IP tomography test survey in the Barrier Main Lode, Broken Hill, Australia

Abstract

In both the domestic and overseas mineral exploration projects of the Metal Mining Agency of Japan (MMAJ), many holes have been drilled in order to core for geological logging and laboratory analysis. However drill holes themselves are seldom used to obtain additional information except for occasional geophysical logging such as resistivity, self-potential and induced polarisation (IP). Therefore the MMAJ has been developing an IP tomography data acquisition system since 1992 for the purpose of effectively utilising drill holes, reducing exploration cost and improving accuracy of geophysical exploration. This system is based on time-domain measurement and adopts a pole-dipole electrode configuration to avoid coupling effects compared to a pole-pole array and to deploy the electrodes more easily than a dipole-dipole array. It consists of a transmitter, receiver, personal computer, controller, non-polarisable electrodes, ground cables and borehole cables, and has the following features: (1) It acquires resistivity and chargeability data at 10 sites simultaneously; (2) It acquires very accurate data because of an operational amplifier at each electrode in the borehole cable and the relay box; (3) It acquires the data automatically through the controller, computer and relay boxes. In 1997, the MMAJ conducted an IP tomography test survey in order to examine the applicability of the system for mineral exploration in the Barrier Main Lode, located northeast of Broken Hill, NSW, Australia. The quality of the acquired data was excellent because of high sensitivity of the system and absence of culture noise such as power lines. The data was analysed with the inversion techniaue based on the alpha centers method and the non-linear least square method. In the reconstructed resistivity and chargeability images, the ore body appears as a low resistivity zone and as a high chargeability zone. The reconstructed images are consistent with the result of IP logging although the reconstructed resistivity is lower than that obtained by IP logging. It is confirmed that the reconstructed chargeability image can delineate a massive sulphide ore deposit such as the Barrier Main Lode more clearly than the reconstructed resistivity image.