Borehole siting in an African accelerated drought relief project

In response to recent droughts in central Africa, an accelerated well drilling programme was commissioned in Victoria Province, Zimbabwe. Geophysical techniques involved in the siting of 370 boreholes included electromagnetic profiling and resistivity sounding, the latter using the offset sounding system. An analysis of the field data showed that although observed errors were low, a significant proportion of the soundings were affected by strong lateral changes in resistivity, this probably indicating the presence of a very irregular bedrock surface. Nevertheless, mean regolith thickness and lithology were reliably estimated. Geomorphological features which were found to have the greatest potential for water supply boreholes include valleys and zones of weathering around bornhardts. In these situations geophysical surveys proved useful in locating areas of thickest regolith. Borehole yield could then be estimated from the measured formation resistivity although a precise relationship was somewhat masked by the clay mineralogy of the regolith and the type of bedrock which must be taken into account when siting boreholes.