Electromagnetic (EM) Induction

Electromagnetic (EM) Induction is the most cost effective approach for identifying conductive utilities within a project area. It works by locating EM fields at the ground surface level that are induced by a transmitter onto an underground utility. It is ineffective for locating non-conductive utilities because a magnetic field cannot be achieved. Also, this method should not be leveraged to obtain accurate depth data, and should be applied to discern horizontal position only. EM data can be combined with GPR data to achieve a complete picture of subsurface utilities within a project area and reveal conflicting/inaccurate information.

Types of signal application techniques include:

• Straight Induction: No connection to utility hardware, the transmitter is placed over the position of the suspected utility and the field is induced onto the target line by “spilling” the electromagnetic field through the earth and onto the utility.
• Direct Connect: A signal lead cable from the transmitter is attached to a piece of utility hardware that is connected to the target line. A ground lead attached to a grounding stake allows the signal to return to the transmitter, thus setting up an electric circuit (this is required since without a complete circuit, no field is generated around the pipe or cable).
• Induction Clamp: The clamp is placed around conduits or exposed cables and is generally used for tracing electrical or telecom cabling. Coils of wire in the clamp are energized by the transmitter and produce a magnetic field that is transferred onto the cable. The cable being traced must be fully grounded at both ends of the line in order for this method to be effective.