Radial resistivity/IP surveys using a downhole current electrode

Abstract

The sensitivity and resolution of surface resistivity and Induced Polarization (IP) surveys can be severely restricted by the conductivity and thickness of overburden material. Electrode arrays using a current electrode buried below the overburden can increase the sensitivity and resolution of surveys conducted in such environments. A current electrode is located down a drillhole, and resistivity and chargeability measurements are made on the surface along survey lines radial to the drillhole. Reconnaissance resistivity/IP surveys using the downhole radial electrode array were conducted in the Cambro-Ordovician Mt Windsor Volcanics near Charters Towers in Queensland, an area covered by electrically conductive unconsolidated sediments of the Late Tertiary Campaspe Beds. The surveys demonstrated the effectiveness of the array in detecting anomalies from buried base metal targets. Data were acquired from a number of surveys conducted around a series of adjacent drillholes to provide continuous coverage across the prospective area. The data from individual radial traverses show the sounding response of the ground as well as the ground response along the profile. A method was devised for removing the sounding response, which is local to each drillhole, from each radial traverse so that the data from each traverse could be merged to form a continuous data set. A number of issues have been identified for future research which include the optimum depth of the buried current electrode, removal of the sounding response, and interpretation of the survey data.