Electromagnetics (EM) is a non-invasive geophysical technique that uses electromagnetic induction.  A transmitter coil induces a magnetic field that creates a current in the ground. The current produces a secondary magnetic field that is sensed by a receiver coil.  Data is collected along transects, or optionally integrated with GPS position data, to create anomaly location maps. Typically, signals are received from depths of 15 feet or less using a Geonics EM-31 or EM-61 or from depths of about 70 feet or less using a Geonics GEM 2, multi-frequency EM sensor.

The EM-61 can be hand pulled (above) for small sites or pulled by an ATV (below) for large sites.

The in-phase component is typically used as an indicator of buried metallic objects.  The quadrature-phase component is typically used as a measure of general ground conductivity.  Readings are compiled to make contour maps using computer mapping programs.

 

In both examples, areas of possible metal are indicated in red.

Applications

Use to identify areas of higher conductivity such as landfill cells.  Use in-phase component to identify areas of possible metallic debris.  Can be used to map electrically conductive contaminant plumes and saltwater intrusion.

Qualifications

SEI has been using the EM-31 and EM-61 systems since 1987 to delineate landfill sites and locate buried metallic objects.