Geophysical methods for lineament studies in groundwater exploration. A case history from SE Botswana

Geoexpioration, 27 ( 1991 ) 165-177 Elsevier Science Publishers B.V., Amsterdam

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ABSTRACT Zeil, P., Volk, P. and Saradeth, P., ! 991. Geophysical methods for lineament studies in groundwater exploration. A case history from SE Botswana. Geoexpioration, 27: i65-i 77. Exploration studies for village water supply from basement and consolidated sedimentary rocks in Botswana show that groundwater occurrence is often restricted to linear structural features associated with faulting. The fractured aquifer which develops in this situation is characteristically channel-shaped, of narrow width and variable length. The prevalent cover of Kalahari sands hampers considerably the detection of lineaments from satellite imagery. By presenting aeromagnetic data as raster images, subtle changes in the magnetic field pattern can be resolved. Breaks or level changes due to structural features, such as faults or fracture zones, are displayed indirectly in airborne magnetic surveys. If satellite imagery and aeromagnetic data are processed in the same format (raster) and geographically referenced by the coordinates of grid points (pixeis) they can be inspected simultaneously. The overlay of the two data sets by the use of a Geographic Information System helps to define linear features more accurately than in one data set alone. The location of fracture zones associated with major lineaments can be mapped successfully on the ground with electromagnetic methods (VLF, HLEM ). Multi-frequency horizontal loop systems proved to supply the best guidance to optimal borehole locations if the operating frequencies and the coil separations are properly adjusted to the local geological environment. Even though an inclined borehole, sited according to the results of this investigation, did not intersect a major fault, the high degree of fracturing in a depth range of 50 to ! 00 m, together with the circulation losses encountered, correlate with our interpretation.

INTRODUCTION

Exploration studies for village water supply (required yield 1-10 m3/h) from basement and consolidated sedimentary rocks iF, Botswana show that groundwater occurrence is often restricted to linear structural features associated with faulting. The fractured aquifer which develops in this situation is

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characteristically channel-shaped, of narrow width and variable length. The individual fracture elements belong to a complex tectonic system. Their permeabili:y is possibly related to the former tectonic function ofthe various directions within the system. The selection of suitable borehole locations is successfully assisted by geophysical exploration methods, especially in areas where extensive sand cover hampers the traditional applications of lineament analysis from satellite imagery and aerial photography. Airborne magnetic data exist in most countries and are generally available (in the form of digital data tapes or published maps) to the exploration community. The pattern of these data can reveal information on the relief and texture of the magnetic basement. In this paper we illustrate how an appropriate processing of satellite imagery and aeromagnetic data and their combined analysis by means of a Geographic Information System facilitated the detection of lineaments pertinent to groundwater extraction. The complex internal structure of fractured linear aquifers calls for a detailed investigation on the ground. Electromagnetic methods and, to a lesser degree, resistivity profiling were carried out to resolve the location and geometry of the local fracture system. On the basis ofthe core log of an inclined borehole we discuss the potential of this approach. G E O L O G Y AND H Y D R O G E O L O G I C A L M O D E L

The area of the present study lies at the edge of the Kalahari (Fig. 1 ). The Precambrian basement is exposed only in the eastern part of Botswana. In the study area, Waterberg rocks ( 1800 Ma), which comprise shales, quartzites, sandstones and dolerite, are diseordantly overlain by fluviatile, deltaic shallow-water deposits of Carboniferous to Jurassic age. This sequence, the Karoo, is built up of siltstones, sandstones and mudstones with intermittent coal seams. Post-Karoo aeolian sands of Kalahari deposits cover the remaining 80% of the land surface. These Kalahari beds vary in thickness between 10 and 200 m and commonly begin their sequence with claystones (often showing high salinity ), followed by silcrete, calcrete and unconsolidated sand (Fig. 2). Earlier investigations for water resources for the Jwaneng Diamond Mine northwest of Kanye revealed that the local aquifer is situated in the vicinity of a former delta at the southern shores of a lake of Beaufort Formation, Karoo age. Initially, coarse material was laid down during Karoo times but, with the progressive infilling of the lake, finer sediments followed. After the consolidation of this formation, only the coarse-grained horizons retain enough open pore space to act as an aquifer. As the rivers feeding their waters to this lake drained the highlands to the south of the Km~oobasin, the deposits in the

