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Forward modeling and inversion of viscous remanent magnetization responses in the time domain Cowan, Devin Christopher


In this thesis, I develop forward modeling and inversion methods for better characterizing viscous remanent magnetization (VRM) responses in the time-domain. Despite advances since the 1980s, aspects of modeling the VRM response and its impact on time-domain electromagnetic sensors remain elusive. Using Néel relaxation models, I parameterize the off-time viscous remanent magnetization in terms of an amalgamated magnetic property, which I show can be directly obtained using dual-frequency susceptibility measurements. For a half-space model, I derive empirical expressions for characterizing the relationship between the survey geometry and the cartesian components of the VRM response. Final expressions are verified with a 1D forward modeling code and used to determine the cross-over time at which the transient response within a large circular loop becomes dominated by the VRM signal. In order to predict the VRM responses from 3D geological structures, I develop a linear forward modeling code. This is accomplished by discretizing the Earth into a set of cells and implementing the parameterization of the off-time viscous remanent magnetization. Convergence of the forward model as a function of cell size is tested and the code is validated against a 1D code. Lastly, I develop an inversion approach for recovering the distribution of magnetically viscous materials from a set of field observations. A multitude of survey geometries are used to verify that our inversion is robust. A workflow is then presented for removing the VRM response from a set of survey data.

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Attribution-NonCommercial-NoDerivatives 4.0 International