Borehole radar measurements aid structure geological interpretations

Transcription

1 Ž. Journal of Applied Geophysics Borehole radar measurements aid structure geological interpretations S. Wanstedt ), S. Carlsten, S. Tiren GEOSIGMA, Box 894, S Uppsala, Sweden Received 2 October 1998; received in revised form 25 February 1999; accepted 20 April 1999 Abstract Successful site characterization for a repository of nuclear waste or underground construction in general provides basic data concerning engineering aspects of repository design with impact on both the efficiency of the repository in isolating waste and in constructing the repository. Three-dimensional Ž 3D. visualization of data is an essential step in the development of descriptive hydrologic and rock-mechanical models of fractured rock systems. Modeling of fractures and fracture zones in 3D is usually based on the correlation of fracturing in drillcores and on outcrops. Difficulties with this procedure arise when the vertical or horizontal separation between fractures is large. Directional radar surveys help decrease the uncertainty in correlation. At great depths, such as is the case when investigating potential nuclear waste repositories, the errors present as borehole deviations, and dip determinations of structures as well as the varying characteristics of geological features, may make interpretations virtually impossible. Tomographic radar measurements help improve the 3D modeling because zones with anomalous properties can be traced across the investigated plane or volume. This leads to a further decrease in uncertainty and eventually to better models. The comparison of directional reflection surveys and tomography shows that the accuracy of single-hole surveys is reasonably good. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Borehole investigations; Directional borehole radar; Tomography; Structural geology 1. Introduction The objectives of a site investigation for a tunnel are to provide information for the assessment of the technical and economical advantages of alternative schemes, the selection of the most suitable alternative, the preparation of an adequate and economical design, and to allow difficulties that may arise during construction, modification or repair to be foreseen and pro- ) Corresponding author. Tel.: q ; fax: q ; sw@geosigma.se Ž. vided for West et al., Potential nuclear waste repositories comprise an especially complicated kind of site investigation because they will be located at a depth of about 500 m below the ground surface. For obvious reasons, investigations commence at the surface with whatever information is available there. In every exploration operation, the investigator eventually finds the situation where he needs detailed information about features at depth. The only way to obtain this information, without excavating, is to drill a hole through the rock volume to be investigated r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž. PII: S

2 228 S. Wanstedt et al.rjournal of Applied Geophysics 43 ( 2000) Drilling, regardless of the method employed, involves a sizeable expense in the evaluation of a site. Hence, it is important to extract as much information as possible from the boreholes. This paper deals with the difficulties in extrapolating geologic surface and subsurface information when investigating large, deep volumes of rock and how such difficulties can be resolved, or at least decreased, with borehole radar measurements. 2. Deep underground repositories The investigations prior to the siting of a repository for radioactive waste involve isolating or detecting a volume of rock that fulfils some basic requirements. According to the Swedish Nuclear Waste Management Ž SKB., the main objectives of the initial site investigation are Ž SKB, 1992.: Ø To define the site-specific position of a rock volume for a repository; Ø To plan the surface and subsurface facilities; Ø To provide a basis for a preliminary sitespecific performance assessment; and Ø To provide information to ensure safe and efficient underground activities. The Swedish bedrock is usually highly competent with very low porosity. Hence, the fractures and fracture zones are fundamental concepts in the site characterization as the structures control the groundwater transport and the mechanical stability. In other words, most of the work is concerned with determining the geometry of these structures. A Swedish repository will be located in Precambrian crystalline rocks. It will be a multibarrier system intended to minimize the probability of radioactivity escaping to the surface. At a depth of approximately 500 m, the storage will be protected from natural and human disturbance, supposedly maintaining favorable conditions for isolating waste without further human aid. The waste is to be enclosed in steel-coated copper canisters deposited in vertical holes about 1.5 m in diameter. A final full-scale repository might contain as many as 4500 canisters and thereby covering an area of about 1 km 2. Fig. 1 shows a hypothetical repository Ž about 10% of full-size. placed at 500 m depth below the island Aspo. The three-dimensional Ž 3D. fracture zone model in the figure, which governs the layout of the facility, was developed from surface-based investigations of the Aspo Ž Tiren et al., A fracture has to fulfil certain requirements to be included in the model, such as being possible to extrapolate and identify in more than one location, borehole or at the surface. The extrapolation of features was essentially performed with the directional borehole radar. 3. Extrapolation of geological data According to the definition, fractures are discontinuities with mechanical or tectonic origin. In geology, the term is used to describe everything from microfractures to large faults. Further, the fractures can have several different chemical and physical properties: thus, the geophysicist who wants to detect and classify fractures using borehole geophysics stands before a very challenging task. One reason is that no geophysical method reacts directly to the fracture. Instead, the geophysicist has to determine how each fracture affects the measurements recorded with the different probes. To simplify investigations concerning fractures, it is often assumed that the strike of the fracture is orthogonal to the drillhole and filled with some fluid, usually water. Furthermore, the fracture surfaces are assumed to be smooth and perfectly plane. The 3D modeling of fractures and fracture zones is usually based on the correlation of fractures in drillcores and surface structures. Remote analysis and surface geophysics help determine the location of surface structures. The correlation of surface data and borehole struc-

