Electromagnetic responses from seismically excited targets A: Piezoelectric phenomena at Humboldt, Australia

Abstract

Certain materials respond to a seismic stress by becoming electrically polarized. There are several different mechanisms for creating the polarization including piezoelectricity, the electrokinetic or E effect, and the triboelectric effect. We have carried out a number of laboratory and field experiments to investigate geophysical exploration techniques based on such phenomena. This paper focuses on an experiment at Humboldt, Australia, at which we made our first credible observations of piezoelectricity under field conditions. The experiment used a compressed-air jackhammer as a source and four 50-m ungrounded long-wire antennas for detectors. Despite the fact that the signals are significantly contaminated with noise, principally from a nearby radio transmitter, they demonstrate conclusively the presence of electrical responses. Seismic energy equivalent to a single hammer blow interacted with the quartz body to produce electrical fields with strengths from microvolts to tens of microvolts at the antennas. These results are in accord with earlier, unpublished results of Alan Boyle (CRA Exploration Pty Ltd). Our interpretation is limited to two dimensions, but the results suggest a quartz structure dipping at a shallow angle beneath the line of shot points. Later arrivals show a source close to the centre of the antenna array, for which there may be explanations other than piezoelectricity. Thus, our experiments support the many Soviet claims that such phenomena can form the basis of valid prospecting tools.