Simultaneous 1D inversion of loop–loop electromagnetic data for magnetic susceptibility and electrical conductivity

Abstract

Magnetic susceptibility affects electromagnetic (EM) loop–loop observations in ways that cannot be replicated by conductive, nonsusceptible earth models. The most distinctive effects are negative in‐phase values at low frequencies. Inverting data contaminated by susceptibility effects for conductivity alone can give misleading models: the observations strongly influenced by susceptibility will be underfit, and those less strongly influenced will be overfit to compensate, leading to artifacts in the model. Simultaneous inversion for both conductivity and susceptibility enables reliable conductivity models to be constructed and can give useful information about the distribution of susceptibility in the earth. Such information complements that obtained from the inversion of static magnetic data because EM measurements are insensitive to remanent magnetization. We present an algorithm that simultaneously inverts susceptibility‐affected data for 1D conductivity and susceptibility models. The solution is obtained by minimizing an objective function comprised of a sum‐of‐squares measure of data misfit and sum‐of‐squares measures of the amounts of structure in the conductivity and susceptibility models. Positivity of the susceptibilities is enforced by including a logarithmic barrier term in the objective function. The trade‐off parameter is automatically estimated using the generalized cross validation (GCV) criterion. This enables an appropriate fit to the observations to be achieved even if good noise estimates are not available. As well as synthetic examples, we show the results of inverting airborne data sets from Australia and Heath Steele Stratmat, New Brunswick.

Evaluating the Effects of Magnetic Susceptibility in UXO Discrimination Problems

“…Forward modelling in 1D is solved in the frequency domain in the standard propagation matrix formalism [Farquharson et al, 2001]. Let us consider a circular transmitter loop of radius , carrying a current , and at a height above a 1-D layered earth.…”

Fast Computation of the Forward Solution in Controlled-Source Electromagnetic Sounding Problems

“…Thus, sizeable zones of buried magnetically permeable metals or ferrimagnetic minerals, especially magnetite and/or titanomagnetite, distributed in rocks throughout the crust, can be located using simultaneous inversion for conductivity and magnetic permeability [8,11]. As an example, Figure 10 illustrates the H zfield response in presence of a two-layer earth model consisting of a 0.01-S/m magnetic layer overlying a nonmagnetic 0.3-S/m half-space, with the permeability contrast µ 2 /µ 1 = 1/µ r 1 taken as a parameter.…”

“…Information may be the thicknesses or the EM properties of the various layers. One way to collect CSEM data is to measure the frequency-domain EM coupling between a transmitting and a receiving insulated wire loop at a set of discrete frequencies, typically spanning several decades [2,3,5,6,8,9].…”

“…The controlled-source electromagnetic (CSEM) sounding method consists of acquiring information about the subsurface structure of a layered earth medium by measuring the EM fields produced above its surface by an intentional source [1][2][3][4][5][6][7][8]. Information may be the thicknesses or the EM properties of the various layers.…”

Airborne EM Surveys Applied to Industrial Land-use Legacy Assessments in the UK

“…In the absence of such information we can use modern geological subsurface information on coal seams to reveal a close correspondence between the arcuate conductivity anomaly and the trace of potential voids arising from worked coal seams. The 2-frequency EM survey data were converted to models of subsurface conductivity using a smooth 1D model inversion procedure (Farquharson et al, 2003). The models were then assembled into a continuous voxel-based conductivity volume.…”