Pulsed Sounding Data Transformation into Apparent Electrical Resistivity for the Cryolithozone Monitoring Problem

Abstract

In light of the ongoing global climate changes, the timely study of cryolithozone objects is crucial to prevent potential natural and man-made disasters. Geophysical methods are also widely used to investigate permafrost strata. The transformation of sounding data into apparent electrical resistivity (AER) is a common procedure for electromagnetic methods for exploring the geological environment and allowing one to quickly obtain general information about its structure. The measurement system for pulsed electromagnetic monitoring of the cryolithozone that is discussed in the article consists of a set of field sources and receivers that are mounted inside nonconductive housings and immersed in two different wells. A method has been proposed for converting the pulsed sounding data into apparent resistivity for all recording times. The transformation algorithm is based on selecting a resistivity of a homogeneous conducting half-space so that the signal for this resistivity corresponds to the measured signal. To develop the algorithm, the behavior of signals was studied and their transformations in half-spaces with an arbitrary resistivity were plotted. Examples are provided to determine apparent resistivity in models of the thawing upper layer of frozen rock at different distances between wells. It has been shown that at early times, when the signal reaches its maximum value and becomes measurable, the apparent resistivity provides a qualitative description of the geoelectric model, while the resistivity of the thawed layer can be accurately determined. The obtained values of the apparent resistivity are necessary for understanding the depth of thawing and allow the development of a reliable starting model for the subsequent inversion of pulsed sounding data with precise spatial localization of the boundary between frozen and thawed rocks.