Time-domain electromagnetic (TEM) soundings were made using a Geonics PROTEM system. A square transmitter loop measuring approximately 40 meters on a side was used. The loop was connected to a Geonics EM-47 transmitter which produced a 50-percent duty-cycle square wave. The current in the transmitter loop creates a primary magnetic field. When the transmitter current is interrupted, the decaying primary magnetic field induces a circulating current flow in the ground below the transmitter loop. The decay of this induced current system is controlled by the electrical resistivity of the ground below the transmitter loop.

The decaying current system produces a secondary magnetic field, which is sensed by a receiver coil located at the center of the transmitter loop and recorded by the receiver. The recorded signal is called a transient. Many transients are recorded and averaged to reduce noise in the data and to estimate measurement error. The averaged data are converted to apparent resistivity to allow comparison of one sounding to another.

The data are modeled as one-dimensional, layered-earth models. The electrical resistivity and thickness of model layers is determined by least-squares parameter estimation.