Comparison Of Em61 Anomaly Response Signatures From Different Tx/Rx Coil Configurations

Abstract

The U.S. Army Engineering & Support Center, Huntsville sponsored a project to collect and evaluate EM61 electromagnetic data over a controlled test site using 4 different coil configurations. A standard EM61 MK1 with a 1m x 1m coil was used as the base-line coil configuration and is referred to as Configuration #1 in our evaluations. Response characteristics from three multi-coil configurations were compared to this baseline. Configuration #2 consisted of two 1m x 0.5m coils configured to produce a 1m x 1m footprint. Configuration #3 consisted of two 0.5m x 0.5m coils configured to produce a 1m (across-track) x 0.5m (along-track) footprint. Configuration #4 consisted of four 0.5m x 0.5m coils configured to produce a 1m x 1m footprint. Results from this evaluation demonstrate that consistent increases in the signal to noise ratios (SNR) can be achieved when more than one transmitter coil is used. The data also demonstrate that total transmitter foot-print size, and not individual receiver coil size, affects detection capabilities. No increase in anomaly resolution was observed in datasets collected using smaller receiver coils. Our evaluations show that an average increase of 85% in SNR can be achieved when more than one TX coil is used, and that SNR significantly decreases with smaller overall TX coil sizes over deeper items. Standard, factory-produced EM61 MK1 units were used for all tests. All multi-coil configurations were synchronized to energize the transmit coils in parallel. The same electronics packages and wiring harnesses were used for all tests; only the coils themselves were interchanged for the individual tests. These testing parameters produced datasets with very similar response characteristics and it is our conclusion that the observed differences in anomaly characteristics are due solely to the different coil configurations.