Use of magnetotelluric signals from 50 Hz power lines for resistivity mapping of geothermal fields in New Zealand

Electromagnetic (EM) fields radiated from the transmission lines of the Mew Zealand electricity grid have been digitally recorded at test sites near t he Tokaanu geothermal field. Amplitudes and phases of the 50 Hz signals (an d the odd harmonics up to 450 Hz) were determined using a software implemen tation of a phase-locked filter. These data were then analysed to determine the components of the magnetotelluric impedance tensor and the correspondi ng apparent resistivities and phases. At most sites, there was sufficient v ariation in the elliptical polarization of the EM fields to enable the impe dance tensors to be determined in full. Sites where the EM data had been af fected by near-source effects were identified by having large vertical magn etic field components and by being closer to a power line source than about 3-5 skin depths. With the test measurements, the north-eastern part of the Tokaanu geothermal field was successfully delineated giving low resistivit ies (<5 Omega m) on the inside and higher resistivities on the outside, in agreement with the Schlumberger array DC apparent resistivities. The small size of the 50 Hz magnetotelluric equipment and its portable nature make th is method of resistivity measurement suitable for reconnaissance resistivit y mapping in places with difficult access.