Rapid approximate inversion of airborne TEM

Lee

2021

Exploration Geophysics

Right running head: Field example of Gaussian conductivity variation Table of contents (TOC) short summary:Using higher-order moments of the electromagnetic response, it is possible to invert data to find a conductivity model that varies smoothly with depth described by a Gaussian function. The Gaussian can approximate thin sheets, thick sheets and more general variations. In the case of our field area, we obtained results more consistent with geological control using the general variation. This is a preprint of the paper published as Richard S. Smith and Terry J. Lee, 2021, Multiple-order moments of the transient electromagnetic response of a one-dimensional earth with finite conductance -the Gaussian variation applied to a field example, Exploration Geophysics,

Section: Introductionmentioning

confidence: 99%

Multiple-order moments of the transient electromagnetic response of a one-dimensional earth with finite conductance – the Gaussian variation applied to a field example

Lee

2021

Exploration Geophysics

“…More sophisticated approaches involve using spherical models to generate discrete conductor sections (Smith and Salem, 2007), in analogy with conductivity depth sections (Macnae et al, 1991). The zeroth-order moments (or resistive limit) has also been used for modelling multiple spheres, as there is no interaction at the resistive limit and the moments can simply be summed (Hyde, 2002;Fullagar, 2010, 2012;Fullagar and Schaa, 2014;Fullagar et al, 2015). The ratio of successively higher-order moments can be used to estimate the time constant of a decay (Smith and Lee, 2002a;Guo et al, 2013).…”

Section: Introductionmentioning

confidence: 99%

Multiple-order moments of the transient electromagnetic response of a one-dimensional earth with finite conductance – theory

Lee

2020

Exploration Geophysics

“…Discrete conductor models have important applications for interpretation of data acquired using electromagnetic geophysical methods. Common models include the plate and sphere, which are commonly used for interpretation of borehole, ground, and airborne electromagnetic (BHEM, GEM, and AEM) data (Dyck et al, 1981;Dyck and West, 1984;Lamontagne et al, 1988;Macnae et al, 1998;Schaa, 2010;Smith and Wasylechko, 2012;Fullagar et al, 2015;Macnae, 2015;Vallée, 2015). In particular, the free-space discrete conductor model is attractive, especially in resistive envi-ronments such as large crystalline terranes due to the ease of computation (Annan, 1974;Smith and Lee, 2001).…”

Section: Introductionmentioning

confidence: 99%

Approximate semianalytical solutions for the electromagnetic response of a dipping-sphere interacting with conductive overburden

Desmarais

2016

GEOPHYSICS

Electromagnetic exploration methods have important applications for geologic mapping and mineral exploration in igneous and metamorphic terranes. In such cases, the earth is often largely resistive and the most important interaction is between a conductor of interest and a shallow, thin, horizontal sheet representing glacial tills and clays or the conductive weathering products of the basement rocks (both of which are here termed the "conductive overburden"). To this end, we have developed a theory from which the step and impulse responses of a sphere interacting with conductive overburden can be quickly and efficiently approximated. The sphere model can also be extended to restrict the currents to flow in a specific orientation (termed the dipping-sphere model). The resulting expressions are called semianalytical because all relevant relations are developed analytically, with the exception of the time-convolution integrals. The overburden is assumed to not be touching the sphere, so there is no galvanic interactions between the bodies. We make use of the dipole sphere in a uniform field and thin sheet approximations; however, expressions could be obtained for a sphere in a dipolar (or nondipolar) field using a similar methodology. We have found that there is no term related to the first zero of the relevant Bessel function in the response of the sphere alone. However, there are terms for all other zeros. A test on a synthetic model shows that the combined sphere-overburden response can be reasonably approximated using the first-order perturbation of the overburden field. Minor discrepancies between the approximate and more elaborate numerical responses are believed to be the result of numerical errors. This means that in practice, the proposed approach consists of evaluating one convolution integral over a sum of exponentials multiplied by a polynomial function. This results in an extremely simple algorithmic implementation that is simple to program and easy to run. The proposed approach also provides a simple method that can be used to validate more complex algorithms. A test on field data obtained at the Reid Mahaffy site in Northern Ontario shows that our approximate method is useful for interpreting electromagnetic data even when the background is thick. We use our approach to obtain a better estimate of the geometry and physical properties of the conductor and evaluate the conductance of the overburden.