Three-dimensional modelling of magnetotelluric data from the Rotokawa geothermal field, Taupo Volcanic Zone, New Zealand

The resistivity structure of the Rotokawa geothermal system in New Zealand's Taupo Volcanic Zone has been determined by 3-D modelling of data from a closely spaced (64 measurement sites) magnetotelluric (MT) survey. 3-D conductivity models were constructed using trial and error forward modelling of the phase-tensor data and 3-D inverse modelling of the impedance tensor data. Both the forward and the inverse resistivity models show good consistency. The most interesting feature of these models is a resistive (similar to 100 Omega m) zone within the otherwise conductive material of the geothermal system. This zone coincides with the high temperature (300-335 degrees C) core of the geothermal system in which seismicity induced by fluid injection occurs and may mark the zone of fracture permeability that is feeding high temperature fluid into the geothermal system from deeper levels.