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From 2D to 3D Models of Electrical Conductivity based upon Magnetotelluric Data: Experiences from two Case Studies
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Magnetotelluric measurements are among the few geophysical techniques capable of imaging the structure both in the shallow subsurface as well as the entire crust of the Earth. With recent technical and computational advances it has become possible to derive three-dimensional inversion models of the electrical conductivity from magnetotelluric data, thereby overcoming the problems arising from the simplified assumption of two-dimensionality in conventional two-dimensional modeling. The transition from two-dimensional to three-dimensional analysis requires careful reconsideration of the classical challenges of magnetotellurics: galvanic distortion, data errors, model discretization and resolution.This work presents two examples of magnetotelluric investigations, where a new three-dimensional inversion algorithm has been applied. The new models are compared with conventional two-dimensional models and combined with the results of other geophysical techniques like reflection seismics and electrical resistivity tomography. The first case presents magnetotelluric investigations of the Kristineberg mining area in the Skellefte district, northern Sweden. This study is part of a joint geophysical and geological project to investigate the present structure and evolution of the whole district. Together with reflection seismic and surface geological information a three-dimensional conductivity model, derived through the inversion of magnetotelluric data, was interpreted. A comparison with two-dimensional modeling gives insights into the capabilities and challenges of three-dimensional inversion strategies with respect to data sampling and model resolution.The second case presents a study of remediation monitoring  with geophysical methods after an oil blow-out in Trecate, Italy. A three-dimensional conductivity model was derived from radiomagnetotelluric measurements. In addition, two-dimensional joint inversion of radiomagnetotelluric and electrical tomography data was performed. Compared with electrical resistivity tomography, radiomagnetotelluric data is more sensitive to conductors and the derived three-dimensional inversion model resolves the vadose zone and the underlying aquifer.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2012. , 58 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 890
National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-165143OAI: oai:DiVA.org:uu-165143DiVA: diva2:472017
Public defence
2012-02-17, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2012-01-27 Created: 2012-01-03 Last updated: 2012-02-15Bibliographically approved
List of papers
1. MT measurements in the western part of the Paleoproterozoic Skellefte Ore District, Northern Sweden: a contribution to an integrated geophysical study
Open this publication in new window or tab >>MT measurements in the western part of the Paleoproterozoic Skellefte Ore District, Northern Sweden: a contribution to an integrated geophysical study
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2009 (English)In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 475, no 3-4, 493-502 p.Article in journal (Refereed) Published
Abstract [en]

A 2D conductivity model of the Kristineberg area in the Skellefte Ore District, Northern Sweden, has been derived from new magnetotelluric measurements. This complements an intensive geophysical and geological study of the area, including reflection seismics, gravity and aeromagnetic data modeling as well as geological field observations. In a pilot study, 20 broadband MT stations were installed in May 2007 along a 20 km long north–south profile. Dimensionality analysis shows that a 2D interpretation of the data is justified, although the presumed geoelectric strike direction of N75°E is not consistent over the whole profile. The new conductivity model of the upper crust agrees well with the results from the seismic studies. Interpreting both independent data sets confirms the major features from the previous model, such as the thickness of the Revsund granites in the south, the existence of a structural basement with metasedimentary origin, and gives new insight into the nature of the volcanic rocks and their possible mineral content.

Place, publisher, year, edition, pages
Elsevier, 2009
Keyword
Magnetotellurics, Skellefte Ore District, Structural basement, Seismic reflection, Revsund granite
National Category
Earth and Related Environmental Sciences Geophysics
Identifiers
urn:nbn:se:uu:diva-114503 (URN)10.1016/j.tecto.2009.06.011 (DOI)000273524100007 ()
Available from: 2010-02-16 Created: 2010-02-16 Last updated: 2017-12-12Bibliographically approved
2. Imaging the Kristineberg mining area with two perpendicular magnetotelluric profiles in the Skellefte Ore District, northern Sweden
Open this publication in new window or tab >>Imaging the Kristineberg mining area with two perpendicular magnetotelluric profiles in the Skellefte Ore District, northern Sweden
2013 (English)In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 61, no 1, 200-219 p.Article in journal (Refereed) Published
Abstract [en]

