Coaxial Electromagnetic Sensor for UXO Detection

A new broadband electromagnetic (EM) sensor, called GEM-5, for shallow earth investigation including detection and discrimination of UXO has been developed. The sensor consists of a central transmitter coil and two coaxial receiver coils located at the same vertical distance above and below the transmitter coil. The difference in output between the receiver coils would be zero in free space. This particular configuration provides a high degree of primary field cancellation, reduction of power-line or other environmental noise, and immunity from noise induced by sensor motion in the earth’s magnetic field. We have first studied the detection capability of the sensor. The EM responses are numerically simulated using a sphere model for UXO-like objects, and a half-space for geology. Algorithms to invert the EM data into apparent susceptibility and conductivity show that the sensor can detect small metal targets. Transforming the EM data into the apparent conductivity can enhance the anomalies from metal targets and reduce geological noise. Our field test indicates that the gradiometer measurements yield a lower static and motion noise over the critical low frequencies than that of existing sensors, and detect all targets at a seeded UXO test site.