COMBINING ADVANCES IN EM INDUCTION INSTRUMENTATION AND INVERSION SCHEMES FOR UXO CHARACTERIZATION

Abstract

Several experimental time-domain EM induction instru- ments have recently been developed for unexploded ordnance (UXO) detection and characterization that use multiple transmitting and re- ceiving coil combinations. One such system, the US Geological Sur- vey's ALLTEM system, is unique in that it measures both the electro- dynamic response (i.e., induced eddy currents) and the magneto-static response (i.e., induced magnetization). This allows target characteri- zation based on the dyadic polarizability of both responses. This pa- per examines the numerical response of the ALLTEM instrument due to spheroidal, conductive, and permeable UXO targets; and to con- ductive and optionally viscous magnetic earth. An inversion scheme is presented for spheroidal targets that incorporates fully polarimet- ric measurements for both magneto-static and electro-dynamic excita- tions. The performance of the inversion algorithm is evaluated using both simulated and surveyed data. The results are examined as a function of the number of coil combinations, number of instrument lo- cations, and uncertainty in sensor location and orientation. Results from the speciflc cases tested (prolate spheroids lying horizontally) show that 1) that collecting data from more than 12 sensor locations or from more than four coil combinations reduced the chances that inversion solutions would be from a local minimum, and 2) that un- certainties in position greater than 3cm or in orientation greater than 10 degrees cause errors in the estimated spheroid principal lengths of greater than 100%. Soil conductivities less than 1S/m contribute neg- ligible interference to the target response, but viscous magnetic soils with permeabilities greater than 10 i6 MKS units do cause detrimental interference.