Why are depths shown on maps labeled “approximate depth”?
A. For each frequency, a skin depth, or depth of penetration has been estimated from a nomogram, courtesy of Geophex, relating frequency, soil conductivity and skin depth. These estimates are only approximations and are dependent on the soil conductivity, texture, moisture and temperature. Manufacturers of EM equipment assume a homogeneous half space for estimation of depths. The assumption of a homogeneous half space is the same as assuming uniform soil conditions (conductivity, moisture, temperature, texture and mineralogy) from the surface to the depth of penetration. However, soil is generally heterogeneous and typically can consist of multiple layers, each of which can have varying electromagnetic properties.