3 S. Wanstedt et al.rjournal of Applied Geophysics 43 ( 2000) 229 Fig. 1. Layout of a potential repository located at 500 m depth governed by 3D structural model of Aspo, Sweden ŽTiren et al., Fractures have to fulfill certain requirements to be included in the model, such as being possible to extrapolate and identify in more than one location, borehole or at the surface. The extrapolation of features was essentially performed with the directional borehole radar. tural information along the hole going downwards is usually not very difficult near the surface but becomes increasingly complicated as holes become deeper. At great depths Žc. 500 m., it is extremely difficult to determine which one of all the zones along the hole is the one that correlates with the surface expression, Fig. 2. The strike of a structure is generally well defined, based on the surface data, but the dip of the interpreted structure is at best Žbased solely on surface data. determined with an accuracy of "108. Assume that there is a zone with an interpreted dip of 60"108 westwards and that the mean spacing between fractured sections is constant. The number of equally possible interpretations is a function of the orientation of the borehole and the distance between the borehole and the structure at the surface, Fig. 2, where the average distance between minor zones is assumed to be 15 m, while it is 30 m between major zones. When a borehole dipping 608 towards the structure is drilled 50 m away from the structure there are two possible solutions in the most favorable case. The borehole will intersect the zone at 40 m depth. If, however, this example is repeated for the same zone at 500 m depth, there are several Ž ) 10. equally possible solutions within a borehole length interval from 470 to 708 m Žcorresponding to vertical depths of 410 to 615 m.. Although the zones of fractured rock do not occur as uniformly in real

4 230 S. Wanstedt et al.rjournal of Applied Geophysics 43 ( 2000) Fig. 2. Potential error in the location of a zone at depth due to errors in borehole deviation measurements and dip determinations of zones at the surface Ž Tiren et al.,

5 S. Wanstedt et al.rjournal of Applied Geophysics 43 ( 2000) 231 rock, there will be several possible interpretation of the location of a structure at great depths. 4. Borehole radar Ground penetrating radar Ž GPR. is a term that covers a group of high-frequency electromagnetic methods Ž EM.. All these methods involve transmitting very high frequency EM waves and recording the resulting signals from reflections within the medium or from transmission passing through the media. The technique can be used to examine and locate interfaces in solid or liquid media, which exhibit changes in electrical properties. The exact technique employed, transmission or reflection, being determined on the basis of the object examined and the medium involved. The borehole radar is a special case of conventional GPR with several features that distinguish it from the surface tools. The most important is perhaps that the borehole itself enables emplacement of the antennas quite close to the objects to be investigated, resulting in precise target responses. Furthermore, the receiver and transmitter antennas may be lowered into different holes, so called cross-hole measurements. The information obtained with this setup often complements the single-hole reflection surveys. Two different properties of radar wave propagation normally determined are velocity and attenuation. The propagation of electromagnetic waves through rocks and soils is mainly a function of the dielectric constant and the electrical conductivity. The potential of a feature in the bedrock to show as a distinct radar reflector or anomaly depends primarily on the contrast in the dielectric constants of the feature and that of the bedrock Ž Olsson et al., Because of the large contrast in dielectric constant between bedrock Ž about 5. and the water-filled pores Ž dielectric constant for water is 81. in the fractured rock the fractures are well indicated. Fig. 3. Principle of results from two types of borehole radar antennas, conventional dipole Ž left. and directional antenna Ž Tiren et al., The prior only gives the intersection angle while the latter gives both intersection angle and direction.