New magnetotelluric (MT) data from two perpendicular profiles in the Kristineberg area, northern Sweden, were analysed and modelled. In the Skellefte Ore District, the Kristineberg volcanic hosted massive sulphide (VHMS) deposit mine is one of the largest and deepest (1250 m). Seventeen broadband magnetotelluric stations were installed along two existing seismic reflection lines. The profiles were 6 and 12 km long with 500 m and 1 km site spacing, respectively. The obtained MT transfer functions in the period range of 0.0015–200 s are of fairly good quality. Detailed strike and dimensionality analysis reveal consistent but period dependent, strike directions, indicating a change in the geoelectrical strike with depth. From the two-dimensional inversion of the determinant of the impedance tensor, two stable conductivity models with good data fit were obtained. The addition of seismic reflection information from the co-located survey, improved the data fit of one of them. Extensive sensitivity analyses helped to delineate the well resolved regions of the models and to determine the position of pronounced boundaries. The results are in good agreement with previous studies, especially regarding the presence of a deep conductor interpreted as a structural basement to the district. They also reveal with more detail the configuration of the main geological units of the Skellefte Ore District, especially of the ore bearing volcanic rocks and the embedded alteration zones.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2013
Keyword
Mining, Magnetotelluric, 2D inversion
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-163596 (URN)10.1111/j.1365-2478.2011.01040.x (DOI)000312548300015 ()
Projects
VINNOVA 4D modeling of the Skellefte District
Available from: 2011-12-13 Created: 2011-12-13 Last updated: 2017-12-08Bibliographically approved
3. The upper crustal 3-D resistivity structure of the Kristineberg area, Skellefte district, northern Sweden revealed by magnetotelluric data
Open this publication in new window or tab >>The upper crustal 3-D resistivity structure of the Kristineberg area, Skellefte district, northern Sweden revealed by magnetotelluric data
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2013 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 192, no 2, 500-513 p.Article in journal (Refereed) Published
Abstract [en]

A 3-D model of the crustal electrical resistivity was constructed from the inversion of magnetotelluric data in the Kristineberg area, Skellefte district, the location of one of Sweden's most successful mining activities. Forward modelling of vertical magnetic transfer data supports our model which was derived from the magnetotelluric impedance only. The dominant features in the 3-D model are the strong conductors at various depth levels and resistive bodies of variable thickness occurring in the shallower subsurface. The deepest conductor, previously associated with the Skellefte crustal conductivity anomaly, is imaged in the southern part of the area as a north-dipping feature starting at similar to 4 km depth. Several shallow conductors are attributed to graphite in the black shales defining the contact between the metasedimentary rocks and the underlying metavolcanic rocks. Furthermore, an elongated intermediate depth conductor is possibly associated with alteration zones in the metavolcanic rocks that host the ore occurrences. The most prominent crustal resistors occur in the southern and northern part of the area, where their lateral extent on the surface coincides with the late-orogenic Revsund type intrusions. To the east, a resistive feature can be correlated to the early-orogenic Viterliden intrusion. The 3-D model is compared with two previous 2-D inversion models along two perpendicular profiles. The main electrical features are confirmed with the new model and previous uncertainties regarding 3-D effects, caused by off-profile conductors, can be better assessed in 3-D, although the resolution is lower due to a coarser model discretization. The comparison with seismic sections along two north-south profiles reveals structural correspondence between electrical features, zones of different reflectivity and geological units.

Keyword
Crustal structure, Electrical properties, Magnetotelluric
National Category
Geophysics Natural Sciences
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-163597 (URN)10.1093/gji/ggs044 (DOI)000313623100004 ()
Available from: 2011-12-13 Created: 2011-12-13 Last updated: 2017-12-08Bibliographically approved
4. 2D joint inversion of RMT and ERT data versus individual 3D inversion of full tensor RMT data: An example from the Trecate site in Italy
Open this publication in new window or tab >>2D joint inversion of RMT and ERT data versus individual 3D inversion of full tensor RMT data: An example from the Trecate site in Italy
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2012 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 77, no 4, WB233-WB243 p.Article in journal (Refereed) Published
Abstract [en]

Tensor radiomagnetotelluric (RMT) and electrical resistivity tomography (ERT) data were acquired along 10 parallel lines to image electrical resistivity of the vadose and the saturated zone in an area near Trecate, 45 km west of Milan in Italy. In 1994, the area was exposed to an oil contamination caused by a tank explosion and has since been subject to monitoring and remediation programs. For the first time, we have examined a 3D inversion of full tensor RMT data and have compared the results with 2D joint inversion of RMT and ERT data. First, a synthetic 3D resistivity model with similar variations close to those measured at the Trecate site was generated for the comparison. The synthetic tests showed that resistivity models from 2D joint inversion of ERT and RMT data contain more details closer to the surface compared to the models from the 3D inversion of tensor RMT data. High resistivity structures are better resolved by the 2D joint inversion, whereas the more conductive features are better recovered by the 3D inversion. In the next step, the ERT and RMT data collected in the Trecate site were modeled with the same approaches used in the synthetic modeling. Using the measured tensor RMT data, it was possible to carry out full 3D inversion to study the underlying geology. Comparison between the resistivity models from both inversions with the lithological data from the existing boreholes, resistivity models from the inversion of crosshole resistivity data, and water content models from magnetic resonance soundings measurements showed that the electrical resistivity, depth to the top and thickness of the water saturated zone is modeled more accurately With the 3D inversion.

National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-165142 (URN)10.1190/GEO2011-0525.1 (DOI)000307799500022 ()
Available from: 2012-01-03 Created: 2012-01-03 Last updated: 2017-12-08Bibliographically approved

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