Lawrence Berkeley National Laboratory

A prototype active electromagnetic system has been developed for detecting and characterizing UXO. The system employs two orthogonal vertical loop transmitters and a pair of horizontal loop transmitters spaced apart vertically by 0.7 m. Eight vertical field detectors are deployed in the plane of each of the horizontal loops and are arranged to measure offset vertical gradients of the fields. The location and orientation of the three principal polarizabilities of a target can be recovered from a single position of the transmitter-receiver system. Further characterization of the target is obtained from the broadband response. The system employs a bipolar half sine pulse train current waveform and the detectors are dB/dt induction coils designed to minimize the transient response of the primary field pulse. The target transient is recovered in a 40 mu sec to 1.0 msec window. The ground response imposes an early time limit on the time window and system/ambient noise limits the late time response. Nevertheless for practical transmitter moments and optimum receivers the size and the ratio of conductivity to permeability can be accurately recovered. The prototype system has successfully recovered the depths and polarizabilities of ellipsoidal test targets.