FEM (Frequency-Domain EM)
Definition
A class of instruments that measure ground conductivity from above-surface, without electrode contact, by inducing eddy currents with an oscillating EM field. Fast survey coverage but limited depth resolution compared to contact resistivity.
- Full name
- Frequency-domain electromagnetic method
- Measures
- Ground conductivity
- Contact
- None, above-surface
- Mechanism
- Induced eddy currents from oscillating EM field
Common questions
Why would a forensic search team choose FEM over resistivity survey when FEM has limited depth resolution?+
FEM instruments can be walked or towed across ground quickly without planting electrodes, making them well suited to rapidly screening large search areas for anomalies before committing time to slower, more depth-resolving resistivity grids over the narrowed area of interest.
What kind of buried-evidence anomaly does FEM actually detect?+
It responds to contrasts in ground conductivity, which can arise from disturbed soil moisture retention, buried metallic objects, or decomposition-altered soil chemistry, showing up as a localized conductivity anomaly rather than producing a direct image of what is buried.
Related terms
- Apparent Resistivity
- The resistivity value computed from a measurement assuming the ground is homogeneous. In practice the ground is layered and variable, so apparent...
- Fluxgate Gradiometer
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- Magnetic Susceptibility
- A measure of how strongly a material is magnetised by an external field. Topsoil heated by fire or biological activity has elevated...
- Twin-Electrode Resistivity
- A resistivity configuration using one mobile pair of contact electrodes and one remote pair. Current is injected and the voltage drop across...
- Wenner Array
- A four-electrode resistivity configuration where all four electrodes are equally spaced. Depth of investigation is approximately equal to the electrode spacing. Provides...