and the presence of channeling features in the rock matrix.
M.J. White / Engineering Geology49 (1998) 185 194
193
Fig. 7. Three-dimensional model of the concentration of sulphate in the groundwater. This model is based on borehole measurements which have been interpolated in three-dimensional space with respect to the geological structure. That is, the borehole data have been interpolated separately north of the uranium vein, within the vein and to the south of the vein. The three-dimensional model demonstrates that the highest levels of sulphate occur within the main mineralized vein and supports a hypothesis that the source of sulphate in the groundwaters of Berrocal is pyrite, mineralized within the vein. The figure illustrates domain based interpolation. The accurate deterministic representation of the structure of crystalline rocks is largely dependent on the scale of information. During site characterization, data on faults and fractures is based on a large scale. With the development of the underground facility or the excavation of preliminary underground research laboratories (URLs), more localized three-dimensional information is available. The fracture visualization examples illustrated in this paper are probably not at the appropriate scale for near-field engineering analyses. However, with the development of repository programmes at a more local scale, information will become available and similar models to the large-scale ones presented here may be produced for the near-field. Another potential application is the use of structural models being used in the identification and analysis of structural domains.
As well as being of use in determining the threedimensional geology for engineering calculations, geological modelling systems may be used as a postprocessor. In this capacity, these tools can be used to view the results of three-dimensional simulations and the structure of stochastic fracture networks. The advantage of using this technology, as opposed to other currently available visualization tools is that the results can be presented in models integrated with the real geology and the repository layout. This would lead to a more effective communication of the results of modelling.
6. Conclusions This paper has illustrated the fact that advanced techniques exist for the analysis and visualization
194
M.J. HTUtc, t'n,,,im, erin~ Geolo~,,l" 49 ~ 199~¥) Ibm5 194
of geological data. Deterministic fracture networks a n d models of rock a n d fluid properties have been presented for the E1 Berrocal granite. These models can be used to u n d e r s t a n d the geological setting and to both provide the framework for and c o m m u n i c a t e the results o f three-dimensional c o m p u t a t i o n s . The coupling of such tools with c o m p u t a t i o n a l software should be a priority for the effective characterization of potential repository sites. Careful application of this forefront technology will allow the detail of engineering calculations to be advanced.
Acknowledgment The models discussed in this paper were constructed from data collected by a large n u m b e r of individuals who worked o n the E1 Berrocal Project. In particular, data has been collected a n d interpreted by the following; Rocio C a m p o s , Jordi Guimerfi, P a l o m a G 6 m e z a n d Luis P6rez del Villar. The work described in this paper was performed under contract to Pedro H e r n a n of Enresa, who are t h a n k e d for their permission to publish.
References BOrgesson, L., Hernelind, ,I., 1995. DECOVALEX I 'lesl Case 2: Calculation of the Fanay Augeres THM Test Thermomechanical Modelling of a Fractured Rock Volume. SKB Technical Report 95-28. SKB, Stockhohn, Sweden. Briggs, I.C., 1974. Machine contouring using minimum curvature. Geophysics 39 ( I ), 39 48. Dynamic Graphics, 1992. EarthVision 2.0 User's Guide. Dynamic Graphics. Alameda, CA.
