Electromagnetics (FDEM):

Frequency Domain ElectroMagnetic induction (FDEM) involves generating an electromagnetic field which induces current in the earth which in turn causes the subsurface to create a magnetic field. By measuring this magnetic field, subsurface properties and features can be deduced.

Common applications of FDEM include the following. Identifying aquifers and aquatard and detecting the presence of water or salt water intrusion in groundwater investigations. Mapping aggregate deposits for quarry operators. Detecting metal objects and mapping leachate in environmental investigations. Mapping permafrost and other geologic features in geotechnical engineering.

The basic principle of operation of the FDEM method is illustrated in the above figure. A transmitter coil radiates a frequency fixed electromagnetic field which induces electrical currents (termed eddy currents, Je) in the earth below the coil. These eddy currents in turn generate a secondary magnetic field (Bs). The receiver coil detects and measures this secondary field. The instrument output, calibrated to read in units of terrain conductivity (apparent conductivity), is obtained by comparing the strength of the quadrature phase component of the secondary field to the strength of the primary field. The apparent conductivity measurement represents a weighted average of subsurface conductivity from the ground surface to the effective depth of exploration of the instrument. Since electrical conductivity of soil correlates strongly with soil properties, FDEM is a powerful tool for mapping soils and changes in soil types.

FDEM has distinct advantages over many other techniques. Because no contact with the ground is required, FDEM can cover a large area quickly and therefore economically. In addition, the lack of ground contact minimizes exposure of workers to potential contaminates in environmental investigations. Depths of 50 meters can be mapped, however, greater depths involve greater expense and some resolution may be sacrificed. The depth of exploration depends on the separation between the transmitter coil and the receiver coil, as well as on the coil orientation (coil axis/dipole horizontal or vertical).

The measured magnetic fields induced in the earth can tell us a great deal about subsurface conditions. After the magnetic field data is recorded and processed, a geophysicist can interpret the data deducing likely subsurface geologic features. Based on this understanding a model of the subsurface is then created for the customer.