Rapid groundwater assessments in gneissic settings using transient electromagnetics - Examples from Togo

Towed transient electromagnetics (tTEM) is applied in the Plateau Region of Togo for rapid and high-resolution characterization of groundwater systems surrounding six villages. Past drilling efforts in the region often resulted in unsuccessful or low-yielding boreholes, due in large part to sparse hydrogeologic data at the scale relevant for borehole siting surrounding each village. Towed transient electromagnetics systems are shown to be well suited to addressing this data sparsity issue, enabling the mapping of large regions surrounding each village in relatively short field times (two days per village in this work). Comparison with borehole lithology logs reveals that a towed electromagnetic system can accurately characterize the thickness of the weathered zone in gneissic settings, as well as delineate elevated saturation fractured zones within the gneissic bedrock in several cases. Boreholes drilled based on tTEM surveys were able to intercept fresh groundwater resources in each of the investigated villages.