ENRESA, 1996a. El Berrocal Project. Charactrization and ~alidation of natural radionuclide migration processes under real conditions of the fissured granitic environment. European Commission Contract Number F12W/CT91/0080. Topical Reports Volume I. Geological Studies. ENRESA, Madrid. Spain. ENRESA, 1996b. El Berrocal Project. Charactrization and \,alidation of natural radionuclide migration processes under real conditions of the fissured granitic environment. European Commission Contract Number FI2W/CT91/0080. Topical Reports Volume 111. Laboratory Migration Test and in situ Migration Test. ENRESA, Madrid, Spain. ENRESA, 1996c. El Berrocal Project. Charactrization and validation of natural radionuclide migration processes under real conditions of the fissured granitic environment. European Commission Contract Number F12W/CT91/0080. Topical Reports Volume IV. Hydrological Modelling and ('ode Development. ENRESA, Madrid, Spain. ENRESA, 1996d. El Berrocal Project. Charactrization and validation of natural radionuclide migration processes under real conditions of the fissured granitic environment. European (ommission Contract Number F12W/CT91/0080. Topical Reports Volume II. Hydrogeochemistry. ENRESA. Madrid, Spain. Miller, W.M., Rivas, P., Perez del Villar, L., Gomez. P.. de la Cruz, B., Hernan, P., Carrera, J., Guimera, J., Holmes. D., lvanovich, M., Avogadro, S., Vinson, J.-M., Bruno, J., 1994. The International El Berrocal Project. Fourth International Conference on the Chemistry and Migration Behaviour of ,\ctinides and Fission Products in the Geosphere, SC, USA, 12 17 December 1993. R. Otdenberg Verlag, Mtinchen, pp. 8o5 812. White, M.J., del Olmo, C. The Application of Geological Computer ModellingSystems to the Characterisation and Assessment of Radioactive Waste Repositories. Proceedings of the International Conference on Deep (Jeological Disposal of Radioactive Waste, Canadian Nuclear Society, Winnepcg, Manitoba. Canada, pp. 3-2235 3-244. White M. J.. Murray. T., del Olmo, C.. Chapman, N.A., 1996. Modelling the Geological Environment of Radioactive Waste Repositories The Use of Computer Mapping, Modelling and Visualization Tools in Site Characterisation. Proceedings of the European TopScal '96 Conference, Stockholm, Sweden, Volume 11, pp. 156 159.
Engineering Geology 49 ( 1998 ) 185 194
Visualization of the E1 Berrocal granite: application to rock engineering M.J. White
QuantiSci, Melton Mowhray, UK
Abstract
This paper outlines the visualization of the El Berrocal granite using a computer-based geological modelling system, EarthVision, and discusses the application of this visualization to engineering aspects of waste disposal in crystalline rocks. The El Berrocal Project was an international study with the aim of understanding and modelling the migration processes which have controlled the distribution of naturally occurring radionuclides in a fractured granitic environment. EarthVision was used to provide three-dimensional geological models of the site structure and properties. Modelling of the site structure concentrated on the development of visualizations of the main discontinuities in the granite. These included a model of the main mineralized structures, a model of the regional fracture network, models of local fracture networks between borehole clusters and a visualization of the mineralogy of the fractures in individual boreholes. These fracture models were visualized with the boreholes and access gallery to the mine. In addition, the fracture network in the region of a large scale tracer test was visualized with the injection and extraction zones for the tracer test. Three-dimensional interpolations of the rock and fluid structure were undertaken. A model of the hydraulic conductivity illustrated large-scale variations in hydraulic conductivity and channelling effects in the tracer test zone. A model of the sulphate concentrations in the groundwater illustrated the interpolation of spatial data based on structural domains. The visualizations of the geology of the El Berrocal granite illustrate that, despite limitations, geological modelling can be a powerful and graphic tool in rock engineering. The use of computer visualizations can provide the three-dimensional structural framework for computations, can aid decision making during the construction phase of waste repositories and can be useful in understanding and analysing the results of numerical calculations. © 1998 Elsevier Science B.V. All rights reserved.
KeyworEv Geologic modelling; Visualization: Fractured rock: Waste disposal: Granitic rock
!. Introduction
The El Berrocal Project was an international study with the aim o f understanding and modelling the migration processes which have controlled the distribution o f naturally occurring radionuclides in a fractured granitic environment (Miller et al., 1994). The investigations used an a p p r o a c h 0013-7952,/98/$19.00 '.~'~1998 Elsevier Science B.V. All rights reserved. Pll: $ 0 0 1 3 - 7 9 5 2 ( 9 7 )00048-3
which fully integrated the structural, hydrogeological and geochemical features o f the site. As part o f the integration o f these investigations a geological modelling system, EarthVision ( D y n a m i c Graphics, 1992) was used to provide threedimensional geological models o f the site structure and properties (White and del Olmo, 1996; White et al., 1996). This paper outlines the visualization
186
11..I. HT~it<" l'Sixim,crm,,.. (i<,ol.:.,v 49 : lYY,S': l,~'5 104
of the E1 Berrocal granite using EarthVision and discusses the application of this visualization to engineering aspects of waste disposal in crystalline rocks.
