Magnetotelluric Soundings and Structure of the Tokaanu Geothermal Field, New Zealand.

Abstract

Magnetotelluric soundings have been made at 15 locations in and around the Tokaanu geothermal field in the North Island of New Zealand. Modelling and inversion of the data along two transects clearly identify the locations of the eastern and southeastern boundaries of the field. Within the field a conductive layer at variable depth is underlain by a more resistive layer. This is in turn underlain by low resistivity which persists to considerable depth. Across the field boundary a somewhat less conductive layer exists at around 0.5-1.0 km depth. This is similar to the conductivity structure associated with the Broadlands-Ohaaki geothermal field and reported by INGHAM (1991a). Additional geophysical information suggests that the total area covered by the Tokaanu field is of the order of 30 square km. The existence of low resistivity to depth is interpreted to correspond to the upwelling of geothermal fluid and associated hydrothermal alteration. No indication is obtained of any geothermal link between the Tokaanu field and the Motuoapa Springs to the northeast.