Applications of Controlled Source and Natural Source Audio‐Frequency Magnetotellurics to Groundwater Exploration

Audio-frequency magnetotellurics (AMT), using either a controlled source (CSAMT) or natural source (NSAMT), has become an efficient, cost-effective tool for groundwater exploration. Advancements in field equipment have improved data quality and increased data acquisition speed, and the availability of 2D inversion modeling has significantly improved data interpretation. Since depth of investigation is not related to the receiver electric field dipole size, AMT can be used as either a highresolution tool (using short dipoles) or as a reconnaissance tool (using large dipoles). Several recent field examples are presented, including fractured bedrock targets, in which lateral resolution is important, as well as reconnaissance-style basin mapping, in which speed and economic efficiency is critical. In one project, located in Tule Desert, Nevada, CSAMT was used successfully to map an undeveloped basin, and was instrumental in subsequent court hearings to support water right applications to develop groundwater resources from this basin. Introduction and Background Several geophysical methods can validly be proposed for subsurface mapping in groundwater exploration projects, including seismic, gravity, magnetics, and various electrical/electromagnetic methods. Each method has advantages and disadvantages, of course, based on specific survey needs and site characteristics. One survey technique which we have been using on an increasing basis is audiofrequency magnetotellurics (AMT), in both the controlled source mode (CSAMT) and natural source (NSAMT). CSAMT in particular has seen widespread use in the minerals exploration industry since 1978 when the first commercial system became available, but until recent years, has only occasionally been applied to groundwater exploration. Advances in field equipment have reduced survey costs, and improved interpretational methods have increased the confidence of the method results. In our experience, AMT often provides more cost-effective, higher lateral and vertical resolution resistivity data at greater depths than other electrical and electromagnetic techniques, particularly in areas of difficult terrain, restricted access, or environmentally sensitive areas. Briefly, AMT is a surface-based electromagnetic sounding technique that uses a fixed grounded dipole as a signal source (CSAMT), or alternatively, the naturally-occurring fields of the earth/atmosphere system (NSAMT). AMT can be considered a subset of the magnetotellurics (MT) method, first described in detail by Cagniard (1953); MT using a controlled signal source was developed later by Goldstein and Strangway (1975). In the case of CSAMT, the transmitter signal source usually consists of a grounded electric dipole, typically 1,200 to 1,800 meters (approximately 4,000 to 6,000 feet) in length, located 3,500 to 9,000 meters (11,500 to 29,500 feet) from the area where the measurements are recorded. In the case of NSAMT, naturally-occurring electromagnetic fields are used as the signal source. At the receiver site, grounded dipoles detect the electric field (parallel to the transmitter if one is being used) and magnetic coil antennas sense the perpendicular magnetic field. The ratio of orthogonal, horizontal electric and magnetic field magnitudes (e.g. Ex and Hy) yields the apparent resistivity (in ohm-meters):