3-D HLEM Modeling of Tropical Weathering: A Trial from a Granitic Area of N-W Rajasthan, India

In a sand-covered granite terrain of northwestern Rajasthan, India, a five-frequency HLEM survey along a 10-km traverse line generated several clear and strong anomalies. Subjected to a joint constrained 1-D layered earth inversion, the IP and OP frequency soundings at each point of observation yields either no solution or one which is inconsistent with the adjacent point. On the other hand, a 3-D model fitting of these anomalies with a tabular body in a layered host leads to a more meaningful interpretation, suggesting the presence of (1) pockets of weathered conductive material and (2) resistive intruding dykes, embedded in a semi-weathered layer, as the cause of the observed anomalies. The locations of weathered pockets are probably determined by pre-existing weak structures such as joints, fractures and faults, which facilitated movement of groundwater and hence weathering. Thus covered features deep in the bedrock such as intrusives, joints, fractures and faults, which are not directly detectable by the HLEM method, being under a more conductive overburden, are indirectly detected through their imprints left in the overlying weathering profile.