Influence of Large - Scale Resistivity Setting on Interpretation of Resistivity Within Geothermal Areas

Abstract

Resistivity surveys using the Schlumberger traversing and multiple-source bipole-dipole methods show a sharpNNE trending discontinuity in electrical resistivity marking the margin between the eastern side of the CentralVolcanic Region and the Kaingaroa Plateau which represents a 5 - 10 wide band of normal faulting. South of Reporoa, the resistivity discontinuity coincides with the mapped position of the Kaingaroa Fault Zone, but further north it cuts beneath the comer of the Kaingaroa Plateau several kilometres east of the mapped positions of the Kaingaroa Fault and Scarp. Three distinct resistivities zones exist in the region: 350 - 1500 for the greywacke rocks beneath the Plateau, about 30 for the volcanic fill of the Taupo-Reporoa and 2 - 5 for the geothermal fields. Results from three multiple-source bipole-dipole surveys at using sourceswithin the field,just outside the field, and 15 away on the Kaingaroa Plateau show that the tensor apparent resistivity invariants measured in the Same resistivity zone as the current source are close in value to actual ground resistivities. But 'static shifts' of apparent resistivity occur when the receiver is in a differentzone the source. The very large 'static shifts' (an order of magnitude) observed in and near the Ohaaki Field from the source on the Kaingaroa Plateau are due to the influence of the large-scale lateral resistivity discontinuityat the edge of the high resistivity region beneath the Kaingaroa Plateau.