Geophysics: Electromagnetic Shallow Mapping

Electromagnetic (EM) methods include commonly employed geophysical techniques used for environmental and geotechnical studies. EM methods fall into two categories: frequency domain and time domain. Frequency domain measures the amplitude and phase of an induced electromagnetic field. Time domain measures the decay time of an electromagnetic pulse induced by a transmitter. EM surveys measure variability in subsurface conductivity, which can be naturally occurring (differing lithologic materials), or man-made (soil/groundwater contaminants or buried metal, such as buried metallic debris or wastes, drums, or underground storage tanks).

EM surveys use an EM-61 time domain metal detector, which consists of a horizontal loop transmitter and receiver with a secondary receiver mounted above it to allow depth estimates to targets and to reject surface responses. The EM-61 is capable of detecting relatively deeply buried metallic objects of significant size and is relatively insensitive to cultural interference such as metal fences and overhead power lines.

Survey lines are spaced appropriately considering the objective of the survey. EM data readings are collected over the survey area, and digitally recorded using a handheld or palmtop computer. Data can be downloaded in the field and results mapped for planning excavations, or the results can be used to conduct further geophysical evaluations using additional complementary methods, such as ground penetrating radar, to check the EM results.

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