Non-Seismic Geophysical Methods Glossary

electromagnetic method (EM method): A method in which the magnetic and/or electric fields associated with artificially generated subsurface currents are measured. In general, electromagnetic methods are considered to be those in which the electric and magnetic fields in the earth satisfy the diffusion equation (which ignores displacement currents) but not Laplace’s equation (which ignores induction effects) nor the wave equation (which includes displacement currents). One normally excludes methods such as ground-penetrating radar that use microwave or higher frequencies (and which consequently have little effective penetration) and methods that use dc or very low frequencies where induction effects are not important (resistivity and IP methods). Natural field methods (such as Afmag and magnetotellurics) employ natural energy as the source; controlled source electromagnetis methods (CSEM) (such as loop-loop techniques) require a man-made source.

skin depth ($\delta$): The effective depth of penetration of electromagnetic energy in a conducting medium when displacement currents can be neglected. The depth at which the amplitude of a plane wave has been attenuated to 1/e ( or 37 percent ):

skin depth = $\delta$ = $\sqrt{\frac{2}{\sigma \mu \omega }} =503\sqrt{\sigma f}$

where $\sigma$ = conductivity in mhos/meter, $\mu$ = permeability in henries/meter, $\omega$ = angular frequency in radians/ second, and $f$ = frequency. Also called effective depth.