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.
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
- An instrument that measures the vertical gradient of the Earth's magnetic field using two fluxgate sensors mounted on a staff at fixed...
- 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...
Explained in
- Magnetometry and Earth ResistanceA class of instruments that measure ground conductivity from above-surface, without electrode contact, by inducing eddy currents with an oscillating EM field....