On the viability of year-round magnetotelluric measurements at ELF/VLF and their application to groundwater exploration
Goldak, David K.
Magnetotelluric (MT) measurements in the Extremely-Low-Frequency (ELF) and Very-Low-Frequency (VLF) bandwidth have been largely regarded as impractical during the winter months, even at relatively low latitude. The economic impact of being able to make valid MT measurements at ELF/VLF during the winter months at mid-high latitude could be significant as many northern areas conducive to mineral exploration are accessed easiest, sometimes solely, during the winter. In the specific case of mineral exploration on fresh water lakes, electrical and electromagnetic methods are best implemented during the winter when the lake surface is frozen. MT surveys at ELF/VLF may prove to be very effective in such an environment due to fewer problems, compared to controlled-source surveys, in penetrating through the thick conductive overburden formed by the mud water-bottom of the lake. Furthermore, at lower latitudes, such as the mineral areas of south-western U.S.A. and northern Mexico, geophysical exploration is best accomplished during the winter when cooler temperatures are prevalent. It is shown in the present study that magnetotelluric measurements at ELF/VLF are viable on a year-round basis at mid-high latitude, providing the correct recording method is used. Specifically, a recording method which exploits the time localized, transient nature of the largest signals in the ELF/VLF bands of the geoelectromagnetic field is required in order to conduct MT surveys at ELF/VLF in all seasons. Such signals originate in electromagnetic radiation from individual, particularly strong or equivalently, relatively nearby lightning discharges. The contradicting statements made in many previous studies is mainly a consequence of the use of a recording technique which implicitly assumes that the MT source field at ELF/VLF is of a continuing nature, or equivalently, one which requires a high level of natural-source activity in order to be successful, a condition not generally seen in the winter months, especially at mid-high latitude. Previous groundwater exploration studies employing MT measurements at ELF/VLF have taken place dominantly in volcanic environments where aquifers are conductive targets, as opposed to relatively thin, resistive targets, which is the case in sedimentary environments. It is shown in the present study that MT measurements at ELF/VLF are of use for groundwater exploration in the sedimentary environment of southern Saskatchewan, both for shallow and deep exploration. The benefit of using MT for groundwater exploration in sedimentary environments is its ability to more cost-effectively image both as shallow as, and much deeper than, time-domain electromagnetic (TEM) soundings. This allows the geophysicist to obtain not only electrical information about shallow glacial deposits but also structural information regarding pre-glacial deposits, the youngest of which is termed Quaternary bedrock. The latter is more difficult to obtain with DC resistivity or TEM methods and has traditionally been the task of refraction seismology. Therefore, of all the geophysical methods, MT measurements at ELF/VLF are somewhat uniquely positioned in being able to cost-effectively provide not only the shallow electrical information normally obtained with DC resistivity or TEM surveys but also deeper structural information usually obtained with the more expensive refraction seismology technique.