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delta become finer and thinner towards the north where the sediments eventually consist of silts, siltstone and clays, more typical of a deep water environment. In his assessment of the hydrogeology of the Jwaneng wellfield, Buckley (1984a) suggests that there might be a strong link between sedimentation and stn~cture. The interpretation of satellite imagery indicates that the existence of coarse sandstone is not the sole factor leading to high yielding wells; faulting and fracturing might also play an important role. This study produced the first evidence that the most productive sites are those having a structural connection with basement groundwater circulation. For the Letlhakeng area, Bucldey concludes that optimum yields can only be expected where boreholes intersect major structures trending in those directions which show the highest permeability due to open fractures. These were suggested to be aligned along lineations with a northwesterly bearing in

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a lineament study carried out by a consulting company using satellite imagery (Viak, 1983). Drilling results dufin~ further groundwater investigations east of Molegolole have also proven northerly trending lineaments to be highly producti~,e fvr groundwater in this area (BucHey, 1984b). The prevalent cover of Kalahari sands hampers considerably the analysis of lineaments throughout the survey area. Often the recent drainage systems follow tracks of linear features. In those cases lineations can be marked, but mostly for short distances only (Fig. 3 ). THE ROLE OF AEROMAGNETIC DATA

Since the sedimentary cover does not contain any magnetic sources, aeromagnetic data reflect the spatial distribution of the magnetic basement. Drill cores recovered during a coal exploration campaign give evidence of dolerite sills at various depths throughout the area (Fig. 2). In such a situation, faults are commonly displayed indirectly in airborne magnetic surveys (Astier and Paterson, 1989). They can manifest themselves for example as: Sharp "-o'~o"'~ forming a linear boundary between areas of different magnetic level, relief or texture. Disruptions and/or deflections of magnetic trends. These are commonly wrench faults or shears, often with distinguishable lateral movement. How can aeromagnetic data sets be presented in a format most appropriate for this type of qualitative interpretation? The production of the traditional contour maps of geophysical anomaly fields by computer has always involved two stages. The first of these, gridding, is the generation of a regular grid of points from the discrete points along flight lines. An adequate sampling of potential field anomalies is achieved according to Reeves and Wu (1989) if the sampling interval 8x is less than 0.5 s, the source-sensor distance. The source-sensor distance is the vertical separation between the magnetometer sensor and the rocks causing magnetic ano~,:~iies and in practical terms is the sum of the flying height (terrain clearance) and the depth of the magnetic basement. The information loss caused by undersampling anomalies, however, is stili tolerably small for a study on basement topography, as in our case here, when 8x falls in the range 0.5-1.0 s (flight height 300 m, survey linespacing 4 km, bearing of flight lines N-S). The second step is the generation of a plot-file of straight-line segments which cross each of the squares defined by the nodes of the grid. However, the contour map is a static display which does not lend itself to further processing. The originally published contour map for the survey area (Fig. 4) gives no obvious indication of, for example N-S trending lineaments. In this style of presentation the human eye predominantly recognizes areas surrounded by contours - that is, single anomalies - or strong gradients where contours are clustered. "

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Instead of plotting contour lines the gridded data can be visually presented in other ways. On the analogy of satellite imagery, each discrete brightness observation is ascribed to a "pixe!" and displayed as a certain value on a grey scale from black to white or as a colour-coded raster. The high resolution of typical hard-copy devices, such as dot-matrix printers or laser film writers connected to image processing systems, can produce maps with a ground pixel size of 1O0 m or better on a 1:100,000 scale. By presenting aeromagnetic data as raster images, subtle changes in the •---e~-~"°""°+;~field pattern_ can. _ be .resolved._. Breaks or level changes due to structural features, such as faults or fracture zones, which had been mostly con-

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Fig. 5. Geographic Information System product. Structural processing of TM channel 4 (intensity) as shaded relief and aeromagnetic data (hue) with tc~al ~leld amplitudes ranging from 400 nT (violet) to 750 nT (red). L=Lctihakeng, M= Molepolole.

