Near-surface high resolution imaging of a metallogenic zone in the northern fringe of Dalma volcanics in eastern India using electrical resistivity tomography

Electrical resistivity tomography (ERT) is a useful tool to map near-surface conducting anomalies. The detailed ERT survey was taken over an already defined conducting zone on a regional scale through a magnetotelluric (MT) survey, in order to provide better resolution of the subsurface structure within the study area. The survey lines were carried out crossing the delineated conducting zone through MT giving a dense coverage over the area. The ERT survey were carried out along 15 lines covering an area of similar to 1km(2) with a line spacing of similar to 50m in the northern fringe of the Dalma volcanics (DVs). The study utilised the 61-channel cum 64-electrode resistivity equipment, FlashRES-Universal ERT multi-electrode data acquisition system, developed by ZZ Resistivity Imaging, Australia. Data has been acquired both through conventional arrays i.e. Wenner, Schlumberger and ZZ unconventional arrays. Inversion of the data set have been performed using 2.5D finite element conjugate gradient algorithm after performing the quality check. Resistivity models along all the lines were obtained using Wenner, Schlumberger and combination of Wenner, Schlumberger and ZZ arrays. Resistivity models resolved four major zones: (1) resistivity less than 1 m; (2) resistivity 1-10 m; (3) resistivity 10-100 m; and (4) resistivity more than 100 m . The resistivity results corroborate well with the geological succession from the drilling data. The conducting zones with resistivity values ranging from 1-10 m correlates with the Lower Dalma volcanics while the Upper Dalma volcanics corresponds to the regions with resistivity values of less than 1 m. The Upper Dalma volcanics corresponds to the metallogeny while the depth to the top of the ore body is similar to 25m.