Advanced inversion methods for airborne electromagnetic exploration

Abstract

Airborne electromagnetic (AEM) surveys can contribute substantially to geologic mapping and target identification if good‐quality multifrequency data are produced, properly evaluated, and displayed. A set of multifrequency EM data is transformed into a set of apparent resistivity ([Formula: see text]) and centroid depth ([Formula: see text]) values, which then are plotted as a sounding curve. These [Formula: see text] curves commonly provide a smoothed picture of the vertical resistivity distribution at the sounding site. We have developed and checked methods to enhance the sensitivity of sounding curves to vertical resistivity changes by using new definitions for apparent resistivity and centroid depth. One of these new sounding curves with enhanced sensitivity to vertical resistivity contrasts is plotted from [Formula: see text] [Formula: see text] values derived from differentiation of the [Formula: see text] curve with respect to the frequency f. This approach is similar to the Niblett‐Bostick transform used in magnetotellurics. It not only enhances vertical changes in resistivity but also increases the depth of investigation. Sounding curves can be calculated directly from EM survey data and can be used to generate a resistivity‐depth parasection. Based on such a section, it can be decided whether a Marquardt‐type inversion of the AEM data into a 1-D layered half‐space model is adequate. Each sounding curve can be transformed into an initial step model of resistivity as required for the Marquardt inversion. We have inverted data from sedimentary sequences with good results. For data from a dipping conducting layer and a dipping plate, we have found that the results depend on the right choice of the starting model, in which the number of layers should be large rather than too small. Complex resistivity structures, however, often are represented better by using the sounding‐curve results than with the parameters of a layered half‐space.