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Environmental Applications of High-Resolution TEM Methods

Publisher —
Zonge, 1998. Presented at The 4th Meeting on Environmental and Engineering Geophysics, European Section. Barcelona, Spain. September 14-17, 1998.

Authors —
Cris Mauldin-Mayerle, Norman R. Carlson, Kenneth L. Zonge, Zonge Engineering & Research Organization, Inc.

Paper —  [pdf] ENV_TEM_HighResEuropeanEEGS-1998

Though commonly used in minerals exploration, transient electromagnetic (TEM) methods are less common in environmental and engineering applications. Several aspects of the technique make it a very useful tool, including the flexibility of loop sizes and geometries, and the recent improvement in electronics that allow faster transmitter turn-off times, and therefore shallower soundings. Faster electronics also allow the acquisition of all three magnetic field components simultaneously, increasing the amount of information available for modeling and interpretation.
Our discussion includes environmental applications of TEM data in standard geometries (such as in-loop, fixed-loop, etc.) but with higher resolution than usually obtained. In this case, “high resolution” refers to higher resolution spatially (much higher data density than is normally used in minerals exploration TEM) and temporally (much faster turn-off times and faster sample rates). Also of interest is the added benefit of utilizing the late-time TEM data, after the background earth response has decayed, as a deep-sounding metal detection tool.
TEM methods can be extremely useful in high resolution environmental surveys, particularly when the equipment system allows rapid and easy changes in loop geometries to take advantage of the differing sensitivities of loop arrays. Small, discrete targets can be located with in-loop surveys, while boundaries of large targets are often better delineated with fixed-loop methods.
Recording a large number of windows along the decay curve allows the system to be used as both a resistivity sounding tool as well as a deep metal detector, and the decay time constants may eventually prove useful in target characterization. Most significantly, by recording all three magnetic field components, substantial additional information is available to help optimize field surveys and improve interpretation of the data.