Near-surface resistivity surveys of geothermal areas using the electromagnetic method

Abstract

Electromagnetic surveys using the horizontal coplanar loop technique in the Broadlands field, New Zealand, and the El Tatio field, Chile, have provided information on resistivities to a depth of about 30 metres. The survey at Broadlands was largely experimental, while the survey at El Tatio was explorational using the techniques and data reduction methods which had proved satisfactory in New Zealand. The principal advantages of the electromagnetic method lie in its high speed and low cost, which make it an attractive alternative to direct temperature surveys. «In-phaseå and «quadratureå components at each of two frequencies were observed at field stations, giving four estimates of resistivity. These have been combined in the construction of the resistivity maps. In reducing the data it has been assumed that the ground resistivity is uniform down to the depth of maximum penetration. Within the limits set by volume considerations there is good correlation between resistivity down to 30 metres and temperatures measured at half this depth. The shape and areal extent of the low resistivity zone defined by the electromagnetic method at El Tatio are almost indistinguishable from those of the high temperature zone defined by direct temperature measurements.