6 232 S. Wanstedt et al.rjournal of Applied Geophysics 43 ( 2000) 4.1. Directional single-hole surõeys Most borehole antennas are axially oriented dipoles. Consequently, the response of a survey includes information from the entire volume of rock surrounding the hole. In our case, it is necessary to determine the orientation of single features and we have therefore used a directional receiver antenna. It is directional in the sense that it allows for the absolute determination of strike and dip of all planar reflectors. In Fig. 3, the principle of the interpreted results from an omnidirectional dipole antenna is compared to that of the directional antenna. The design of the antenna limits the practical range of angles at which reflectors are determined with the borehole radar. Fig. 4 shows an example of how the spatial density of fractures interpreted depends upon the angle of intersection between the borehole and the fractures. There is no reason to assume that there is a bias in the natural distribution of zones in this rock mass and therefore this distribution can be assumed in other rocks as well. Fig. 4. Distribution of fractures detected by borehole radar relative to the angle of intersection between borehole and structures Ž Tiren et al.,

7 S. Wanstedt et al.rjournal of Applied Geophysics 43 ( 2000) 233 Previous investigations show that the ability of the borehole radar to detect features within the rock mass appears to have a maximum when intersecting structures at an angle of c. 308, while it diminishes rapidly for lower intersection angles and more slowly for higher intersection angles. At high intersection angles Ž , the ability to detect structures is the lowest. This geometrical relationship has to be considered when planning borehole radar surveys and especially when the target will be investigated from several holes drilled in different directions. The target could be a specific object such as a fracture zone, but it could also be a rock volume. An orthogonal borehole configuration does not optimize the potential of the borehole radar, as there will be orientations of structures poorly sampled by all boreholes Ž Tiren, Since the reflection coefficient is highly dependent on the variation in water-content in the rock mass the radar reflection surveys result in selective picking of features. Bedrock features that give rise to reflectors need to be of a certain size and preferably, but not always, are the effect of water-saturated porosity. The radar reflectors of concern in this study are generally planar and large in extent Ž20 40 m or more. structures. At the intersection of radar reflectors and borehole, the core logs often express increased fracturing. Locally the fractures appear to be sealed Tomography The principle of cross-hole surveys is that the transmitter and receiver are located in such a manner that direct rays traveling between them pass through the medium. For each possible antenna configuration, a trace is recorded at the receiver. Each trace corresponds to a raypath between the boreholes. Along each raypath, the travel time and the amplitude of the first arriving direct wave is determined. Both arrival times and amplitudes can be analyzed with tomographic inversion. Fig. 5. Dipole radar component of single hole surveys in the two holes together with velocity tomogram from plane delineated by the two holes.

8 234 S. Wanstedt et al.rjournal of Applied Geophysics 43 ( 2000) Tomography and reflection surveys are affected a bit differently by variations in the rock mass. For example, a wide, severely fractured and water-bearing zone will probably cause a reflection at either of the boundaries. The minimum velocity in the tomogram, on the other hand, will probably be located in the center of the zone Žwhere the porosity and water content reaches a maximum Description of measurement setup Borehole radar measurements were performed in two co-planar holes 509 and 301 m deep, respectively, at the Romuvaara investigation site in Finland. Single-hole directional surveys were performed in each hole. Furthermore, Fig. 6. Interpreted reflectors from directional antenna surveys in the two boreholes, plotted in the tomographic plane. Numbers to the left in plot are reflectors found in the left borehole Ž RO-KR4. Ž Carlsten and Wanstedt, Tomographic inversion involves dividing the interval between the boreholes into segments and assigning approximate values of the dielectric constant or attenuation to each segment in an iterative manner. The values are adjusted by means of the amplitudes and travel times of the rays passing through each segment, calculations are continued until the travel times and attenuations correspond to the measured values. The resolution of the method is a complex function of the wavelength, the transmitter and receiver point spacing Ž along the respective holes., as well as the distance between the boreholes. The resolution is on the order of meters. Fig. 7. Interpreted reflectors from directional antenna surveys in the two boreholes, plotted in the volume Ž 3D. surrounding the tomographic plane.

9 S. Wanstedt et al.rjournal of Applied Geophysics 43 ( 2000) 235 cross-hole measurements were performed between the holes. The distance between the holes is 73 m at the surface and about 201 m at depth. The use of a directional tool enables the determination of location, strike and dip of reflectors in the vicinity of the hole. The transmit- Fig. 8. Velocity tomogram between the two boreholes with interpreted structures. Dashed lines are possible structures picked from the velocity tomogram.

