Applying three-dimensional inversion to the frequency-domain response converted from transient electromagnetic data for a rectangular fixed loop

Li, Dajun; Shan, Xuanlong; Weng, Aihua

doi:10.1016/j.jappgeo.2021.104489

Journal of Applied Geophysics

2022

DOI: 10.1016/j.jappgeo.2021.104489

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Applying three-dimensional inversion to the frequency-domain response converted from transient electromagnetic data for a rectangular fixed loop

Dajun Li

Xuanlong Shan

Aihua Weng

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Cited by 2 publications

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“…Over the past decades, innovations in numerical calculations have driven remarkable progress in the threedimensional (3D) forward modeling of CSEMs, achieving successful breakthroughs (Constable, 2010;Ansari and Farquharson, 2013;Börner et al, 2015;Oldenburg et al, 2020;Werthmüller et al, 2021). The common methods used for developing the 3D forward modeling of CSEMs include finite element techniques (Tonti, 2002;Um et al, 2012;Cai et al, 2014;Rochlitz et al, 2019;, finite volume methods (Haber and Ascher, 2001;Ren et al, 2017;Peng et al, 2018), finite difference methods (Newman and Alumbaugh, 1995;Weiland, 1996;Li et al, 2022), integral equation techniques (Hursán and Zhdanov, 2002;Tang et al, 2018), and spectral element methods (Huang et al, 2019;Xu and Tang, 2022). Furthermore, numerous scholars have dedicated research efforts to enhance the efficiency of numerical solutions in 3D forward modeling for CSEMs.…”

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A general forward solver for 3D CSEMs with multitype sources and operating environments

Wang

et al. 2023

Front. Earth Sci.

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To determine the electromagnetic (EM) fields of different three-dimensional (3D) controlled-source electromagnetic methods (CSEMs) using the same parameters of the forward solution, by explicitly considering the commonalities, we present a general 3D forward modeling solver for CSEMs with multitype sources and operating environments. The commonality of the solver is reflected in two aspects. First, the solver is based on a frequency-domain (FD) vector Helmholtz equation for determining the scattered electric field. The different types of sources are imposed on the right-hand term of the equation, expressed as background Green’s function. Second, sources of any CSEM can be composed of electric dipole (ED) or magnetic dipole (MD) superposition. Thus, the focus of the 3D forward modeling of CSEMs is reduced to determining the EM fields of ED or MD sources for the background medium. The quasi-minimal residual (QMR) method is used to solve the large sparse complex linear system. Once the FD EM fields have been calculated, the time-domain (TD) response can be obtained using the cosine/sine transformation. The numerical results show that the relative error is less than 5% between the 3D numerical and analytical solutions, which verifies the accuracy of the solver. We further study the difference between the real (bent) and theoretical (straight) wires. We suggest that the shape of the source must be considered for TD and FD CSEMs with a wire source during data processing and inversion. The last example investigated the characteristics of FD EM fields from a finite-length wire and TD EM fields from a rectangular fixed loop on the same conductive tilted disk model buried in resistive sediments. According to the numerical results, we recommend FD CSEMs with a wire source for detecting deep anomalies.

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A general forward solver for 3D CSEMs with multitype sources and operating environments

Wang

et al. 2023

Front. Earth Sci.

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Interpreting Surface Large-Loop Time-Domain Electromagnetic Data for Deep Mineral Exploration Using 3D Forward Modeling and Inversion

Cheng

Yang

Luo³

2022

Minerals

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Surface transient electromagnetic (TEM) data with large transmitter loops for deep mineral exploration are often complicated by the non-trivial coupling between extended sources and arbitrarily oriented geological targets. This case study reports a TEM field data set acquired across terranes with strong lateral inhomogeneity, which is responsible for the high inconsistency in TEM data patterns along the survey line, as well as for the negative TEM transients (sign reversal) at some near-central loop stations. 3D forward modeling and inversion, as maturing tools in recent years, offer unique opportunities to extract as much geological information from such data as possible. 3D forward simulations of representative synthetic models found that the phenomenon of sign reversal at some TEM stations is associated with compact conductors enclosed by the transmitter loop and receivers that are in the loop, but off the conductor–a situation that is common in large-loop TEM and can only be explained by 3D models. However, 3D inversion of the field data with a uniform subspace as the initial and reference model fails to converge, another point of evidence that 3D inversions of large-loop TEM data are more likely to be subject to stability issues. Our solution is to warm-start the inversion with the representative model in the forward simulation experiments as the initial model, so the ill-posed 3D inversion can escape from local minima. Finally, the vertical contact structure in our 3D-inversion model is verified by a resistivity cross section of the CSAMT method. Our case study demonstrates the demand and capability of 3D electromagnetic modeling and inversion for high-resolution deep mineral exploration. It also provides an easy-to-follow template for carrying out 3D interpretation for complex geology in practice.

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Applying three-dimensional inversion to the frequency-domain response converted from transient electromagnetic data for a rectangular fixed loop

Cited by 2 publications

References 44 publications

A general forward solver for 3D CSEMs with multitype sources and operating environments

A general forward solver for 3D CSEMs with multitype sources and operating environments

Interpreting Surface Large-Loop Time-Domain Electromagnetic Data for Deep Mineral Exploration Using 3D Forward Modeling and Inversion

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