Long-Offset Transient Electromagnetic Measurements at the Wairakei Geothermal Field

Abstract

Transient electric field measurements were made in a line crossing the northwestern boundary of the Wairakei geothermalfield. Three grounded bipoles were used as current sources enabling the apparent resistivity tensors to be determined at each measurement site. Source-receiver separations ranged between 7 and 10 km. Despite high levels of electrical noise instantaneous apparent resistivity tensors could be determined reliably between 0.1 and 4 s. The apparent resistivity image derived from these data shows that, beneath the southern part of the geothermal field, higher resistivities underlie a layer of much lower resistivity about 1 thick.