Electrical resistivity and magnetic investigations of the geothermal systems in the Rotorua area, New Zealand

Abstract

Electrical and magnetic data are used in an investigation of a 450 km 2 region in order to delineate the Rotorua City Geothermal system and determine its relationship with other geothermal systems in the region. Electrical resistivity mapping of the region has been carried out using Schlumberger array measurements with nominal current array (half) spacings of 500 m (586 measurements) and 1000 m (410 measurements). In addition continuous profiling using an equilateral dipole array provides close spaced data along 6.5 km of profile in Lake Rotorua. Modern draped aeromagnetic data (200 m line spacing, 60 m ground clearance) has been made over about 100 km 2 covering two areas. Where aeromagnetic data could not be obtained, these data have been augmented with previously measured ground magnetic (vertical force) and waterborne (total field) data. Three distinct regions of low (< 30 Ωm) apparent resistivity are delineated. The southern of these outlines the Rotorua City Geothermal System which has an area of about 18 km 2, with the northern third covered by Lake Rotorua. The boundary of the system is characterised by a rapid lateral change in apparent resistivity which can be modelled as a single, near vertical zone in which the distance between hot and cold water is very narrow. Magnetic properties also change in the vicinity of the discontinuity in some areas, consistent with hydrothermal alteration having destroyed the magnetite in the rocks of the geothermal system. Hot water is believed to be rising, driven by buoyancy forces across the whole of the low resistivity region. There is some indication, particularly in the south, that the boundary between hot and cold fluids dips away from the field. A second low resistivity zone (the East Lake Rotorua anomaly) with an area of about 8 km 2, is believed to outline a second independent geothermal system, with surface manifestations on Mokoia Island, and on the eastern shore of the lake. High heat flow in lake bottom sediments, and a reduction in magnetic signature over this region supports this conclusion. A third resistivity low under the west of Lake Rotorua has no associated thermal features and is believed to be a fossil hydrothermal system. There is no apparent relationship between the location of the geothermal systems and the Rotorua caldera. The aeromagnetic measurements have delineated several highly magnetic bodies which cannot be linked with surface geology. These are believed to be caused by buried rhyolite dome complexes at shallow depth.