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Seismics, 2D and 3D Inversion of Magnetotellurics: Jigsaw pieces in understanding the Skellefte Ore District
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]

The Skellefte District (SD) is one of the richest metallogenic mining areas in Sweden. The main deposits consist of volcanic-hosted massive sulphides (VHMS) rich in zinc, copper, lead, gold and silver, that have been explored and mined for more than a century. Considering that technological advancements allow deeper mining, and that today new discoveries rarely occur, renewed efforts are now directed at locating targets at greater depths.  

Thus, current exploration strategies need to be adapted, and a better understanding of regional scale structures is necessary. To address these questions the project VINNOVA 4D modeling of the Skellefte District was launched. Its main purpose is to unravel the regional structures and tectonic setting of the SD. To accomplish this, new geological and geophysical data have been acquired in two key localities.

This thesis presents the contribution from 2D and 3D inversion of magnetotelluric (MT) data and seismic reflection data. The main findings include: conductive hydrothermally altered zones within the otherwise resistive rocks of the Skellefte Group, the depth extension of early and postorogenic intrusions, prominent shear zones in the central part of the district, and enhanced reflectivity and conductivity at the base of the Skellefte Group throughout the SD.

Even though the application of these methods is challenged by the complex geological setting of the SD, it is shown that after a careful processing and analysis of the data, they are able to provide a robust image of the deep subsurface. Additionally, the combination of reflection seismics and MT has proved to be a powerful tool for hypothesis testing and to develop the general understanding of the configuration and history of the SD.

Furthermore, two 3D inversion models of MT data are presented and compared with the results of standard 2D determinant inversions. The 3D procedure shows significant improvements in data fit and is able to constrain better the observed model features. Although 3D inversion of MT data is not yet a run of the mill scheme and issues like model assessment and galvanic distortion effects need to be further addressed, results from complex environments with areal coverage, are already superior to those from 2D inversions.

Abstract [sv]

Skellefteåfälten är ett av de viktigaste malmdistrikten i Sverige. Malmkropparna består av vulkaniskvärda Massiva Sulfider (VHMS) rika på Zink, Koppar, Bly, Guld och Silver, och har utforskats och brutits i mer än ett sekel. Med tanke på att de senaste tekniska framstegen tillåter djupare brytning, och att nya upptäckter är ovanliga idag, riktas nya ansträngningar mot att lokalisera malm på större djup.

Aktuella prospekteringsstrategier måste därför anpassas, och en bättre förståelse av regionala strukturer är nödvändig. För att lösa dessa frågor lanserades projektet VINNOVA 4D modeling of the Skellefte District. Dess främsta syfte är att utreda de regionala strukturerna och det tektoniska läget av Skelleftefältet. För att uppnå detta, har nya geologiska och geofysiska data insamlats vid två viktiga platser i distriktet.

Denna avhandling presenterar bidrag från inversionsmodellering i 2D och 3D av magnetotelluriska (MT) data samt resultaten av en reflektionsseismisk profil. De viktigaste resultaten är: bra ledande hydrotermiskt förändrade zoner inom de annars resistiva bergarterna i Skellefte-gruppen, djupet till tidiga och postorogeniska intrusioner, framstående skjuvzoner i den centrala delen av området, och ökad reflektionsförmåga och konduktivitet vid basen av Skellefte-gruppen i hela fältet.

Även om tillämpningen av dessa metoder utmanas av fältens komplexa geologiska läge, visas det efter en noggrann bearbetning och analys av data att de ger en robust bild av den lite djupare berggrunden. Dessutom har kombinationen av reflektionsseismik och MT visat sig vara ett kraftfullt verktyg för hypotesprövning och för att utveckla den allmänna förståelsen av Skelleftefältet och dess historia.

Därutöver presenteras två 3D inversionsmodeller av MT data och jämförs sedan med resultaten från 2D determinantinversioner. 3D tekniker visar betydande förbättringar av datapassform och begränsar observerade anomalier bättre. Även om 3D inversion av MT data ännu inte är en vanlig teknik och frågor som modellbedömning och galvaniska distorsionseffekter måste behandlas ytterligare, är resultat från komplexa miljöer med lagom yttäckning redan överlägsna.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. , 46 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 946
Keyword [en]
Magnetotellurics, 2D inversion, 3D inversion, hard rock seismics, mineral exploration, integrated geophysics, 4D modeling
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
URN: urn:nbn:se:uu:diva-178060ISBN: 978-91-554-8409-5 (print)OAI: oai:DiVA.org:uu-178060DiVA: diva2:541954
Public defence
2012-09-07, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Projects
VINNOVA 4D modeling of the Skellefte District
Available from: 2012-08-17 Created: 2012-07-26 Last updated: 2013-01-22Bibliographically approved
List of papers
1. 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
2. MT and reflection seismics in northwestern Skellefte Ore District, Sweden
Open this publication in new window or tab >>MT and reflection seismics in northwestern Skellefte Ore District, Sweden
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2013 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 78, no 2, B65-B76 p.Article in journal (Refereed) Published
Abstract [en]

A seismic reflection and MT survey was carried out along a 27-km long transect in northwestern Skellefte District, as part of a bigger 3D modeling project. The main motivation for the data acquisition is to elucidate the geologic relationship between the known mineralizations in the Adak mining camp to the north and in the well studied Kristineberg area south of the transect. The seismic reflection data were acquired with a VIBSIST system, and show reflectivity down to 3 s. Apart from the conventional processing for crystalline environments, the seismic data was also subject to an azimuthal binning procedure and cross-dip analysis, allowing the orientation of planar reflectors in 3D. Regarding the MT data, it is primarily of good quality along the 17 installed sites. The inversion of the determinant of the impedance tensor yielded a stable 2D resistivity model, dominated by resistors corresponding to the postorogenic intrusions along the transect. Adding the location of the analyzed seismic reflectors in the MT inversion rendered an integrated model that facilitated a preliminary joint interpretation of the data sets. Overall, the results are in good agreement with surface observations and reveal a crude configuration of the geologic units below the transect. The most prominent outcomes are the lateral and depth extent of the large postorogenic intrusions in the area reaching to 5- or 6-km depth, the dimensions of the nearly vertical Brännäs gabbro extending to 6-km depth, and the presence of enhanced conductivities along the transect at about 10 km depth. The latter is probably related to the deep conductor previously identified in the district.

Place, publisher, year, edition, pages
Society of Exploration Geophysicists, 2013
Keyword
2D, magnetotelluric, mining, imaging, inversion
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-178056 (URN)10.1190/geo2012-0169.1 (DOI)000319858000002 ()
Projects
VINNOVA 4D modeling of the Skellefte District
Available from: 2012-07-26 Created: 2012-07-26 Last updated: 2017-12-07
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 and 3D MT in the central Skellefte Ore District, northern Sweden
Open this publication in new window or tab >>2D and 3D MT in the central Skellefte Ore District, northern Sweden
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(English)Manuscript (preprint) (Other academic)
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-178057 (URN)
Projects
VINNOVA 4D modeling of mineral belts
Available from: 2012-07-26 Created: 2012-07-26 Last updated: 2013-01-22

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