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The electrical conductivity of the Hallandsås Horst, Sweden;: A controlled source radiomagnetotelluric study
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. (Electromagnetic studies)
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. (Electromagnetic studies)
(English)Manuscript (Other (popular science, discussion, etc.))
Abstract [en]

We studied the behaviour of controlled source responses of 2½-D synthetic models based on controlled source tensor magnetotelluric (CSTMT, 1 – 12 kHz) and radio magnetotelluric (RMT, 14 – 250 kHz) data sets collected along profiles on the Hallandsås Horst, Southern Sweden with a view to study the depth extent of possible near surface fractures zones that might intersect a tunnel under construction. Based upon 2½-D forward modelling of the TE- and TM-mode RMT data, a simplified 2D model was constructed such that a dipping conductor was continued down to a depth of 140 m, which is the depth of the tunnel. Based on this extended model, we calculated both plane wave and controlled source responses for the frequency range 1-250 kHz corresponding to the source location in the study. Finally, we inverted both data sets under plane wave assumptions. When comparing synthetic controlled source and plane wave data in the frequency range 1 – 12 kHz, we find that the CSTMT transfer functions generally deviate from the plane wave condition for frequencies less than 6 kHz. The phases are much more distorted than apparent resistivities. The TM mode phases, in particular, are generally distorted for frequencies less than 6 kHz. Regarding the vertical magnetic fields, we find that at all frequencies the real part of tippers are distorted by source effects, but the tipper component along the profile direction is much less affected by the off-profile lying source than the tipper component orthogonal to the profile direction. Based upon synthetic model studies of combined CSTMT and RMT (in short; CSRMT) responses, we find that determinant data are less distorted than individual TE- and TM-mode data. On the other hand, TE- and TM-response have greater resolving power and used with care (selecting data that are weakly distorted by source effects), the corresponding models can resolve the conductor down to tunnel depths.

Keyword [en]
Controlled-source tensor magnetotelluric, Radiomagnetotelluric, CSRMT, 2D inversion, forward modelling, near field effects
National Category
Geophysics
Research subject
Earth Science with specialization in Environmental Analysis; Hydrology
Identifiers
URN: urn:nbn:se:uu:diva-102731OAI: oai:DiVA.org:uu-102731DiVA: diva2:216652
Note
Submitted to Near Surface Geophysics, in revision. Available from: 2009-05-11 Created: 2009-05-11 Last updated: 2010-12-21
In thesis
1. Controlled Source Radiomagnetotelluric (CSRMT) Applications in Environmental and Resource Exploration
Open this publication in new window or tab >>Controlled Source Radiomagnetotelluric (CSRMT) Applications in Environmental and Resource Exploration
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

An integrated use of radio magnetotelluric (RMT) and controlled source tensor magnetotelluric (CSTMT) measurements, the so-called CSRMT method, has been employed in environmental and resource exploration studies. A number of case histories, including a groundwater investigation in glacial deposits, a study of fracture zones for geotechnical purposes and a mining exploration study of a copper deposit, are presented in this thesis in order to illustrate the usefulness and capability of the CSRMT method. The resolutions of the estimated models using various types of data are studied. Magnetotelluric transfer functions are used to analyze the dimensionality, the near surface resistivity distortions and the near field effects in the case of CSTMT data analysis. The near field effects in CSTMT data have also been identified by performing 2½D forward modelling.

Data analysis, dimensionality tests and forward modelling show that at the lowest frequencies used the CSTMT transfer functions are generally distorted by source effects, except when the source-receiver distances are sufficient large compared with the penetration depth. Regarding CSTMT transfer functions, apparent resistivities are generally less distorted than phases. TM mode transfer functions are more affected by the sources than TE mode, while tipper vectors generally contain source signatures at all frequencies.

Based on the analysis of dimensionality and source effects 2D inverse modelling of CSTMT and RMT data, as well as their combination, have been performed under the plane wave assumption. The RMT method proved to be a powerful tool for imaging the upper 50 m near-surface, but their penetration depth reduces as a conductive layer structures cover the targets at depth. The penetration depth can be increased by including the CSTMT data in the modelling if the measurements are in the far field range. The resolution of the deeper parts of the models may be improved by performing a joint inversion of TE and TM modes, if the strike direction is well-defined. Alternatively, inversion of determinant data can be performed, since the determinant data are less affected by 3D structures and source effects. However the resolution of the determinant models is somewhat degraded compared to the models inverted from combined TE and TM modes.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2009. 66 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 655
Keyword
Magnetotelluric, radio magnetotelluric, controlled-source tensor magnetotelluric, plane wave, near surface geophysics, sediment, fracture zone, inversion, forward modelling, near field effect, two-dimensional model, three-dimensional model, resolution
National Category
Geophysics
Research subject
Earth Sciences with Specialization Environmental Analysis; Hydrology; Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-102750 (URN)978-91-554-7550-5 (ISBN)
Public defence
2009-06-05, Axel Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2009-05-14 Created: 2009-05-11 Last updated: 2009-05-25Bibliographically approved

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Ismail, NazliPedersen, Laust

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