Geophysical Environmental Surveys

2D RECON SURVEYS

2D RECON SURVEYS PROVIDE A THUMB-PRINT OF ALL
FREE-PHASE HYDROCARBON PLUMES IN AN AREA

In the vicinity of underground storage tanks (USTs) and other hydrocarbon leak sources, anomalous high-resistivity readings usually indicate free-phase petroleum hydrocarbons have leaked into the subsurface. For such cases, free-phase total petroleum hydrocarbons (TPH) is several orders of magnitude more resistive than groundwater and other fluids found in the subsurface. Free-phase TPH is defined as concentrations of TPH > 100 ppm in situ.

Most leaked TPH starts in the Vadose zone and migrates down to and sometimes below the Water Table. For LNAPL plumes, the high resistivity anomaly is near vertical in the Vadose zone and then spreads laterally at the Water Table. For DNAPL plumes, especially the chlorinated hydrocarbon plumes, the high resistivity anomaly is near vertical in the Vadose zone, at the Water Table, and below the Water Table.

The 2D RECON Survey (a surface EM gradiometer technique) evolved from geophysical techniques developed for petroleum and mining exploration. Such techniques are used to detect subsurface zones with unusually high contrasts in electrical resistivity.

2D RECON measurements are made by moving a surface EM gradiometer along closely spaced transects around and over suspected TPH leak areas. The data acquired are processed using proprietary algorithms to filter noise and outline TPH plumes in the Vadose zone, on the Water Table, and in bedrock fractures.

The surface EM gradiometer data acquisition system uses pairs of EM sensor coils normally aligned vertically; one sensor near the ground surface and the other sensor five feet above it. Each pair of coils measures the difference in signal between the coils, i.e., the “signal gradient.”. Resistivity anomalies are indicated by large changes in the voltage gradient. The higher the concentration and/or the thicker the plume, the greater the change in the gradient.

The 180 Hz harmonic of the EM fields generated by industrial and heavy-use powerlines is usually the 2D RECON source. Powerlines approximate infinite line sources. Since the powerlines provide the signal measured, their EM fields are not noise. Industrial powerline noise on or near a survey area prevents the use of the standard EM techniques, such as, the small loop-source to small loop-receiver EM techniques.

A large EM moment (1200 amp-meter2) flat-lying loop source provides power for 2D RECON surveys at sites located more than a half mile from suitable powerlines. The oscillating loop source (a dipole source) is offset a few hundred feet from the area of gradiometer transects.

The 2D RECON Survey has been 90 percent successful in outlining free-phase hydrocarbon plumes. This dual-coil receiver technique, a Pritchard technology, was developed and “Beta Tested” in California in 1988.

2D RECON SURVEY SYSTEM/CONCEPT

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