Electromagnetic methods in applied geophysics

Abstract

Applied electromagnetic research in recent years has been influenced by the growing importance of geothermal energy, coal, and permafrost, in addition to the traditional area of minerals. The interest in near-insulators such as coal and ice encouraged development of radars and other VHF-UHF techniques. Interpreting such measurements required reliable physical properties data for those materials over a frequency range of 6–10 decades. The utility of the high frequency field data has been improved through the use of transient techniques for data acquisition, and data processing schemes similar to those used in reflection seismology. The major developments in the more usual frequency range of applied geophysics (30 Hz—3 kHz) have also dealt with transients. In certain circumstances they appear to have a fundamental sensitivity not readily obtained by discrete frequency methods. Computer modelling of 3-D problems is progressing slowly. Improved 2-D inversion programs are in use, but their capabilities are very limited. Superconductivity plays a role in several new instrument developments. SQUIDs, and SQUID gradiometers have improved considerably since the last Workshop. Robust SQUID magnetometers having noise levels of $$10^{ - 5} - 10^{ - 6} nT/\surd Hz$$ can now be obtained commercially. Gradiometer sensitivities have improved accordingly. A superconducting loop 3 metres in diameter, to be test flown early in 1979, is the prototype of a new low frequency system to map conductivity from a helicopter. It is expected to have greater depth penetration in conductive terranes than the best existing systems, because of the low frequency and anticipated low system noise. A new magnetotelluric procedure, using a remote field reference, reduced the scatter in apparent resistivities and other response functions to a few percent. Further improvements must now be made in modelling and interpreting MT results if we are to benefit from this development.