Audio-Frequency Magnetotelluric Measurements in the Broadlands Geothermal Field Using 50Hz Mains Power as the Signal Source

Abstract

Audio-frequency magnetotelluric measurements using mains power as the signal source were made at sites along three traverses crossing the resistivity boundary of the Geothermal field. The traverses were in the same positions as those used previously for resistivity measurements which were made to locate the resistivity boundary of the geothermal field, and the results from the AMT measurements have been compared with the results from the ivity from low values to higher values (of up to orders of magnitude) occurred at nearly the same locations as obtained from the measurements. The actual values of apparent resistivities were similar to those obtained from measurements, usually differing by a factor of less than 2, but at 4 of the 19 sites the apparent resistivities differed by factors of 3, 3, 4, and 10. Reproducibility was evaluated by making measurements at different times and with different array orientations at two sites. Good reproducibility (the apparent resistivities varied by up to of the mean) was obtained except when the measuring array was oriented close to a null in the electromagnetic field.