C. Bromley, TENSOR CSAMT STUDY OF THE FAULT ZONE BETWEEN WAIKITE AND TE-KOPIA GEOTHERMAL FIELDS, Journal of Geomagnetism and Geoelectricity, 45(9), 1993, pp. 887-896
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.