In electromagnetic (EM) geophysical methods, a wave's ability to penetrate the subsurface depends on the medium's electrical conductivity and the wave's frequency. Attenuation (energy loss) of an EM wave in a conductive medium increases with frequency, while the propagation speed in typical near-surface materials is comparatively less sensitive to frequency over the ranges used in EM surveys.
Low frequency EM waves are attenuated much less than high frequency EM waves as they travel through a (mildly) conductive medium, because the skin depth...
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