Modeling Aquifer Structures With Airborne Em In The Boteti Area, Botswana

As part of a project by the Department of Water Affairs, Botswana to evaluate groundwater<br>resources in the Boteti River area in Botswana airborne EM data were acquired with the TEMPEST<br>system with the objective to map prospective lithologies and structures containing freshwater aquifers.<br>The project area is characterized by a palaeolake system bordered to the south by elevated terrain. Most<br>of the lower-lying area is covered by a thick layer of Kalahari Beds with saline groundwater generally<br>situated less than 20 m below surface. Elevated terrain to the south have Karoo sediments outcropping<br>in places.<br>In the north of the project area AEM data were acquired across palaeolake terrain to locate fresh<br>water zones associated with recharge pans above the saline water table. The derived conductivity<br>structure outlines several shallow, resistive zones that appear to correspond to dry sand; fresh water<br>pockets above the saline water were not located by the boreholes drilled. Further, the AEM results<br>indicate the outline of a brackish to saline aquifer at a depth of 30 m beneath a clay layer. Preliminary<br>borehole data from this aquifer indicate high yields (20-64 m3/h) with TDS values in the range 1550-<br>2700 mg/l. A comparison of borehole information with the modeled conductivity structure indicates the<br>conductivities to be dominated by clay content, water saturation and salinity.<br>In the southeast of the project area AEM data were acquired across elevated terrain to delineate<br>fresh sandstones sandwiched in-between dry alluvium or basalt and mudstone. The comparison of<br>airborne EM and magnetic data with drillhole information indicates that the basalts are characterized by<br>very low conductivities and a strong magnetic response. The underlying sand- and mudstones have<br>intermediate and high conductivities, respectively, and are characterized by horst-graben structures,<br>reflected in the magnetic data and conductivity-depth sections. Resistivity isosurfaces generated from<br>the 3D conductivity data facilitate the spatial appreciation of the geometry of the interpreted structures.<br>Favorable locations for freshwater exploitation interpreted from the TEMPEST data include a shear zone<br>and sandstone units fully preserved beneath basalt caps. Preliminary borehole data indicate yields of 15-<br>54 m3/h of brackish to saline water (TDS 1000-3000 mg/l) in the depth range 56-180 m with the highest<br>yield obtained from fractured sandstone at 180 m depth. The results indicate the conductivities of Karoo<br>lithologies to be dominated by clay content, water salinity and, locally, by the presence of quartz.