| 1. |
- Castillo-Reyes, Octavio, et al.
(författare)
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Meshing strategies for 3d geo-electromagnetic modeling in the presence of metallic infrastructure
- 2023
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Ingår i: Computational Geosciences. - : Springer Nature. - 1420-0597 .- 1573-1499. ; 27:6, s. 1023-1039
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Tidskriftsartikel (refereegranskat)abstract
- In 3D geo-electromagnetic modeling, an adequate discretisation of the modeling domain is crucial to obtain accurate forwardresponses and reliable inversion results while reducing the computational cost. This paper investigates the mesh design forsubsurface models, including steel-cased wells, which is relevant for many exploration settings but still remains a numericallychallenging task. Applying a goal-oriented mesh refinement technique and subsequent calculations with the high-order edgefinite element method, simulations of 3D controlled-source electromagnetic models in the presence of metallic infrastructureare performed. Two test models are considered, each needing a distinct version of approximation methods to incorporatethe conductive steel casings of the included wells. The influence of mesh quality, goal-oriented meshing, and high-orderapproximations on problem sizes, computational cost, and accuracy of electromagnetic responses is investigated. The maininsights of our work are: (a) the applied numerical schemes can mitigate the computational burden of geo-electromagneticmodeling in the presence of steel artifacts; (b) investigating the processes driving the meshing of models with embeddedmetallic infrastructures can lead to adequate strategies to deal with the inversion of such electromagnetic data sets. Based onthemodeling results and analyses conducted, general recommendations formodeling strategies are proposed when performingsimulations for challenging steel infrastructure scenarios.
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| 2. |
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| 3. |
- Rulff, Paula, et al.
(författare)
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Efficient goal-oriented mesh refinement in 3-D finite-element modelling adapted for controlled source electromagnetic surveys
- 2021
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Ingår i: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 227:3, s. 1624-1645
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Tidskriftsartikel (refereegranskat)abstract
- We developed a 3-D forward modelling code, which simulates controlled source electromagnetic problems in frequency domain using edge-based finite elements and a total electric field approach. To evaluate electromagnetic data acquired across complex subsurface structures, software performing accurate 3-D modelling is required, especially for incorporation in inversion approaches. Our modelling code aims at finding a good compromise between the necessary solution accuracy at the points of interest and the general problem size by using a goal-oriented mesh refinement strategy designed for models of variable electric conductivity and magnetic permeability. To formulate an improved error estimator suitable for controlled source electromagnetic problems, we developed literature approaches of mesh refinement further targeting three aspects. First, to generate a roughly homogeneously fine mesh discretization around all receiver sites, our new error estimator weights the adjoint source term by the approximate decay of the electric field with increasing distance from the primal source using the expression for a homogeneous half-space. This causes almost no additional computational cost. Second, the error estimator employed in the refinement approach can be optimized for models with pronounced conductivity and magnetic permeability contrasts as often encountered in, for example, mineral prospecting scenarios by optionally including terms that measure the continuity of the normal component of current flow and the tangential component of the magnetic field across interfaces of abutting elements. Third, to avoid amplitude-dependent over-refining of the mesh, we formulate our element-wise error estimators relative to the local amplitude of the electromagnetic field. In this work, we evaluate the implemented adaptive mesh refinement approach and its solution accuracy comparing our solutions for simple 1-D models and a model with 3-D anomalies to semi-analytic 1-D solutions and a second-order finite-element code, respectively. Furthermore, a feasibility study for controlled-source electromagnetic measurements across ferrous mineral deposits is conducted. The numerical experiments demonstrate that our new refinement procedure generates problem-specific finite-element meshes and yields accurate solutions for both simple synthetic models and realistic survey scenarios. Especially for the latter, characteristics of our code, such as the possibility of modelling extended sources as well as including arbitrary receiver distributions and detailed subsurface anomalies, are beneficial.
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| 4. |
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| 5. |
- Rulff, Paula
(författare)
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Three-dimensional forward modelling and inversion of controlled-source electromagnetic data using the edge-based finite-element method
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electromagnetic geophysical methods are applied to investigate anomalous subsurface structures exhibiting contrasts in electrical resistivity, as for example in mineral exploration or geothermal areas. When evaluating electromagnetic data, one big challenge is to account for full three-dimensional measurement setups and subsurface scenarios. To address this challenge, this work seeks to report on the development of a three-dimensional forward modelling code for controlled-source electromagnetic data and its integration into an inversion framework to enable the computation of three-dimensional resistivity models of the subsurface.This thesis outlines the technical details of the developed forward and inverse modelling routines and describes synthetic tests to verify their effectiveness. The implemented forward modelling routines, based on the linear edge-based finite-element formulation on tetrahedral elements, consist of a standalone code containing a goal-oriented mesh refinement option. The forward modelling code was included with several modules into an existing inversion framework. Electromagnetic field components or impedance tensors and vertical magnetic transfer functions serve as input data for the inversion. Data gradients, required for the used non-linear conjugate-gradient inversion algorithm, were implemented for all input data types. Furthermore, regularisation options were employed and tested. The aforementioned forward and inverse modelling codes were applied for different field settings: A forward modelling study of an ore deposit in central Sweden aimed at finding an adequate receiver distribution for the field measurements. In another modelling project, the mesh design for subsurface models containing steel-cased wells was investigated. Finally, an inversion of controlled-source electromagnetic data collected on a frozen lake in Stockholm for a tunnel-project complemented previously obtained two-dimensional resistivity models with a three-dimensional model. The benefit of the presented research is twofold: First, the new modules added to the inversion framework can be used by the academic community to obtain improved resistivity models. Second, the reported models and field settings provide detailed information for measurement design, meshing of three-dimensional domains and local resistivity distributions of the subsurface allowing to answer geological, hydrological and geotechnical questions of selected field sites.
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