INVESTIGATIONS OF ASYMMETRIC AND INVERTED RESPONSES OF THE EM-31 SYSTEMS

The use of portable two-coil electromagnetic (EM) frequency-domain systems is widespread in the environmental industry as well as in academia. While most were developed as tools to measure the electrical conductivity of the uppermost layers of the earth, much of their use has been in the detection of buried pipes, tanks, and wires. In those applications the fundamental assumption of operation in the low induction number regime is violated, and sometimes unexpected responses can occur. This paper describes work done with the horizontal co-planar (HCP) orientation (also called vertical dipoles) of systems with coil spacing in the range of 1 to 5 meters. One problem examined is the correct location at which to position a reading, relative to the midpoint of the coils, where the instrument time delay (latency) and walking speed are the variables. This has been examined with walking profile surveys as well as compared with stepwise measures made while the instrument is stationary. Another response that has been examined is the development of a double trough signature as the diameter of a linear target increases, or its depth decreases. Further, the first of the two troughs has a greater amplitude than the second when the leading coil is the receiver coil. Finally, we examined the reversal of the sign of the conductivity as instrument height increases. Noteworthy here is that there exists a height (target-to-instrument spacing) at which the reversal takes place, where the conductivity signal is zero. The examples shown will lead to a better understanding and confidence in the use of these instruments and aid in interpretation of data produced in these special situations.