by E. Wightman, T.B. Bakaya, A. Kumar, H. Kumar and I.H. Ngwisanyi

ABSTRACT

Aeromagnetic, airborne electromagnetic surveys and Transient Electromagnetic Soundings were carried out in the Lower Okavango Delta in northwestern Botswana to evaluate the groundwater potential of a 12,500 square kilometers area. The area is covered by thick sediments of the Kalahari Beds and underlain by various bedrock types including the Karoo Supergroup of Carboniferous to Jurassic Age and Damara Supergroup of Palaeozoic to Proterozoic Age. Kalahari Beds sediments and Bodibeng sandstone of Karoo Supergroup were the prime targets for exploration.

The geophysical investigations were carried out in two phases during 1995-1997. The Phase 1 investigations concentrated on evaluating the regional geology and structure. This was done by interpreting aeromagnetic data (70,447 line Kilometers) along with deep TEM soundings. Phase I also included testing the applicability of airborne and ground Transient Electromagnetic (TEM) methods for evaluating the ground water potential of two distinct targets i.e. shallow unconsolidated sediments and the deep underlying bedrock.

The airborne magnetic survey provided information on sediment thickness, depth to the top of the magnetic basement, location of major faults, dykes and sills. This information along with data from existing deep boreholes, all of which were located outside the study area, was used to develop a geological model for the area. This model defined the spatial and vertical distribution of Kalahari Beds and various bedrock types that could be postulated to occur below the sediments. The interpretation also suggested a complex geological and structural environment of the area. Lack of borehole data within the project area constrained the aeromagnetic interpretation model.

A test airborne electromagnetic (AEM) survey over the existing Shashe wellfield along with ground TEM soundings were completed and showed that AEM clearly defined the spatial distribution of fresh water within the sediments and ground TEM precisely defined the vertical extent of fresh water. It was thus recommended that a regional AEM survey be conducted over the western part of the area (15.871 line kilometers that is underlain by thick Kalahari sediments and receives active recharge from the delta outlet distributaries. Phase 1 surveys concluded that the regional AEM interpretation in conjunction with ground TEM soundings are the most pertinent methods for exploring groundwater resources of the Kalahari Beds. For bedrock exploration, aeromagnetic interpretation in conjunction with TEM soundings was used.

Phase 2 commenced with incorporating the results of the AEM survey in a reassessment of the eight exploration areas that were previously delineated on the basis of TM imagery interpretation hydrogeological data and the flow characteristics of the delta distributaries. Using the AEM data the eight exploration areas were redefined into six areas; four of the previously defined areas were retained, one was discarded, one was reconfigured and a new area was added. These six areas were demarcated for ground geophysics. In each area TEM soundings were conducted and a series of geoelectric sections were drawn to infer the lateral and vertical extent of sediments containing fresh water. The maximum thickness of the fresh water within these sediments was interpreted to be close to the river channels and the thickness decreased towards the edges of channels / valleys. The thickness of fresh water in the sediments was interpreted to range from 50 to 140 meters and the resistivity of these sediments ranged from 10 to 70 ohm - meters.

The drilling results confirmed the interpretation of AEM and ground TEM soundings. TEM soundings enabled the delineation of the lithologies within the Kalahari Beds (as units of sand, clay) and as well as characterizing the water quality in these units in terms of fresh, brackish and saline water. The method was proven to be capable of resolving the thickness of fresh water sediments within 5 meters at depths to 100 meters, or to an accuracy of 95%. The resistivity of sediments to a depth of about 100 meters (ignoring the top 5- 10 meter laver) decreases with depth from 70 ohm - meters in the shallow aquifers to 10 ohm - meters in the middle aquifers. Less than 5 ohm - meters resistivity was interpreted for the lower saline aquifers. The change in formation resistivity with depth is attributed to increased water salinity with depth, as a high degree of correlation was ascertained between water salinity and formation resistivity.

In the investigation for bedrock aquifers, the primary target was the Bodibeng Sandstone Formation. Two areas were selected for locating such targets. One in the northeast of the area where aeromagnetic interpretation suggested that basalt could occur below Kalahari Beds and it was assumed that the Karoo Age Bodibeng Sandstone would be underlying Stormberg Basalt. The second was between Thamalakane and Kunyere Faults where previous drilling had indicated the possibility of encountering water of relatively low TDS in Bodibeng Sandstone below a zone having high TDS water. Interpretations of magnetic data and deep TEM soundings were used to locate a deep drill site.

The first deep exploration borehole was completed to a depth of 980m in the northeastern part of the project area. Beneath 720m of basalt the Proterozoic Meta-arkose Ghanzi Group was encountered indicating that the Karoo Age sediments were not deposited in this area. The second deep borehole drilled to a total depth of 247 meters encountered Bodibeng sandstone below 154 meters of Kalahari Beds. All water strikes in both boreholes were saline indicating that prospects of fresh water in the bedrock were poor.

The results of this exploration program have demonstrated the usefulness of the aeromagnetic method for mapping the bedrock structure and the airborne EM method for determining water quality in the overlying sediments in a large area. The airborne EM provided the conductance variations within the Kalahai Beds that were related to fresh and saline water sediments. The regional coverage by airborne EM provided the delineation of the most promising areas for ground geophysics and ensured that no potential area was omitted. The TEM sounding method very precisely demarcated the zones bearing fresh, saline and brackish water in the six exploration areas. It was also better able to decipher the vertical distribution of resistivity than the airborne EM method. Drilling pumping tests and modelling results have proved large reserves of fresh water in these exploration areas. Potential wellfield areas have been delineated for detailed characterization.

In conclusion the Airborne EM survey provided an effective geophysical technique for delineating the lateral extent of shallow aquifers. Using this data in conjunction with ground TEM soundings accurately defined the lateral and vertical extent of fresh groundwater. The results provided the basis for definition of aquifer geometry and water quality for quantification of resources. Aeromagnetic data and deep TEM soundings were used to better understand the bedrock. For additional understanding of the bedrock, exploration methods such as seismic reflection and magnetotellurics are recommended.