Investigations of deep resistivity structures at the Wairakei geothermal field

The Wairakei geothermal field was the proving ground for the use of electrical resistivity methods for geothermal exploration. At this site it was first demonstrated that a large contrast in resistivity existed between geothermal ground and the cold surroundings. Within the top 500 m of the geothermal field, low-resistivity (5–10 Ωm) reflects the effects of both the hot saline water in the pore spaces and the conductive rock-matrix. The first surveys at Wairakei used a Wenner array (a ∼550 m) to measure the resistivity values along tracks throughout the field; contour maps of the resistivities were used to estimate the lateral extent of the geothermal waters at a few hundred metres depth. In the late 1960s the Wenner array was superseded by the Schlumberger array (AB/2 = 500 m and 1000 m), which enabled deeper penetration and better definition of the extent of the geothermal waters. These early surveys showed that the bounds of the geothermal waters were often sharp, leading to the concept that a ‘resistivity boundary’ could be defined for New Zealand's liquid-dominated geothermal fields. As new methods of measuring electrical structure with greater precision became available, Wairakei was often chosen as the testing ground.In 1982, a deep penetrating tensor bipole–dipole survey was made over the northern part of the field to provide close-spaced data across the boundary, leading to a better definition of the boundary and its change with depth. A similar survey over the eastern edge of the field revealed a very sharp boundary with an apparent correlation between its location and a deeper caldera structure. The first application of tensor time-domain resistivity (tensor LOTEM) was made across the north-western edge of Wairakei in 1997 giving both a detailed image of the boundary and showing that it was possible to trace geological structures within the geothermal field. The most commonly used method for resistivity surveying in the 21st century is magnetotellurics (MT), which determines the resistivity of the ground from the currents induced in the earth by natural variations of the earth's magnetic field. While MT has not yet been applied within the Wairakei field, the method has been used to investigate the setting of the geothermal systems of the Taupo Volcanic Zone. These data reveal key elements of the deep source of heat, including the presence of a small fraction of connected molten rock at a depth of about 10 km and the possible pooling of magma at about 15 km.