A global measure for Depth of Investigation in EM and DC Modeling

Abstract

All geologists or geophysicists want to know the depth of investigation (DOI) for their final survey models. For diffusive methods, such as groundbased or airborne EM, there is no specific depth below which there is no information on the resistivity structure, but the question is to what depth the model is most reliable. We present a new robust concept for the calculation of DOI that is valid for any 1D EM geophysical model. The method is based on the actual model output from the inversion and includes the full system response, contrary to assuming e.g. planar waves over a homogeneous halfspace. Equally important, the data noise and the number of data points is integrated in our calculation. Our methodology is based on a recalculated sensitivity (Jacobian) matrix of the final model and it can thus be used on any model type for which a sensitivity matrix can be calculated. Contrary to other sensitivity matrix methods we define a global and absolute threshold value contrary to defining a relative, say 5%, sensitivity limit. The threshold limit will apply to all 1D inverted data and will thus produce comparable numbers of DOI.