10 236 S. Wanstedt et al.rjournal of Applied Geophysics 43 ( 2000) ter antenna, emitting 60-MHz radar waves, manages to survey a rock volume rock with a radius of 30 to 40 m in reflection mode Ž Fig. 5.. This means that near the surface, it would be possible to record the same reflection from either of the holes if there was an adequately oriented anomaly in the rock volume between the holes. At the bottom of the investigated plane the distance is much too far for the reflection mode investigation. As a result, it is not always easy to correlate zones between the two holes. It is even more difficult to extrapolate a zone accurately when the target is at the surface. Radar waves for the cross-hole survey were generated with 22-MHz dipole antennas to cover the somewhat large distance in the deeper parts of the investigated plane Ž Fig. 5.. In total, 4040 rays were recorded with a minimum length of 76.5 m and a maximum length of m. Data quality of recorded rays was excellent and very few rays had to be omitted Ž1.9% of the velocity rays and 2.1% of the amplitude rays.. found with the directional antenna. There are possibly more structures within the tomogram, 5. Discussion of results When plotted in the plane confined by the two holes, as in Fig. 6, it is difficult to extrapolate and connect reflectors between the holes since there is no dip information. If the interpreted reflectors are plotted in 3D Ž Fig. 7., there is a possibility to connect the various features. Due to the quite large number of reflectors, this is still difficult. The distance is not, unfortunately, the only problem as there is a limited accuracy in the determination. The tomographic inversion of the data resulted in plots of the velocity and amplitude distributions in the plane. In the plots, low velocity or high attenuation characterizes fracture zones across the plane. The features are generally easier to locate in the velocity tomogram Ž Fig. 8.. Six features were interpreted as significant zones and subsequently compared to the zones Fig. 9. Comparison of features interpreted from the velocity tomogram and interpreted reflectors that show how overall structural interpretation can be improved. Labeled lines Ž A G. are features related to tomography. Other lines are reflectors. Width of each line corresponds to dip out of the reflector, wide line dips more than thin line.

11 S. Wanstedt et al.rjournal of Applied Geophysics 43 ( 2000) 237 but they do not appear as clearly. The significant zones delineated by the velocity variations generally correlate with interpreted reflectors, although only a subset of the reflections originate from the low velocity zones Ž Fig. 9.. The low velocity zone in the tomogram around 200 m Ž Figs. 5 and 8., appears to cause several reflections in both boreholes but the directions are contradictory. In fact, there are no reflectors that accurately describe the low velocity zone. With the information from the tomogram, the correct location of the zone can be established. Note that although the boreholes are quite close near the surface, there are not many interpreted reflectors that are found in both holes. There are a few possible reasons for the discrepancy between single-hole surveys and cross-hole surveys. The most important reason in this case is probably the difference in resolution. 6. Conclusions A single-hole radar investigation is clearly a very helpful tool for understanding the structural geology in the rock mass surrounding a borehole. Although the information is oriented, it is difficult to extrapolate reflectors between holes and to the surface. When planning an investigation, it is necessary to be aware of the actual performance of the directional borehole radar. Since the tools does not cover all angles in the same way, the orientation of the holes should be planned taking this into account. Radar tomography treats the structures between the holes a bit more stringently in that there is really no room for error in the dip determination within the plane. Due to the high resolution, zones can be traced across the entire plane between the holes, or up to the surface. Acknowledgements Thanks are due to POSIVA OY for allowing us to publish the results of this investigation. References Carlsten, S., Wanstedt, S., Detailed borehole radar measurements at the Romuvaara site, Finland. POSIVA OY Site Investigation Project Work Report PATU-96-52e, 64 pp. Olsson, O., Falk, L., Forslund, O., Lundmark, L., Sandberg, E., Crosshole investigations results from borehole radar investigations. Stripa Project Technical Report 87-11, an OECDrNEA international projected managed by the Swedish Nuclear Fuel and Waste Management Ž SKB., Stockholm, 160 pp. Žrevised Feb SKB, SKB-91. Final disposal of spent nuclear fuel importance of the bedrock for safety. SKB Technical Report TR 92-20, Swedish Nuclear Fuel and Waste Management Ž SKB., Stockholm, 197 pp. Tiren, S.A., On the use of borehole radar measurements for 3D assessment of structures in rock volumes. In: Proceedings of the 3rd Aspo International Seminar Characterization and Evaluation of Sites for Deep Geological Disposal of Radioactive Waste in Fractured Rock, Oskarshamn, Sweden, June 10 12, SKB Technical Report TR 98-10, Swedish Nuclear Fuel and Waste Management Ž SKB., Stockholm, pp Tiren, S.A., Beckholmen, M., Voss, C., Askling, P., SITE-94: Development of a Geological and a Structural Model of Aspo. SKI Report 96:16, Swedish Nuclear Power Inspectorate Ž SKI., Stockholm, 198 pp. West, G., Carter, P.G., Dumbleton, M.J., Lakes, L.M., Rock mechanics review site investigations for tunnels. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 18,