cealed between contour lines, can now be enhanced by considering the magnetic relief as though it was a topographic relief. As both data sets - satellite imagery and aeromagnetic d a t a - are in the same format (raster) and both geographically referenced by the coordinates of the grid points (pixels) they can be inspected simultaneously. Even if the pixel resolution is different in TM (30 m) and aeromagnetics ( 100 m) the

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overlay of the two data sets by the use of a Geographic Information System helps to define linear features more accurately than in one data set alone (Fig. 5). The combined presentation is achieved by using a structural processing of TM channel 4 (shaded relief) as the intensity component, along with the hue image derived from magnetic amplitudes ranging from 400 nT (violet) to 750 nT (red). This type of processing corresponds to "colouring in" the higher spatial resolution TM data with the colour information derived from the aeromagnetic raster image. Assuming a relatively constant thickness and magnetisation of the dolerite sill over the area under study (Fig. 2), an interpretation of the magnetic pattern yields estimates on varying depth levels of this magnetic horizon. Areas of relatively strong magnetic amplitudes correspond to shallow basement, whereas a thicker sedimentary cover is indicated by a weaker magnetic response. GROUND FOLLOW-UP SURVEY

A major lineament detected during the combined interpretation of airborne magnetic data and satellite imagery (Fig. 3, A-A' ) was chosen on the grounds of easy access and with the intention to investigate this feature in detail using different geophysical survey methods. Before the implementation of this study, 10 holes were sited in the area on the basis of airphoto interpretation and geoelectrical survey results. The last hole drilled was located close to this lineament and produced the highest yield (6 m3/h) obtained so far in the survey area (BH 4695, water struck at a depth of 84 m, section in Fig. 7). Resistivity profiling was carried out along an E-W bearing test profile (Fig. 3, line 0N). A Schlumberger configuration with a constant separation of 200 m between current electrodes and 25 m between voltage electrodes was used to record the data at the bottom of Fig. 6. Two resistivity lows were detected: at the intersection with lineament A-A' between - 6 0 0 and - 8 0 0 and from - 2 2 0 0 to -2500 where a second lineation was located on satellite imagery. The average resistivity of the blocks separated by these lineaments decreases from east to west, probably indicating an increase of the Kalahari cover in this direction. DC ge~electrical methods are often used in groundwater exploration programs, primarily due to the fact that instrumentation is inexpensive and easy to operate. In semi-arid terrain this is however counterbalanced by difficulties in achieving a sufficient galvanic coupling with the ground through highly resistive dry sands at surface. The use of additional electrodes or the reduction of contact resistance by watering slows down field operations. Furthermore, information on the geometry and depth location of two-dimensional features, such as steeply dipping fracture zones, are not readily derived from resistivity data.

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The application of electromagnetic methods offers several advantages in this situation. Since the contact with the subsurface is inductive, a fast progress of measurements is achieved. Successfully applied in mineral exploration, EM methods are very sensitive to vertical conductive targets. Besides locating conductors accurately, the interpretation of EM data with regard to depth, dip and conductance is routinely carried out with the aid of model graphs or nomograms. As an additional asset, the most commonly used EM instruments need only 1 (VLF) or 2 (HLEM) operators in comparison to resistivity surveys where at least 4 to 6 persons are required. At the location of lineament A-A', the VLF-survey produced two small anomalies (Fig. 6, centre) where the inphase component (tilt angle) changes its sign to negative readings ( - 540 and - 6 7 0 ) . Relatively good conductance is indicated by the reversed change of the out-of-phase component at - 540. At low response levels, such as recorded here, anomalies are better defined after the application of a linear filter to the inphase readings (Fraser, 1969). Considering a thickness of 20 m for the Kalahari beds and the limked penetration depth of the VLF signal waves at the average resistivity of 50 ~ - m of this layer, these anomalies could indicate local accumulations of conductive clays where fractures have produced depressions at the surface of the K~rroo rocks. The location of fracture zones associated with major lineaments can thus be mapped with VLF. Multi-frequency horizontal loop electromagnetic (HLEM) measurements improve the accuracy for the determination of the location of conductive zones as indicated by data collected at coil separations of 50, 75 and 100 m (Fig. 6,

