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The 2D DEPTH SCAN, like the EOL, is a single frequency method that evolved from the EMP (EM Pulse) time-domain method developed at Newmont Mining's Research Lab. All methods are surface-source to borehole-receiver (STBH) methods.
For environmental studies, 2D DEPTH SCANs are used to outline subsurface zones that have large contrasts in electrical resistivity. In the vicinity of USTs (Underground Storage Tanks) and other sources of hydrocarbon leaks, very-high gradient resistivity values usually indicate free-phase hydrocarbon plumes. Free-phase hydrocarbons are several orders of magnitude more resistive than the groundwater and other fluid
resistivities.
The 2D DEPTH SCAN uses a fixed low frequency EM signal generator to induce a long wavelength magnetic field below a large, flat-lying, loop. The loop has a moment equaling 1200
ampere x meter x meter. The source's magnetic flux induces image sources ("eddy currents") at the boundaries where resistivity change occurs. A 2D DEPTH SCAN is used to measure the resistivity gradient across such features as the
Water Table.
Each in-hole sensor pair is fixed to straddle a specific depth in a borehole, e.g., straddling the
Water Table. The voltage gradient measured by each sensor pair provides a voltage that reflects the resistivity.
The voltage measured for
each transmitter location is passed through inverted-notch filters, then integrated and digitally sampled, then stored on disc. Then the large loop source is moved to a new grid location and the process is repeated. Measurements are made for closely spaced source grid locations. The grid is rectangular. The grid spacing is nominally five (5) feet. The data are processed into resistivity anomaly profiles and maps.
The 2D DEPTH SCAN, a Pritchard technology, was developed and "Beta Tested" in California in 1988.
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