At early time, the time derivative of the response of isolated conductive bodies to step function excitation decays as t-1/2 (under a quasi-static approximation). One simple parametric form for the response with correct early time behaviour is k'(1 + t1/2/a1/2)-beta e-t/gamma. For a conducting magnetic sphere, parameter values are determined from the high and low frequency limit responses, together with two time scales taken from the form of the analytic solution. Parameters alpha and gamma correspond to transition times, for transition from an early time t-1/2 derivative response, and to late time exponential decay. For conducting spheres with high permeability, increasing the permeability moves the transition from early time behaviour earlier in inverse proportion to the relative permeability mu r, and increases the time constant of the late time decay in proportion to mu r. Magnitude parameter k' corresponds to the difference between high frequency and low frequency limit responses.

A non-invasive, wide-band electromagnetic (EM) impedance difference system for shallow subsurface electrical structure characterization in environmental and engineering problems has been developed at the Lawrence Berkeley National Laboratory (LBNL). Electrical parameters of interest are electrical conductivity and dielectric permittivity that are deduced from the impedance difference data. The prototype system includes a magnetic loop transmitter, which operates between 0.1 MHz and 100 MHz, an electrical dipole antenna for observing the electric field, and a loop antenna for measuring the magnetic field. All antennas are mounted on a cart made of non-metallic material for easy movement of the whole array for profiling. Surface EM impedance difference is obtained by taking the difference of the ratios of the electric fields to the magnetic fields at selected frequencies at two different levels. Numerical simulations will be presented to verify this new approach. A set of the impedance difference data acquired at the University of California s Richmond Field Station compares reasonably well with simulation results based on a model obtained with the resistivity method and in situ TDR (time domain reflectometry) measurements.

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.