2. The EarthVision modelling system
Geological modelling systems are computerbased mapping, analysis and visualization systems. They concentrate on the development of static three-dimensional block models which provide a descriptive representation of the surface and subsurface. In these models discontinuities (faults, fractures, joints, veins and dykes) and lithological contacts are represented by a regular xy grid. At each xv point in the two-dimensional grid, the elevation z is defined as a variable. The properties of the rock and the fluids which occupy the pore volume can also be represented by interpolation and extrapolation of known values. Property modelling is commonly undertaken onto a three-dimensional array of either points or blocks. In the points approach, the three-dimensional array is treated in a manner analogous to a two-dimensional grid and is assumed to be varying smoothly from point to point in the array. EarthVision uses this approach to threedimensional property modelling.
3. EarthVision modelling of the structure of El Berrocal
Visualization of the structure of the E1 Berrocal pluton concentrated on the following aspects of the investigations. 3.1. Visuali-alion o/lhe m#wra/ized SIFltcllo'd,~' ~Fi:4. 1: This model was used to illustrate the relationship of mineralized veins and dykes to the boreholes and the access gallery. Different amounts of data were available tk~r the modelling of these mineralized features. The mineralized quartz vein (Fig. 1 ) had been mapped at the surface, in the access gallery and in boreholes 13, 15 and 16. The Tarica Dyke ( Fig. 1 ) had been mapped at the surface and in borehole 11. The barite vein ( Fig. 1 ) had been
mapped at the surface only. For each of the mineralized structures two-dimensional grids were created lk)r both the top and the base from the input data. Thus, the features were represented with a finite thickness. Due lo the differences in the input data. different interpolation algorithms were used to generate the two-dimensional grids. The mineralized quartz vein grids used a minimum tension algorithm (Briggs, 1974), the Tarica Dyke used a combination of two planar grids and the grid for the barite dyke was created by planar extrapolation of surface data. 3.2.
I%ualization qf the re viomd/i'acture network
This model used data from surlace and regional (remote sensing) mapping which were correlated with borehole measurements (ENRESA, 1996a). Single grids represented the fracture planes, not only as planar surfaces, but also as curved surfaces. EH echelon fractures were realistically represented m this model (Fig. 2). This model was also used to illustrate the relationship of the structure to the boreholes and the access gallery. 3.3. l,Tsualizations of the geomet D" qlfi'acture Hclworks around borehole clusters (F~,,. 3) During the Berrocal Project, an acoustic televiewer (TLV) probe was used to investigate the geometry of fractures in three main borehole clusters (ENRESA, 1996a). The distance between boreholes in these clusters is approximately 20 m. These clusters are: I l) boreholes 11 and 12: {2) boreholes 2, 13, 15 and the access gallery: (3) boreholes 14, 16 and 18. The TLV probe measures the interval transit time of an acoustic wave through the rock. Thus, fractures and fracture zones are identified by their increased travel times. The data measured by the TLV probe were separated into three orders depending on this transit time. The fractures with the largest travel times (and, by assumption, the largest aperture) were compared between boreholes ( ENRESA, 1996a). Correlated fractures were considered to be those which could be projected to
M.J. White / Engineering Geology 49 (1998) 185-194
187
Fig. 1. Geological model of E1 Berrocal showing the geometry of the main veins and dykes in the granite host rock and their relationship to the boreholes and access gallery. Numbers shown are those of the boreholes.
within 5 m of a fracture of the same order in another borehole. A similar correlation was performed between the boreholes and the surface. Fig. 3 illustrates the geometry of the correlated fractures in the region of boreholes 14, 16 and 18.
3.4. Visualizations o f the fracture network in the tracer test zone (Fig. 4) The TLV data were used for a more specific task in the region of boreholes 2, 13 and 15, i.e.
