Experiences with a local loop VLF transmitter for geological studies in the canadian nuclear fuel waste management program

Abstract

Very low frequency electromagnetic (VLF-EM) methods have been useful for detecting weak conductors such as overburden infilling resistive bedrock valleys or water-filled fractures and shear zones in the highly resistive Precambrian granitic terrain of Canada. Since VLF-EM methods detect conductors only when their strikes are roughly perpendicular to the direction of the primary magnetic field; at least two VLF sources with orthogonal magnetic field directions are required for a total conductor mapping of an area. At several areas in western Ontario and eastern Manitoba such orthogonal sources are not available; the primary magnetic field directions in that region generated by available U.S. Navy VLF stations are generally only north-south. In order to overcome this problem, a portable local loop VLF transmitter was designed and built at the Geological Survey of Canada. Surveys with the portable local loop transmitter proved that this source was a reliable substitute for Navy VLF stations when the survey area was 3–10 km from the loop source. Identical conductor positions were mapped when the local loop was placed in such a way that its primary magnetic field was aligned in the same direction as that from a U.S. Navy VLF station. In areas where orthogonal VLF magnetic fields were not available, the use of the local loop source resulted in the identification of additional conductors, the locations of which agreed well with results from surface geological mapping.