The time domain method is a method that uses the inductive properties of a transient primary electromagnetic field to measure the ground response or resistivity of the material through which the field passes after the primary field is turned off. This method has its history in mineral exploration; however, it is now used for many environmental and engineering types of applications. For shallow applications Spectrum uses the Geonics EM-61 MK-2 high sensitivity metal detector (EM-61). This instrument is highly effective for the lateral delineation of metallic objects ranging in size from 3-inch military ordnance to 300-foot-long utilities, and can detect a buried 55-gallon drum under up to 9 feet of cover under favorable conditions. The EM-61 is particularly useful where there is a large area to investigate, as an extremely large number of measurements can be made quickly and cost effectively. The EM-61 is very effective in open areas of industrial sites such as chemical plants, refineries and tank farms or at demolished facility locations where no surface expression of the former buildings or utilities exist and utility location using standard methods is very limited.
For deeper applications Spectrum uses the Geonics EM-47 or EM-57 TEM system. These instruments measure the vertical resistivity variation of subsurface materials, and are capable of imaging to significant depths. TEM methods are useful for the vertical delineation of geologic or hydrogeologic contacts in cases where the target is conductive and there is a strong contrast in electrical resistivity (or conductivity – its reciprocal) between one unit and the next. As such, TEM methods are effective for the delineation of sand/shale or sand/clay interfaces or the interface between freshwater and saltwater or freshwater and high-TDS-water aquifers. TEM methods using the EM-47/57 system are especially useful for sites where the space available is small (precludes the use of electrical resistivity), the target is deep (200 feet to 500 feet or more), or for sites where there is a large area to investigate and a limited budget, as many 1D measurements can be made quickly and relatively cost-effectively. In addition, lateral delineation of geologic or hydrogeologic features can be accomplished by stringing together several 1D TEM measurements in a linear fashion to create electrical cross sections.
This method can be used for location of metallic utilities, location of metallic USTs and buried drums, delineation of high-TDS or saline groundwater and placement of water wells.