The Colorado School of Mines time-domain electromagnetic (EM) sounding system makes use of a grounded length of cable powered with high-amplitude current square waves to generate an EM field for probing the earth. The vertical component of magnetic induction is detected at a sounding site located at a relatively large distance compared to the desired depth of investigation. With a source moment of a million ampere meters or greater, offset distances of several tens of kilometers can be achieved easily, providing depths of investigation of up to 10 km. The recorded induction field versus time curves are routinely interpreted by comparison with computer-generated theoretical curves for a layered earth. Megasource EM surveys have been carried out at The Geysers in northern California and near Yakima in central Washington, providing apparently meaningful information on the electrical structure in these areas at depths as great as 10 km.FIG. 1. Principal components of the Colorado School of Mines time-domain EM sounding system. FIG. 2. Typical TDEM signals converted from digital form. The signal at the top is a strong signal with little noise apparent, while the lower signal is heavily contaminated with noise.