GEM-3: A Monostatic Broadband Electromagnetic Induction Sensor

We have designed, fabricated, and field-tested a new, unique, monostatic, broadband, electromagnetic sensor for subsurface geophysical investigation. The sensing unit consists of a pair of concentric, circular coils that transmit a continuous, broadband, digitally-controlled, electromagnetic waveform. The two transmitter coils, with precisely computed dimensions and placement, create a zone of magnetic cavity (viz., an area with a vanishing primary magnetic flux) at the center of the two coils. A third receiving coil is placed within this magnetic cavity so that it senses only the weak, secondary field returned from the earth and buried targets. This monostatic configuration has many advantages including (1) compact sensor head, (2) a large transmitter moment, (3) high spatial resolution, (4) no spatial distortion of an anomaly common to bistatic sensors, (5) circular symmetry that greatly simplifies mathematical description, and, therefore, (6) simplified forward and inverse modeling processes. Three prototype GEM-3 units have been built and tested at various environmental sites, including those containing unexploded ordnance and land mines.