TENSOR CSAMT STUDY OF THE FAULT ZONE BETWEEN WAIKITE AND TE-KOPIA GEOTHERMAL FIELDS

Closely spaced tensor CSAMT soundings have been conducted along three traverses perpendicular to the Paeroa Fault, which links the Waikite a nd 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 i nto a triangular set of three dipoles, located at least 6 km from the sounding sites. DC resistivity measurements (using tensor bipole-quadr ipole 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 a nd the use of three current directions for least-squares tensor analys is were also demonstrated. Disadvantages included sensitivity to elect ric field distortions (static effects) and cultural noise. A large loc alised galvanic distortion, caused by an earthed water pipe, was obser ved in both the CSAMT and bipole-quadripole (DC) data sets. Interpreta tion 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 foun d in a nearby deep borehole. Similar structures have been found to ext end well beyond the recognised shallow boundaries of several other TVZ geothermal fields, and may indicate widespread lateral movement of hy drothermally altering fluids at these deeper levels.