The structure of the Mokai geothermal field based on geophysical observations

Abstract

The Mokai geothermal area, about 25 km northwest of Taupo, has only minor thermal manifestations with a natural heat output of about 6 MW. In late 1976 a roving dipole resistivity survey was made in the area surrounding Mokai Springs. Since then an extensive Schlumberger resistivity survey has been made using two spacings (nominally 600 m and 1200 m) covering an area of about 4 B.V.n2 around the Mokai area. Interpretation of all the resistivity data suggests the area of Mokai geothermal field below which high temperatures may be expected at depth, is about 12–16 km2. Low resistivities from soundings within this area indicate highly conductive (saline) geothermal fluids at a depth of about 60 m, although below this the resistivity increases, suggesting a lowering of porosity. The interpretation of resistivity measurements suggests a considerable subterranean flow of geothermal fluids northward from the Mokai geothermal field, and a further 76 MW of heat is estimated to be carried by springs and seeps into the deeply incised Waipapa Stream. A second low-resistivity region occurs at the Waikato River, about 12 km north of the geothermal field, with an area of 3–4 km2. There are indications that this anomaly is caused by hot geothermal fluids and it is suggested here that these geothermal fluids have their origin at the Mokai geothermal field. On this basis an estimate of the total heat output of the Mokai geothermal area is 400 ± 160 MW.