A Novel EM Gradiometric Surveying System for Geophysical Reconnaissance


Alexey V. Veryaskin1,2, Francis A. Torres2, Timo P. Vaalsta1, Ju Li2, David G. Blair2

Trinity Research Labs1
School of Physics2, University of Western Australia

35 Stirling Highway, Nedlands
Perth WA6009, Australia



SUMMARY

Interferometric principles are widely used in precision physics experiments and/or in advanced laboratory-based phase measurement systems. Phase resolution of such systems is a few orders of magnitude higher compared to that of standard mixer-based quadrature demodulators or lock-in technique. The first attempt of applying interferometric signal processing to transmitter-target-receiver based electromagnetic (EM) surveying in geophysical prospecting is described. It is shown that it is possible to build an EM single carrier surveying system that is, firstly, immune to amplitude variations of both the primary and the secondary EM fields, and, secondly, can directly measure phase variations between the primary and secondary EM fields. Its inherent phase noise floor, if limited by the interferometer itself, can be as low as tens of nanoradians/√Hz or below -140 dBc/√Hz level. A practical example of an EM gradiometric surveying system based on an interferometric principle and operating in the Extremely Low Frequency (ELF) range, is presented. The system has been tested in regional outback Australia, in the presence of a highly conducting overburden, in the search of a nickel sulphide deposit.

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