ELECTRICAL-CONDUCTIVITY STRUCTURE OF THE BROADLANDS OHAAKI GEOTHERMAL-FIELD, NEW-ZEALAND

Audiomagnetotelluric and magnetotelluric soundings have been obtained at 11 sites on a south-north line across the Broadlands-Ohaaki geothermal field. The orientation of the line is approximately parallel to the eastern boundary of the Central Volcanic Region of the North Island of New Zealand and makes anything other than one-dimensional quantitative modelling difficult. As the data quality is generally good and the measured apparent resistivities are isotropic at high frequencies it is believed that one-dimensional modelling gives, in this instance, an accurate picture of the structure of the geothermal field. This is supported by the excellent agreement between the structure obtained and the available borehole, geological and geophysical information. The modelling results clearly show a very low resistivity layer which may be identified with the upper aquifer of the geothermal reservoir. Beneath the field the depth to this layer varies from 100 m to approximately 450 m. This low-resistivity region associated with the reservoir appears to extend well to the south of the field at a depth of approximately 1.5 km. In contrast, the northern boundary of the field is relatively abrupt. Beneath the field the very low resistivity zone is underlain by a resistivity of approximately 20-omega-m which extends to several kilometres deep and which in at least the upper 1 km can be identified with the circulation of geothermal fluids. There is also some tentative evidence of a second conducting region, possibly marking the base of the crust, at a depth of approximately 20 km.