188
,14. J. Illfite
l~Stt~meerm,. , Geolo,,..v 49 (1998) 185
194
Fig. 2. Model of the regional fracture network at El Berrocal. The data have been derived from remote sensing ( Landsat-MSS, spot and aerial photographs) and surface mapping, and have been tied to relevant borehole information.
enhancing the understanding of the hydrogeology of a tracer test. The tracer test was undertaken by injecting a marker fluid in boreholes 13 and 15 and extracting in borehole 2 ( E N R E S A , 1996b). The fracture network model for this borehole cluater was visualized with the boreholes and the injection and extraction zones. This illustrated how these zones were connected by the major fractures {Fig. 4). It was noted that the injection zone which illustrated the greatest response in the extraction zone (the upper zone in borehole 13) was not connected to the extraction zone in borehole 2 by any major fractures. This supported groundwater modelling observations, which indicated that the
groundwater flow did not occur only in the fractures with the largest apertures.
3.5. Visualization of the Mineralogy otthe Fractures (F& 5) The fracture network models described above were combined with analyses of the mineralogy of the fractures ENRESA, 1996a. During these analyses, the fractures were classified with respect to the infilling mineralogy. Sericite- and smectite-filled IYactures: sericite, smectite and kaolinite fractures: clay and carbonate fi'actures ( 1 0 50% carbonate):
M..L White / Engineering Geology 49 (1998) 185-194
189
4. Modelling of rock properties In addition to visualizing the geometry and properties of fractures, three-dimensional modelling of the rock and fluid properties was undertaken. Two aspects of the rock-water system were investigated: firstly, the hydraulic conductivity (k); and secondly, the hydrochemistry. A model of the variation in log k in the tracer test zone is illustrated in Fig. 6. This model was constructed by interpolation of 162 borehole k measurements (ENRESA, 1996c) using a minimum tension algorithm (Briggs, 1974). In Fig. 6, areas of the model with values of log k less than - 1 0 have been removed. This allows visualization of the variability of the log k model. Large variations in log k and channelling features are present as would be expected in a low-permeability granite. However, there are areas, away from the boreholes, where there is significant smoothing of the data. Therefore, the primary use of this model is to illustrate variation in k close to the boreholes. A model of the sulphate concentrations in the groundwater is illustrated in Fig. 7. This is based on 65 borehole measurements (ENRESA, 1996d) which have been interpolated in three-dimensional space with respect to the geological structure. That is, the borehole data have been interpolated separately north of the uranium vein, within the vein and to the south of the vein. This process is automated in EarthVision. The three-dimensional model demonstrates that the highest levels of sulphate occur within the main mineralized vein and supports a hypothesis that the source of sulphate in the groundwaters of E1 Berrocal is pyrite within the vein.
Fig. 3. Visualization of the correlated fractures in the vicinity of boreholes 14, 16 and 18.
and a fracture set with greater than 50% carbonate were recognized. Each fracture set was colourcoded with respect to the dominant mineralogy as illustrated in Fig. 5. This provided a visualization of the spatial variation in fracture mineralogy.
5. Application of geological modelling and visualization tools to engineering geology A primary application of geological modelling in site characterization studies is the provision of geological representations of the site to computational software tools (White et al., 1996). These can be used to accurately define the geological structure and rock mass properties of the area analysed. However, there are several limitations to
190
M.J. I~V/lile ' Engineering Geoh~g)' 49 / 1998) 185 194
Fig. 4. Visualization o1" lhe c o r r e l a t e d IYacturcs m 111c Iracer lest z o n e between b o r e h o l e s 2. 13 a n d 15.
the structural and the rock and groundwater property models illustrated in this paper which may limit their suitability as pre-processors for rock engineering calculations and must be taken into
account when drawing conclusions from the results. The fracture network visualizations are deterministic models derived from TLV probe data. As
M.J. White / Engineering Geology 49 (1998) 185 194
191
Fig. 5. Visualizationof the mineralogyof fractures in borehole 17 at El Berrocal:(a) sericite-and smectite-filledfractures; (b) sericite-, smectite- and kaolinite-filledfractures; (c) clay and crabonate fractures (10 50% carbonate); and (d) fractures with > 50% carbonate.
