Magnetotelluric imaging of the Ohaaki geothermal system, New Zealand: Implications for locating basement permeability

Closely-spaced broadband MT (magnetotelluric) data measured on a profile through the Ohaaki geothermal system show that the basement greywacke directly beneath the geothermal field is resistive. There is no evidence in these MT data for the existence of an intrusive heat source directly beneath Ohaaki. However, these data confirm that a dipping conductive zone links the near surface low resistivity anomaly associated with the geothermal field to a deep region of low resistivity that is offset to the northwest. The resistivity of the dipping conductor is consistent with a zone of fracturing within the basement meta-sediments, and is interpreted to show the location of upwelling high-temperature fluids that supply the Ohaaki geothermal field from a deep magmatic source.