Magnetotelluric results from the Ohaaki Geothermal System, Taupo Volcanic Zone, New Zealand

The Ohaaki geothermal system in the Taupo Volcanic Zone currently supports a 100 MW power station and has long history of geophysical investigation using electrical methods since its first delineation by DC resistivity mapping. Although the shallow (<1 km) resistivity structure is well known from detailed DC resistivity mapping using long-wire Schlumbeger resistivity surveys and from long-offset tensor bipole-dipole surveys little is know about the deep structure of geothermal system below the deepest wells, ~ 3km. At these depths the geothermal reservoir is contained in the meta-sedimentary basement rocks (greywackes) which have very poor permeability. Magnetotelluric (MT) studies at the Rotokawa geothermal field about 15 km to the southwest, suggest that the deep high temperature part of the geothermal system (also hosted in greywacke) is anomalously resistive. Here we report the findings from a 20 km long profile of 28 broad-band MT measurement stations through the centre of the Ohaaki geothermal system. Although the near-surface low-resistivity anomaly marking the Ohaaki system is a 3-D feature, phase tensor analysis of the MT data show that at longer periods the MT response is quasi 2-D. 2-D inverse modelling of the MT data identify a narrow (~600 m) near vertical (dyke-like) zone of high conductivity on the south-eastern side of the geothermal field. The geochemistry of the gas from samples taken from this side of the geothermal field have a distinct volcanic signature suggesting that we may be imaging the source of these volcanic gases.