such, they do not represent statistically defined networks in which the components of fracture length, orientation and truncation are adequately represented. They are, in fact, simple extrapolations from the fractures recorded in boreholes. These deterministic fracture networks also suffer from sampling errors associated with the orientation of the boreholes and uncertainty in the features detected by the TLV probe. In certain situations, however, deterministic fracture models are used as the framework for three-dimensional calculations (e.g. Brrgesson and Hernelind, 1995) The three-dimensional models of the rock and groundwater properties are also subject to limitations. They are derived from relatively simple distance-weighted algorithms which do not purport to represent the true mathematical variation in the measured borehole data. However, the models are useful in illustrating the gross variation in the modelled properties over the scale investigated. The sulphate model in particular illustrates the influence
of geological structure on the hydrochemistry (Fig. 7). This is, in part, due to the capabilities of the software to interpolate data in relation to the structure. The sulphate model is relatively simple but there is no limit to the number of domains which can be constructed for property interpolation. Despite limitations, visualization of mapped fractures can be an important part of the engineering component of developing waste repositories. At some stage in developing a repository, engineers have to make up their mind if they should place repository tunnels at certain locations in the rock mass. These decisions will certainly be based on borehole information, with structural information combined with predictions of rock mass response to thermal and mechanical loading and groundwater flow. Geological modelling and visualization can provide a useful aid in these decisions. Analysis tools, such as volumetric calculations, allow repository plans to be readily validated with respect to the geological structure.
192
M.d. White, En,~ineerin,g (;eoloKL' q9 (lg9~;) 185 194
Fig. 6+ Three-dimensional model of the hydraulic conductivity in the tracer test zone. Areas of the model where log hydraulic conductivity is less than - - l 0 have been removed. This model illustrates gross variations in hydraulic conductivity and the presence of channeling features in the rock matrix.
M.J. White / Engineering Geology49 (1998) 185 194
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Fig. 7. Three-dimensional model of the concentration of sulphate in the groundwater. This model is based on borehole measurements which have been interpolated in three-dimensional space with respect to the geological structure. That is, the borehole data have been interpolated separately north of the uranium vein, within the vein and to the south of the vein. The three-dimensional model demonstrates that the highest levels of sulphate occur within the main mineralized vein and supports a hypothesis that the source of sulphate in the groundwaters of Berrocal is pyrite, mineralized within the vein. The figure illustrates domain based interpolation. The accurate deterministic representation of the structure of crystalline rocks is largely dependent on the scale of information. During site characterization, data on faults and fractures is based on a large scale. With the development of the underground facility or the excavation of preliminary underground research laboratories (URLs), more localized three-dimensional information is available. The fracture visualization examples illustrated in this paper are probably not at the appropriate scale for near-field engineering analyses. However, with the development of repository programmes at a more local scale, information will become available and similar models to the large-scale ones presented here may be produced for the near-field. Another potential application is the use of structural models being used in the identification and analysis of structural domains.
As well as being of use in determining the threedimensional geology for engineering calculations, geological modelling systems may be used as a postprocessor. In this capacity, these tools can be used to view the results of three-dimensional simulations and the structure of stochastic fracture networks. The advantage of using this technology, as opposed to other currently available visualization tools is that the results can be presented in models integrated with the real geology and the repository layout. This would lead to a more effective communication of the results of modelling.
6. Conclusions This paper has illustrated the fact that advanced techniques exist for the analysis and visualization
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of geological data. Deterministic fracture networks a n d models of rock a n d fluid properties have been presented for the E1 Berrocal granite. These models can be used to u n d e r s t a n d the geological setting and to both provide the framework for and c o m m u n i c a t e the results o f three-dimensional c o m p u t a t i o n s . The coupling of such tools with c o m p u t a t i o n a l software should be a priority for the effective characterization of potential repository sites. Careful application of this forefront technology will allow the detail of engineering calculations to be advanced.
Acknowledgment The models discussed in this paper were constructed from data collected by a large n u m b e r of individuals who worked o n the E1 Berrocal Project. In particular, data has been collected a n d interpreted by the following; Rocio C a m p o s , Jordi Guimerfi, P a l o m a G 6 m e z a n d Luis P6rez del Villar. The work described in this paper was performed under contract to Pedro H e r n a n of Enresa, who are t h a n k e d for their permission to publish.
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