Electromagnetic Signal Attenuation (Skin Depth)

In electromagnetic (EM) methods, the electrical conductivity of the earth plays a pivotal role in the EM signal penetration and depth resolution that can be obtained.  High electrical conductivity removes, or attenuates, energy from the EM wave through the work done by moving electrical charge.  High conductivity materials effectively absorb the EM energy and limits or attenuates its flow, or penetration, deeper into the earth.  This attenuation of the EM signal is exponential with depth and is described by the skin depth relationship.

Skin depth is the depth at which the EM signal attenuates to 1/e or approximately a third.  Skin depth is typically measured in meters and is related to the frequency (f) of the EM wave and the electrical conductivity (σ) of the media (i.e., geologic materials) the EM wave is propagating through.

skin depth = 500 √ [1 / (σ f)]

Skin depth is inversely proportional to conductivity (σ) and frequency (f).  Higher electrically conductive earth materials and higher frequency EM wave result in shallow EM wave penetration.  The frequency of the wave presents a trade-off for resolution and depth.  The higher the frequency, the higher the resolution and lower penetration (smaller skin depth), while the lower the frequency the deeper the signal penetration (larger the skin depth) and lower resolution.