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Radiomagnetotelluric two-dimensional forward and inverse modelling accounting for displacement currents
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Department of Physics, Faculty of Science, Mahidol University, Rama VI Rd., Rachatawee, Bangkok 10400, Thailand.
2008 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 175, no 2, 486-514 p.Article in journal (Refereed) Published
Abstract [en]

Electromagnetic surface measurements with the radiomagnetotelluric (RMT) method in the frequency range between 10 and 300 kHz are typically interpreted in the quasi-static approximation, that is, assuming displacement currents are negligible. In this paper, the dielectric effect of displacement currents on RMT responses over resistive subsurface models is studied with a two-dimensional (2-D) forward and inverse scheme, that can operate both in the quasi-static approximation and including displacement currents.Forward computations of simple models exemplify how responses that allow for displacements currents deviate from responses computed in the quasi-static approximation. The differences become most obvious for highly resistive subsurface models of about 3000 Ohm*m and more and at high frequencies. For such cases, the apparent resistivities and phases of the transverse magnetic (TM) and transverse electric (TE) modes are significantly smaller than in the quasi-static approximation. Along profiles traversing 2-D subsurface models, sign reversals in the real part of the vertical magnetic transfer function (VMT) are often more pronounced than in the quasi-static approximation. On both sides of such sign reversals, the responses computed including displacement currents are larger than typical measurement errors.The 2-D inversion of synthetic data computed including displacement currents demonstrates that serious misinterpretations in the form of artefacts in inverse models can be made if displacement currents are neglected during the inversion. Hence, the inclusion of the dielectric effect is a crucial improvement over existing quasi-static 2-D inverse schemes. Synthetic data from a 2-D model with constant dielectric permittivity and a conductive block buried in a highly resistive layer which in turn is underlain by a conductive layer are inverted. In the quasi-static inverse model, the depth to the conductive structures is overestimated, artefactual resistors appear on both sides of the conductive block, and a spurious conductive layer is imaged at the surface.High-frequency RMT field data from Ävrö, Sweden, are re-interpreted using the newly developed 2-D inversion scheme which includes displacement currents. In contrast to previous quasi-static modelling, the new inverse models have electrical resistivity values comparable to a normal-resistivity borehole log and boundaries between resistive and conductive structures which correlate with the positions of seismic reflectors.

Place, publisher, year, edition, pages
2008. Vol. 175, no 2, 486-514 p.
Keyword [en]
Magnetotelluric
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:uu:diva-97758DOI: 10.1111/j.1365-246X.2008.03902.xISI: 000260134900006OAI: oai:DiVA.org:uu-97758DiVA: diva2:172817
Available from: 2008-11-07 Created: 2008-11-07 Last updated: 2016-01-15Bibliographically approved
In thesis
1. Improvement and Assessment of Two-Dimensional Resistivity Models Derived from Radiomagnetotelluric and Direct-Current Resistivity Data
Open this publication in new window or tab >>Improvement and Assessment of Two-Dimensional Resistivity Models Derived from Radiomagnetotelluric and Direct-Current Resistivity Data
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Two-dimensional (2-D) models of electrical resistivity are improved by jointly inverting radiomagnetotelluric (RMT) and direct-current resistivity (DCR) data or by allowing for displacement currents in the inversion of RMT data collected on highly resistive bedrock. Uniqueness and stability of the 2-D models are assessed with a model variance and resolution analysis that allows for the non-linearity of the inverse problem.

Model variance and resolution are estimated with a truncated singular value decomposition (TSVD) of the sensitivity matrix. In the computation of model errors, inverse singular values are replaced by non-linear semi-axes and the number of included eigenvectors is increased until a given error threshold is reached. Non-linear error estimates are verified with most-squares inversions. For the obtained truncation levels, model resolution matrices are computed. For RMT data, non-linear error appraisals are smaller than linearized ones. Hence, the consideration of the non-linearity in RMT data leads to reduced model errors or enhanced model resolution.

The dielectric effect on RMT data is investigated with a new 2-D forward and inverse code that allows for displacement currents. As compared to the quasi-static approximation, apparent resistivities and phases of the impedance tensor elements are found to be significantly smaller and the vertical magnetic transfer function exhibits more distinct sign reversals. More reliable models of electrical resistivity are obtained from areas with highly resistive bedrock, if displacement currents are allowed for. In contrast, inversions with a quasi-static scheme introduce artefactual structures with extremely low or high resistivities.

A smoothness-constrained 2-D joint inversion of RMT and DCR data is presented. The non-linear model variance and resolution analysis is applied to single and joint inverse models. For DCR data, the errors estimated by most-squares inversions are consistently larger than those estimated by the non-linear semi-axes, indicating that DCR models are poorly resolved. Certain areas of the joint inverse models are better resolved than in the single inverse models.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2008. 82 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 572
Keyword
Electromagnetic geophysics, radiomagnetotellurics, direct-current resistivity, displacement currents, dielectric effect, non-linearity, inversion, regularization, two-dimensional model, model variance, model resolution, data resolution
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-9366 (URN)978-91-554-7334-1 (ISBN)
Public defence
2008-12-05, Axel Hambergsalen, Department of Earth Sciences, Villavagen 16, 75236 Uppsala, Sweden, 10:00
Opponent
Supervisors
Available from: 2008-11-07 Created: 2008-11-07 Last updated: 2016-05-13Bibliographically approved

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Kalscheuer, ThomasPedersen, Laust B.

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