Identification and estimation of the subsurface anisotropy from the 2D electrical resistivity tomography surveys

Abstract

This research was dedicated to examining regions rich in schist rock near the Singhbhum shear zone in Ghatshila, Jharkhand. The aim was to detect schist rocks that were sheared, fractured, and highly foliated in both shallow and deeper layers. Electrical resistivity tomography (ERT) measurements were conducted using a 2 × 21 electrode configuration, with nine profiles covering inter-electrode spacings ranging from 3 m to 10 m. A recently developed software called Anisotropic DC resistivity Forward and Inverse (ADCFI) was employed to conduct 2D isotropic and anisotropic inversion of the collected data. The 2D interpreted sections along the profiles indicated non-continuous resistivity values at their intersections. Furthermore, areas demonstrating irregular resistivity values showcased anisotropy coefficients exceeding unity, indicating significant anisotropy in these particular zones. The irregular resistivity patterns additionally provided further evidence for the existence of substantial anisotropic behavior within the region.

The outcomes of the 2D anisotropic inversion conducted in Ghatshila unveiled significant anisotropy coefficients beyond a depth of 20 m. This depth correlated with the presence of layers containing chalcopyrite, suggesting stratified deposition originating from a volcanogenic setting. Furthermore, the existence of schist rocks in shallow borehole depths contributed to the observed anisotropic tendencies. Notably, regions with heightened anisotropy demonstrated thicker layers in the isotropic section compared to the anisotropic section across all profiles. Anisotropy coefficient values derived from areas abundant in schist rock in Ghatshila were approximately 2.00. This substantial anisotropy was attributed to the inherent foliation and schistosity of the dominant rock type, namely schist.