Approximate Inversion of AIrborne Em Data From a Multilayered Ground¹

A complex transfer function c (or generalized skin depth) can be derived from data for the secondary magnetic field measured by a dipole system with small coil spacing at height h above the ground. This function has a useful property: For a uniform or layered ground, the real part of c yields the‘ centroid depth’ z * of the in‐phase current system as a function of frequency. This parameter can be combined with the apparent resistivity ρ a derived by conventional methods. The function ρ a ( z *), if known over a broad frequency range, yields a smoothed approximation of the true distribution ρ ( z ) without an initial model. The relations between ρ a ( z *) and ρ ( z ) are studied for a number of multilayer models. An example of the application of the ρ a *) algorithm to data from a groundwater survey is given.