Very Low Frequency EM Survey
The VLF method uses very low frequency EM signals propagating from external sources, such as from military radio stations located throughout the United States and internationally, to locate water bearing fractures and/or otherwise conductive mineralization or contaminant zones in bedrock. The government-sponsored stations propagate signal, up to 1000 Watts at frequencies varying between 15 to 30 KHz. These signals propagate well along water-filled or mineralized fractures that are oriented in strike with these transmitting base stations. For instance, VLF signal generated at the base station in Cutler, Maine (NAA) would propagate well in those northeast-southwest oriented fractures for surveys area near eastern, Massachusetts, Rhode Island, and Connecticut, making them detectable using a VLF receiver, such as the WADI , manufactured by ABEM. To measure the maximum EM response from a fracture, however, VLF survey lines would have to be oriented perpendicular to the conducting fracture. Hence, a significant disadvantage to VLF surveying with just a passive receiver is its reliance on base stations that routinely go down for maintenance or may be closed due to budget constraints, and its reliance upon these sources that they will be in the strike of the water bearing fractures that need to be detected.
To circumvent this problem, ABEM has created the portable VLF transmitter TX27, especially designed to give the geophysicist the ability to choose the location and orientation of the VLF source. The TX27 transmitter consists of a VLF generator feeding into a long grounded wire to produce an appropriate primary magnetic field. A portable power source of 50 to 400 Hz, 115V, 300 Watts is needed to supply power to the generator. Other manufacturers, such Geonics with its EM-16, have developed VLF receivers with their own source. The source also feeds a large grounded wire loop that is laid out over an area encompassing several hundreds of meters and up to a kilometer. Often, in urban areas, placement of the source is infeasible due to interference from numerous cultural sources, including pipelines, overhead electrical, chain link fences, and to property boundary constraints. For this reason, VLF is generally not feasible in urban environments, and borehole methods become more reliable and cost-effective for water bearing fracture detection.