Electrical Resistivity Surveys of the Rotokawa Geothermal Field, New Zealand

Abstract

Electrical resistivity surveys of the Rotokawa geothermal field, using the Schlumberger traversing, Schlumberger soundings and multiple-source bipole-dipole measuring methods show a low resistivity anomaly associated with the field. The resistivity structure within the field was inferred from the soundings and by applying 3-D computer modelling to the bipole-dipole data. The lowest resistivities (ca. 2 Szm) occur at shallow depths (down to 250-400 m depth) representing the hydrothermal reservoir comprising high temperature thermal fluids in altered silicic pyroclastic rocks. Below this conductive layer, resistivity increases (to c. 20 am) reflecting the denser rhyolites, ignimbrites and andesites encountered by drilling and the presence of resistive clay species. At the edge of the field the observed lateral increase of resistivity, to about 700 m depth, defines a resistivity boundary zone which encloses a conductive region of area about 17-28 km2.