GEM-2: A New Multifrequency Electromagnetic Sensor

A new broadband electromagnetic sensor, GEM-2, has been tested at several environmental sites for subsurface investigation. A hand-held, digital, multi-frequency sensor based on an earlier, similar helicopter-towed sensor, GEM-2 operates in a frequency range of 90 Hz to 22 kHz, and can transmit an arbitrary waveform containing multiple frequencies. The unit is capable of transmitting and receiving any digitally-synthesized waveform by means of the pulse-width modulation technique. Owing to the arbitrary nature of its broadcast waveform and high-speed digitization, the sensor can operate either in a frequency-domain mode or in a time-domain mode. Depth of exploration for a given earth medium is determined by the operating frequency. Therefore, measuring the earth response at multiple frequencies is equivalent to measuring the earth response from multiple depths. Hence, such data can be used to image a 3-D distribution of subsurface objects. Results from several environmental sites indicate that the multifrequency data from GEM-2 is far superior in characterizing buried, metallic and non-metallic targets to data from conventional single-frequency sensors.