A joint TEM-HLEM geophysical approach to borehole siting in deeply weathered granitic terrains

The accurate location of aquiferous fracture zones in granite beneath a > 5 0 m thick weathered mantle in semi-arid regions is a major hydrogeological problem, It is expected that the zone of intensive fracturing will be more susceptible to weathering and thus be characterized by the thickest develop ment of saprolite, a good electrically conductive target for deep-probing e lectromagnetic systems. The single-loop transient electromagnetic (TEM) tec hnique is well known to have the capability for detecting concealed steep m ineralized targets in mining environments and can be adapted to this hydrog eological problem. We propose that combining the conventional frequency-dom ain horizontal-loop electromagnetic (HLEM) and single-loop TEM is an effect ive practical approach to locating concealed aquiferous fracture zones. In the supporting case studies presented here, we deployed multifrequency HLEM profiling (with 50 m transmitter-receiver separation) and TEM soundings wi th contiguous 10 or 20 m sided loops along the survey lines in a granitic t errain affected by deep (> 50 m) weathering in northeast Brazil. A somewhat layered structure consisting of resistive hardpan/leached zone, conductive saprolite, and resistive basement is identifiable in the typical TEM depth sounding data. We obtained coincident HLEM and TEM anomalies at all the si tes, enabling a relatively straightforward selection of potential drilling positions. Simple resistivity-depth transformation of the TEM data was done for each site, yielding an approximate section from which drilling depths were estimated. All of the boreholes located were successful. Although our results appear to indicate that the single-loop TEM method could be used in dependently for borehole siting in deeply weathered granitic terrains and t hat the weathering profile over granite can be mapped using TEM depth sound ings of appropriate observational bandwidth, we recommend a joint electroma gnetic approach for optimal well siting.