D.C. resistivity surveys of the Broadlands geothermal region, New Zealand

Abstract

The size and shape of the hot water reservoir of the Broadlands geothermal field has been investigated to depths of about 3 km using D.C. resistivity methods. Since the resistivity of water saturated rocks decreases with increasing temperature, porosity, salinity and also with the presence of conductive clay minerals, ground resistivity is a good target for investigating geothermal areas. Two surveys made using the fixed spacing Wenner method with electrode spacings of 550 m and 180 m gave resistivity information for shallow depths. Information from greater depths was obtained from several dipole surveys in which electric field strengths were measured at a number of points about fixed current dipole sources. The true resistivities of structures in the ground have been deduced from the apparent resistivities by allowing for the disturbing effects of resistivity boundaries. Theoretical models of a hemispherical sink and a vertical boundary were used to assess these boundary effects. Maps are presented showing resistivity sections at various depths below the surface. The volume of rocks with low resistivity defining the hot water reservoir is approximately circular in section and has a nearly vertical boundary over most of its perimeter. Resistivities within the field gradually increase with depth.