The following methods are used to detect USTs and/or associated piping:
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Time Domain Electromagnetics (TDEM) – High Sensitivity Metal Detection
The equipment used during a UST search depends on several variables including site conditions and UST construction materials. When the surface cover of the site is primarily soil or asphalt covered and steel USTs are suspected, time domain electromagnetic (TDEM) methods will be used at the majority of cases. A TDEM survey is conducted using the Geonics EM-61 high sensitivity metal detector.
On smaller sites a grid is established by the geophysical survey crew while GPS can be collected simultaneously with EM-61 data on larger projects. The EM-61 data are processed and a contour map is generated and reviewed for anomalies typical of those associated with USTs.
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Ground Penetrating Radar
Ground penetrating radar is utilized in areas covered with reinforced concrete or where data collection using high sensitivity metal detection (TDEM method), will encounter too much interference from above ground metallic features. GPR will be used in an attempt to delineate the surface trace of detectable fiberglass or steel underground storage tanks (USTs) or evidence of a former tank cavity. Spectrum employs the Sensors and Software Noggin Smart Cart® ground penetrating radar system (GPR). This data can then be post processed utilizing a variety of programs to provide a sequence of map views of the data. The data are displayed as “layers” or “slices” of varying depths below the ground surface. These maps are reviewed for UST-like signatures as well as linear features that may indicate the presence of conduits.
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Electromagnetic (EM) Utility Location
Passive and active EM utility-locating methods are used in an effort to identify possible sources of EM-61 and GPR anomalies and to delineate the surface trace of detectable underground utilities and abandoned piping that may be associated with USTs.
Passive locating is possible when electrically conductive conduits are energized by ambient radio frequencies (RF) that are often produced by 50/60 cycle electrical, radio, audio, television, communication, and other sources of RF transmissions. A receiver tuned to these frequencies can be used to locate the re-radiated signal emitted by the conductor (e.g., conduit). Several types of utilities may act as conductors of RF transmissions including metallic pipes, direct burial wire, and non-metallic piping that contain a metallic conductor such as wire.
Active locating is initiated by applying an EM signal at a known frequency (between 512Hz and 200 kHz) on a conduit exposed at the surface. A receiver, tuned to these frequencies, is then used to locate the signal maxima of the applied signal. These maxima are marked on the ground with spray paint or lumber crayon and the trend of the utility can then be observed on the ground surface.
