Approximate interpretation of Airborne Electromagnetic data using a half-space model



When an alternating electromagnetic field propagates through the ground, it induces electric currents in any conductor in its path. The strength of the induced currents mainly depends on the electrical resistivity of the conductor concerned and the frequency with which the primary field is alternating. Generally speaking, the currents are stronger for smaller resistivity and higher frequency.
Airborne Electromagnetic (AEM) surveys can be used extensively for mapping and interpretation of geological targets. If the data were available for many frequencies, the resolution of such maps will be better.
With modern AEM equipment, the secondary magnetic field of the current induced in the ground is measured at a rate of ten readings per second and for several frequencies simultaneously. The large amount of data collected during a survey must be converted into a set of apparent resistivity and centroid depth values which will provide at least a rough picture of the vertical resistivity distribution at each sampling point. The centroid depth for each frequency is an estimate of the depth in the half-space at which there is maximum induced current flow. This is the depth for which there will be maximum sensitivity to resistivity variations.
Here we have used a complex transfer function c (or generalized skin depth) derived from data for the secondary magnetic field measured by a dipole system with a small coil spacing "r" at a height "h" above the ground. It is known that the data measured above layered ground with AEM systems which satisfy h?3.3 r, can generally be interpreted in terms of an equivalent half-space.
The complex transfer function ‘c’ has useful properties: for a uniform or layered ground, the real part of c yields the ‘centroid depth’ (z*) and the imaginary part of c yields the apparent resistivity (?a) as a function of frequency. So, if the function ?a(z*) is known over a broad frequency range, it yields a smoothed approximation of the true resistivity distribution ?(z) without an initial model. The relations between ?a(z*) and ?(z) are studied for a number of multilayer models. We also examine the method on real data which are collected over Kalat-e-Reshm area in southeast of Semnan province. Results show that the method can determine the existence of conductive and resistive structures but it cannot deliver detailed information about subsurface structure. Thus, it is recommended to use this method as a reconnaissance method not an exact detailed method.