Geophysical characterization for dolerite dykes (dimension stone) reserves exploration using electrical resistivity tomography (ERT) in Shergarh Oghi, Mansehra, Pakistan

Abstract

Due to rapid increase in demand of building stones the geophysical prospecting of potential dimension stone (Dolerite Dykes, commonly known as “Black Granite”) deposits is of great significant. Dimension stone is widely distributed in the Hazara Valley of KPK, Pakistan. The primary aim is to characterize subsurface structural features and resistivity values crucial for dimension stone deposits, guiding the selection of viable mining zones. Employing electrical resistivity tomography (ERT), an electric current was introduced through electrodes embedded in the ground, demonstrating efficiency, cost-effectiveness, and data coverage. Conducted with the SuperSting R8/IP/SP instrument from AGI USA, the survey utilized a dipole-dipole array with 56 electrodes spaced at 20-meter intervals along eleven profiles. The 2D-ERT technique, or resistivity imaging, employed EarthImager software from AGI USA for data inversion, revealing distinctive low and high resistivity signatures along all profiles. Shallow high resistivity values indicated boulder material, while shallow low resistivity values pointed to subsurface dolerite dyke material. Profiles exhibited deep high resistivity values, suggesting subsurface granitic soft bedrock. EarthImager™ 3D software enabled inferred resource estimations, identifying a volume of 4,349,744 cubic meters and a total tonnage of 13.04 million tons for dolerite (dimension stone) material. The 3D-ERT inversion results further enhanced the understanding of the spatial distribution of the dolerite deposits, providing high-resolution images of subsurface resistivity, which facilitated the identification of economically significant zones. Based on geophysical study it is recommended to drill six confirmatory boreholes of a minimum 150 m depth to assess and confirm dolerite dyke zones/anomalies for future mine planning in the study area.