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top). The 50 m separation is too small to detect conductivity changes caused by fracturing in the sedimentary rocks. The decrease of signal level for the high frequencies from west to east, however, clearly reflects thinner overburden to the east of the lineament. With increasing coil separation - and therefore greater penetration - two anomalies appear at the locations already

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marked during the interpretation of the VLF data. The different response for primary signals of 3037 and 337 Hz indicates that the conductances involved are low. For further HLEM surveys readings needed only be taken at the highest frequencies (adapted to the low induction numbers related to narrow fracture zones). Nomograms published for conductive dipping thin bodies and two layer models for half space were used to derive the interpretation in Fig. 7. For both conductive zones a dip towards the east is indicated by the asymmetry ofthe anomalies recorded with a 100 m coil spacing. To test the validity of such a model, an inclined cored hole was drilled as indicated in Fig. 7. Even though the borehole did not intersect a major fault, as the recovered core shows no signs of high fracture densities coupled with a major disruption of the formation, the high degree of fracturing between 50 and 100 m together with the circulation losses encountered correlate with our model (Fig. 8 ). Tl~e verification of dip angle and direction remains ambiguous since the orientation of the core with respect to the axis of the drillhole was not recorded. However, a statistical analysis of the apparent angles measured at core samples indicates that a majority corresponds to steep angles of true dip. Furthermore this group of fractures shows movement and most probably produces the lineament trace at ground level (Von Hoyer et al., 1985). CONCLUSIONS

The combined analysis of satellite imagery and aeromagnetic data is a useful tool in groundwater exploration. Where linear fracture zones have to be located, areas for ground surveys can be selected accurately and thus time and costs for borehole siting will be reduced to a minimum. A detailed image of the local fracture system is obtained by the interpretation of electromagnetic survey data. Multi-frequency horizontal loop systems proved to supply the best guidance to optimal borehole locations if the operating frequencies and the coil separations are properly adjusted to the local geological environment. ACKNOWLEDGEMENTS

The authors are grateful to the Director of the Geological Survey of Botswana for the permission to publish this article. They also acknowledge the critical review of the manuscript by C.V. Reeves and R.J. Peart. Thanks to A. Siebert, GAF, for preparing most of the figures.

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REFERENCES Astier, J.L. and Paterson, N.R., 1989. Hydrogeotogical interest of acromagnetic maps in crystalline and metamorphic areas. In: G.D. Garland (Editor), Proceedings of Exploration '87.: Third Decennial Int. Conf. on Geophysical and Geochemical Exploration for Minerals and Groundwater, Special Volume 3. Ontario Geological Survey, pp. 732-745. Buckley, D.K., 1984a. Groundwater exploration at Letlhakeng. Phase 1. Rep. DKB/9/84, Department of Geological Survey, Botswana. Buckley, D.K., ! 984b. Geological and hydrogeological information from water borehole drilling in Waterberg rocks at Mochudi and Molepolole 1982-1983. Rep. DKB/6/84, Department of Geological Survey, Botswana. Fraser, D.C., 1969. Contouring of VLF-EM data. Geophysics, 34: 958-967. Shaw, P.A. and De Vries, J.J., 1988. Duricrust, groundwater and valley development in the Kalahari of southeast Botswana. J. Arid Environm., 14: 245-254. Reeves, C.V. and Wu Chaojun, 1989. Adequate sampling in magnetic profiling, the resolution of closely-spaced magnetic sources and their importance to image-enhancement techniques for magnetic anomaly maps. Presentation at the 51st Meeting and Technical Exhibition of the European Association of Exploration Geophysicists, West Berlin, 29 May-2 June 1989. Viak, Ltd., 1983. Eastern Botswana regional water study. Rep. to the Dept. of Water Affairs, Government of Botswana. Von Hoyer, M., Keller, S. and Rehder, S., 1985. Core borehole Letlhakeng 1 - Lithological and hydrogeological log. Rep. MVH/4/8~, Department of Geological Survey, Botswana.