Tensor CSAMT Study of the Fault Zone between Waikite and Te Kopia Geothermal Fields.

Abstract

Closely spaced tensor CSAMT soundings have been conducted along three traverses perpendicular to the Paeroa Fault, which links the Waikite and Te, Kopia Geothermal Fields in the Taupo Volcanic Zone (TVZ) of New Zealand. Source current at frequencies of 4 Hz to 4 kHz was injected into a triangular set of three dipoles, located at least 6 km from the sounding sites. DC resistivity measurements (using tensor bipole-quadripole and Schlumberger arrays) were obtained at the same locations for comparison with the CSAMT data. As a technique appraisal, the survey demonstrated some of the advantages of CSAMT soundings, such as: good penetration through resistive overburden, a reliable signal and a high data acquisition rate. The benefits of harmonic analysis processing and the use of three current directions for least-squares tensor analysis were also demonstrated. Disadvantages included sensitivity to electric field distortions (static effects) and cultural noise. A large localised galvanic distortion, caused by an earthed water pipe, was observed in both the CSAMT and bipole-quadripole (DC) data sets. Interpretation of the CSAMT soundings suggests that low resistivity occurs below about 600 m depth between the two fields, correlating with a layer of intense hydrothermal alteration of fractured rhyolites and tuffs found in a nearby deep borehole. Similar structures have been found to extend well beyond the recognised shallow boundaries of several other TVZ geothermal fields, and may indicate widespread lateral movement of hydrothermally altering fluids at these deeper levels.