Cornerstone Provides Subsurface Utility Locating Serives with several different techniques for locating those difficult to find utilities or when asbuilts are not available. we use the following methods to locate utilities and underground storage tanks UST.
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.
GPR methods in areas where non-metallic/non-electrically conductive utilities are expected (e.g. transite, PVC, and/or vitrified clay pipe) and over accessible areas where proposed excavations and borings are proposed. Spectrum Geophysics uses the Sensors and Software Noggin Smart Cart or a MALA ground penetrating radar unit in an effort to identify possible buried utilities. A high frequency radio signal is transmitted into the ground via the antenna. As radio waves propagate into the ground, these signals are reflected off structures with differing electrical properties. These reflected signals are then captured by the receiver and are presented as vertical profiles on the GPR unit. These data are reviewed to identify possible subsurface interferences.
