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.