Two-dimensional inversion of CSAMT data and three-dimensional geological mapping for groundwater exploration in Tongkeng Area, Hunan Province, China

Controlled source audio-magnetotelluric (CSAMT) is an indirect and effective geophysical prospecting method in subsurface prospecting. Although CSAMT investigation tools are equipped with new techniques, it must be noted that the models obtained still do not automatically decipher the reality of groundwater in bedrock with very high resistivity contrasts. Our study proposed a new CSAMT data processing scheme, which combined the 2D modeling and 3D visualization for groundwater exploration in Tongkeng area, Hunan Province, China. To directly highlight exploitable groundwater, we used the open-source and the OCCAM2D inversion software, coupled with the geological information of the area to define the 3D imaging with emphasizing on the network of producing fractures. We deployed three CSAMT profiles with transverse magnetic (TM) mode recorded on 21 frequencies (8192 Hz to 8 Hz). The data were filtered, pre-processed and carefully analyzed from station to station. Analysis of the apparent resistivity and phase maps show the conductive zones (<150 Ωm) and resistive zones (> 500 Ωm) separated by transition zones (150 to 500 Ωm). The 2D models and stacked sections show discontinuities of fractures marked by two major faults and a main fracture in the zone interconnected by minor fractures. A 3D geological map was obtained from the result of the best models of the different lines. The ITER2DAT function was used to export the data from OCCAM2D inversion and then display it to array and depth channels data for each station. Geological and borehole ZK01 data were used to build pseudo stratigraphic drillholes based on the range of very low (<10 Ωm) to high values resistivity structures (>1500 Ωm). The 3D view shows stratigraphic discontinuity that is between carbonaceous formation and the metamorphic granites. The intruded Devonian limestone layer, which is the potential water tank, and the carbonate vein layer are constantly heated by the geothermal gradient of the area.