Forward modelling 3-D geophysical electromagnetic field data with meshfree methods

Long, Jianbo (2020) Forward modelling 3-D geophysical electromagnetic field data with meshfree methods. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Simulating geophysical electromagnetic (EM) data over real-life conductivity models requires numerical algorithms that can incorporate realistically complex geometry and topography. The most successful way to incorporate them is to use unstructured meshes in the discretization of an Earth model. Current mesh-based numerical methods that are capable of using such meshes have inherent drawbacks caused by generating 3-D unstructured meshes conforming to irregular geometries. Such a mesh generation process may become computationally expensive and unstable, and particularly so for EM inversion computations in which the forward modelling may be required many times. In this thesis I investigate the feasibility and applicability of radial basis function-based finite difference (RBF-FD), a meshfree method, in forward modelling 3-D EM data. In the meshfree method, the physical model is represented using only a set of unconnected points, effectively overcoming the issues related to the mesh generation. To improve numerical efficiency, unstructured point sets are used in the computation for the first time for EM problems. The computation is further accelerated by introducing a new type of radial basis function in the RBFFD method. The convergence and accuracy of the proposed RBF-FD method are demonstrated first via forward modelling gravity and gravity gradient data. The computational efficiency of the meshfree method is compared with that of using a more traditional finite element method. The meshfree method is then applied to forward model magnetotelluric data of which the effectiveness is demonstrated using three benchmark conductivity models from the literature. Faithful reproduction of the physics in the EM fields, e.g. discontinuous electric fields across the conductivity contrasts, is achieved by proposing a hybrid meshfree scheme which is a modification to standard meshfree algorithms. The hybrid method is also applied to simulate controlled-source EM data in the frame of both total-field and primary-secondary field approaches, in which the problems in dealing with singular source functions that cause singularities in the EM fields are addressed. For these two approaches, the accuracies of the proposed hybrid meshfree method in forward modelling the controlled-source EM data are demonstrated by using idealized 1-D layered models and a 3-D marine canonical disk model. The successful applications of the proposed meshfree method in modelling the above EM data suggest that the meshfree technique has the potential of becoming an important numerical method for simulating EM responses over complicated conductivity models.

Item Type: Thesis (Doctoral (PhD))
Item ID: 14512
Additional Information: Includes bibliographical references (pages 153-170).
Keywords: Forward modelling, geophysics, electromagnetic, meshfree, numerical modelling
Department(s): Science, Faculty of > Earth Sciences
Date: May 2020
Date Type: Submission
Library of Congress Subject Heading: Electromagnetism--Computer simulation; Meshfree methods (Numerical analysis); Three-dimensional modeling.

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