Line Tracing

Passive mode detects 60 Hz AC current as well as very low frequency (VLF) radio waves transmitted by the US Navy to communicate with nuclear submarines. Passive mode is excellent for indicating that a buried utility is present but can’t distinguish what kind of utility.

Active mode uses a transmitter set to a user-specified frequency that induces an EM signal into the earth that will be picked up by linear conductive features such as wires or pipes. Active mode can identify sections of utilities that are too short to be detected in Passive mode.

Direct-Connect mode is used when a pipe or wire can be clipped on to so that the transmitter is directly sending a user-specified frequency signal on a utility. This allows the geophysicist to identify where specific utilities are and differentiate between multiple utilities, if present. Direct-connect mode also allows the geophysicist to determine the depth of the utility.

Ground Penetrating Radar

Ground-penetrating radar (GPR) is used after all of the modes of the Line Tracer have been used. GPR can image non-conductive utilities such as concrete, terra cotta (clay), or PVC sewer lines that the line tracer can’t detect. In some circumstances GPR can show a trench–the utility may be too deep (sewer) or too small (residential gas line) to show on radar but the change from native soil to trench backfill is different enough to reveal the presence of the buried utility. GPR data are difficult to interpret, hence the need for a trained and experienced geophysicist!