A study of equivalence of influences of varying thickness and conductivity in 3D-processing of airborne electromagnetic data in complex media

Abstract

The paper is devoted to the problem of processing the airborne electromagnetic (AEM) data with a view to developing recommendations on parameterization of a geological medium for carrying out the geometric 3D-inversion of airborne data. The study is performed with the use of finite element 3D-modeling of transient EM fields induced by the helicopter AEM system in complex media followed by the 3D-inversion of airborne synthesized data. The geoelectrical models chosen for investigation are taken as typical of this class of airborne EM surveys: the rugged topography and the conductive inhomogeneous top layer, which overlaps the low-resistive layer containing target conductive bodies. It is shown that, if the variations of the top layer thickness do not exceed 30...35 m in performing 3D-inversion, as a minimum, at its first stage, the top layer can be recovered as equivalent with constant (but recovered) thickness, i.e. when the top layer is parameterized in the inverse problem, in the vector of the unknowns we can include only lateral borders of 3D-inhomogeneities, their conductivity, and layer thickness described by one parameter. It is also shown that, if, in the medium under study, the variations of the top layer thickness are more significant, its bottom border should be recovered in more detail, because otherwise the target objects located in the second layer with the responses of about 15...20 % of the measured signals can be missed, while, in the case of recovering the top layer parameters more accurately, these target objects can be discovered quite confidently.