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Joint inversion of lake-floor electrical resistivity tomography and boat-towed radio-magnetotelluric data illustrated on synthetic data and an application from the Äspö Hard Rock Laboratory site, Sweden
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.ORCID iD: 0000-0003-3819-8182
Geological Survey of Sweden, Uppsala, Sweden.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
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2018 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 213, no 1, p. 511-533Article in journal (Refereed) Published
Abstract [en]

The electrical resistivity tomography (ERT) method provides moderately good constraints for both conductive and resistive structures, while the radio-magnetotelluric (RMT) method is well suited to constrain conductive structures. Additionally, RMT and ERT data may have different target coverage and are differently affected by various types of noise. Hence, joint inversion of RMT and ERT data sets may provide a better constrained model as compared to individual inversions. In this study, joint inversion of boat-towed RMT and lake-floor ERT data has for the first time been formulated and implemented. The implementation was tested on both synthetic and field data sets incorporating RMT transverse electrical mode and ERT data. Results from synthetic data demonstrate that the joint inversion yields models with better resolution compared with individual inversions. A case study from an area adjacent to the Aspo Hard Rock Laboratory (HRL) in southeastern Sweden was used to demonstrate the implementation of the method. A 790-m-long profile comprising lake-floor ERT and boat-towed RMT data combined with partial land data was used for this purpose. Joint inversions with and without weighting (applied to different data sets, vertical and horizontal model smoothness) as well as constrained joint inversions incorporating bathymetry data and water resistivity measurements were performed. The resulting models delineate subsurface structures such as a major northeasterly directed fracture system, which is observed in the HRL facility underground and confirmed by boreholes. A previously uncertain weakness zone, likely a fracture system in the northern part of the profile, is inferred in this study. The fractures are highly saturated with saline water, which make them good targets of resistivity-based geophysical methods. Nevertheless, conductive sediments overlain by the lake water add further difficulty to resolve these deep fracture zones. Therefore, the joint inversion of RMT and ERT data particularly helps to improve the resolution of the resistivity models in areas where the profile traverses shallow water and land sections. Our modification of the joint inversion of RMT and ERT data improves the study of geological units underneath shallow water bodies where underground infrastructures are planned. Thus, it allows better planning and mitigating the risks and costs associated with conductive weakness zones.

Place, publisher, year, edition, pages
2018. Vol. 213, no 1, p. 511-533
National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-327094DOI: 10.1093/gji/ggx414ISI: 000448715000035OAI: oai:DiVA.org:uu-327094DiVA, id: diva2:1129395
Funder
Swedish Research Council Formas, 25220121907Available from: 2017-08-02 Created: 2017-08-02 Last updated: 2019-01-18Bibliographically approved
In thesis
1. Joint inversion and integration of multiple geophysical data for improved models of near-surface structures
Open this publication in new window or tab >>Joint inversion and integration of multiple geophysical data for improved models of near-surface structures
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Geophysical methods are non-invasive and allow an effective way of understanding subsurface structures and their physical properties. One of the main challenges is the often non-uniqueness of the geophysical models and that several different models can explain a dataset to an agreeable fit. Moreover, noise and limitations in resolution, which are inherent to field data, are additional obstacles for obtaining a true physical property model of the subsurface. Facing all these challenges, geophysicists have dedicated their efforts for decades to recover models that represent, as close as possible, the true subsurface. Joint inversion and integration of multiple geophysical data are two main approaches that I studied to better resolve subsurface structures. I further used these approaches, together with new software and hardware implementations for data acquisition and inversion, for near-surface applications. 

In this thesis, radio-magnetotelluric (RMT), boat-towed RMT, boat-towed controlled source MT (CSMT), electrical resistivity tomography (ERT), and first-arrival traveltime tomography are jointly used for quick clay investigations and fracture zone delineation under shallow water-bodies. The joint approach, as compared with any individual method, shows a better ability to both resolve the geological targets and to assist in understanding the subsurface geology that hosts these targets. For examples: by performing the joint inversion of lake-floor ERT and boat-towed RMT data, a fracture zone is better delineated with greater details compared with single inversion; by employing boat-towed CSMT measurements and jointly inverting with boat-towed RMT data, the subsurface structures, especially at greater depth, are better resolved than by inverting each dataset alone.

During my PhD studies, two types of new implementations were employed. (1) Boat-towed data acquisition system was implemented to expand the RMT and CSMT method from land to shallow-water applications. This is significant since many large-scale underground infrastructures are likely to cross these water zones (for example multi-lane train or bypass tunnels, such as the Stockholm bypass). (2) The modification of a well-structured code EMILIA allows joint inversion of boat-towed RMT and lake-floor ERT datasets, and the modification of another well-structured code MARE2DEM can accurately model high frequency CSMT data and handle joint inversion of boat-towed RMT and boat-towed CSMT datasets. Thus, the code modification as another type of new implementation guarantees the success of near-surface applications using the boat-towed RMT and CSMT data acquisition systems.

Studies conducted during my PhD work, included under the SEG-GWB (the Society of Exploration Geophysicists - Geoscientists Without Borders) program and the TRUST (TRansparent Underground STructure) umbrella project, are useful for near-surface applications including, for examples, engineering purposes such as planning of underground infrastructures, site characterization in connection with energy or waste storage, and geohazard investigations.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. p. 65
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1536
Keywords
joint inversion, ERT, RMT, CSRMT, boat-towed, tomography
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-327096 (URN)978-91-513-0018-4 (ISBN)
Public defence
2017-09-18, Hambergsalen, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2017-08-29 Created: 2017-08-03 Last updated: 2017-09-08

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Wang, ShunguoKalscheuer, ThomasMalehmir, AlirezaPedersen, Laust B.

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