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DC Resistivity Mapping with the Multiple-Source Bipole-Dipole Array in the Central Volcanic Region, New Zealand.

1993; Society of Geomagnetism and Earth, Planetary and Space Sciences; Volume: 45; Issue: 9 Linguagem: Inglês

10.5636/jgg.45.897

2185-5765

G. F. Risk, H. M. Bibby, T. G. Caldwell,

Seismic Waves and Analysis

A DC resistivity survey of the eastern margin of New Zealand's Central Volcanic Region (CVR) was made using the multiple-source bipole-dipole technique. The survey employed three distinct current bipoles each of length about 5 km placed in high resistivity ground east of the volcanic region. Electric field strength vectors were measured at 220 receiver stations, with source-receiver offsets of up to 35 km. For each field station, the data for the three sources were analysed together to form an apparent resistivity tensor. A constraint to the interpretation of these data was provided by extensive coverage of shallow resistivity mapping data made with Schlumberger arrays of current electrode half-spacing (AB/2) of 500 m and 1000 m. Two provinces with distinct electrical properties can be distinguished. On the Kaingaroa Plateau to the east, high resistivity ignimbrites (500-1000 Ωm) about 200 m thick overlie greywacke basement rocks whose resistivity is about 300 Ωm to about 3 km depth and increases with depth to greater than 1000 Ωm. To the west, in the Taupo-Reporoa Basin, resistivities axe generally much lower. Away from the geothermal fields, resistivities of 50-100 Ωm are found for the volcanic rocks within about 500 m of the surface; at deeper levels, resistivity decreases to about 20-30 Ωm. The lowest resistivities (2-10 Ωm) are observed in the upper parts of the geothermal fields within the Taupo-Reporoa Basin. A sharp lateral change in electrical resistivity, trending NNE, occurs at the eastern margin of the CVR between the Taupo-Reporoa Basin and the Kaingaroa Plateau. Along the southern parts of the margin, the resistivity discontinuity coincides with the mapped position of the Kaingaroa Fault Zone, but further north it lies beneath what is normally considered part of the Plateau, several kilometres east of both the topographic scarp and the mapped position of the Fault. Two-dimensional numerical modelling of the resistivity data across the margin of the CVR suggests that the Kaingaroa Fault Zone is a 5-10 km wide zone of normal faulting with the greywacke basement down-thrown to the west in a series of steps. The modelling also suggests that resistivity beneath the eastern part of the CVR is about 20-30 Ωm to depths of at least 6 km. Below this level resistivities are less than 300 Ωm, much smaller than the resistivities at comparable depths beneath the Kaingaroa Plateau.