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MT and reflection seismics in northwestern Skellefte Ore District, 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.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Geological Survey of 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. Vol. 78, no 2, B65-B76 p.
Keyword [en]
2D, magnetotelluric, mining, imaging, inversion
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
URN: urn:nbn:se:uu:diva-178056DOI: 10.1190/geo2012-0169.1ISI: 000319858000002OAI: oai:DiVA.org:uu-178056DiVA: diva2:541935
Projects
VINNOVA 4D modeling of the Skellefte District
Available from: 2012-07-26 Created: 2012-07-26 Last updated: 2017-12-07
In thesis
1. Seismics, 2D and 3D Inversion of Magnetotellurics: Jigsaw pieces in understanding the Skellefte Ore District
Open this publication in new window or tab >>Seismics, 2D and 3D Inversion of Magnetotellurics: Jigsaw pieces in understanding the Skellefte Ore District
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
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:nbn:se:uu:diva-178060 (URN)978-91-554-8409-5 (ISBN)
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

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