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. / 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. / VINNOVA 4D modeling of the Skellefte District

Audiomagnetotelluric measurements with (CSAMT) and without (AMT) a controlled source were performed near Gällivare and Kiruna in Swedish Lapland in order to retrieve representative conductivity models of the subsurfaces. Magnetotelluric transfer functions were gained from processed time series’ and subsequently inverted to generate the sought models successfully. Additionally, a strike angle analysis was performed to determine the dimension of the ground structures. That information was used to justify the approaches of 1D and 2D inversions of the data sets and to judge their applicability. In Kiruna, two profiles were installed. One profile is considered to be in line with the strike direction, the other profile was oriented rather orthogonal to the strike direction. In Gällivare, only one profile was installed orthogonally with respect to the strike direction. The strike analysis showed a preferentially 2-dimensional structure for Kiruna’s parallel profile. For the orthogonal oriented profiles from Kiruna and Gällivare, the analysis revealed a 2D (with distortions) to 3D dimension of the ground structures. For the AMT method, it was possible to generate 1-dimensional and 2-dimensional models. Regarding the CSAMT data, it was only possible to generate a 1D conductivitymodel for the subsurface. Due to a significant transmitter overprint, no undistorted start model for the 2-dimensional CSAMT data inversion could be produced. The models from Kiruna had a sufficient data quality and thus resulted in reliable 2D AMT resistivity models with, locally, 2 to 3 layers. However, in combination with the 1D models for AMT and CSAMT, a 3-layer structure was predicted, where a resistive layer is covered by a thin conductive layer and underlaid by a rather conductive basement. For Gällivare’s profile, the data quality was good such that for all inversion methods good results were achieved. The predicted 2-layer models were resolved for for depths between 10m and 10,000m and coincide with the at hand geological maps and cross sections.

In this thesis, four studies using different geophysical electromagnetic methods are presented. In the first study dealing with airborne measurements, the noise response due to the rotation of the aircraft and the aircraft itself as a metallic conductive body on the Earth's electromagnetic response in very low frequency and low frequency band was investigated. The magnetic fields are independent of the aircraft in the VLF band and part of the LF band. But at higher frequencies (above 100 kHz), the signals are more influenced by the aircraft. The aircraft also generates its own noise frequencies which are mixed with the radio transmitter signals. The second and third studies are applications of radio-, controlled source-magnetotellurics and electrical resistivity tomography methods at a quick-clay landslide site in southwest Sweden. The data are processed and modeled in 2D and 3D, and the models are compared with high-resolution seismic and geotechnical data. The obtained results were further validated and refined by performing synthetic tests in the second study. The third study shows that the 3D models provide larger and more continuous volume of the quick clay structure than traditional 2D models. Both studies have shown that integrated application of geophysical methods for landslides is ideal. Quick clays often overlie the coarse-grained layers showing an increase of resistivity values in the models. In the fourth study, a new audio magnetotelluric data acquisition technique is developed and is named moving magnetotellurics (MMT). In this new technique, the magnetic sensors are placed on the ground and only 15 to 20 minutes data are acquired for each station, which usually is enough to cover the frequency range 30-300 Hz. The new technique is more efficient and convenient than the traditional magnetotelluric method, and test measurements have shown that it is an applicable method in shallow depth studies.

Nuclear magnetic resonance (NMR) has become an effective borehole measurement option to assess petrophysical and fluid properties of porous and permeable rocks. In the case of fluid typing, two-dimensional (2D) NMR interpretation techniques have advantages over conventional one-dimensional (1D) interpretation as they provide additional discriminatory information about saturating fluids and their properties. However, often there is ambiguity as to whether fluids detected with NMR measurements are mobile or residual. In some instances, rapid vertical variations of rock properties (e.g. across thinly-bedded formations) can make it difficult to separate NMR fluid signatures from those due to pore-size distributions. There are also cases where conventional fluid identification methods based on resistivity and nuclear logs indicate dominant presence of water while NMR measurements indicate presence of water, hydrocarbon, and mud filtrate. In such cases, it is important to ascertain whether existing hydrocarbons are residual or mobile. The radial lengths of investigation of resistivity, nuclear, and NMR measurements are very different, with NMR measurements being the shallowest sensing. Even in the case of several radial zones of NMR response attributed to different acquisition frequencies and DC magnetic field gradients, the measured signal originates from a fairly shallow radial zone compared to that of nuclear and resistivity logs. Depending on drilling mud being used and the radial extent of mud-filtrate invasion, the NMR response of virgin reservoir fluids can be masked by mud filtrate because of fluid displacement and mixing. In order to separate those effects, it is important to reconcile NMR measurements with electrical and nuclear logs for improved assessment of porosity and mobile hydrocarbon saturation. Previously, Voss et al. (2009) and Gandhi et al. (2010) introduced the concept of Common Stratigraphic Framework (CSF) to construct and validate multi-layer static and dynamic petrophysical models based on the numerical simulation of well logs. In this thesis, the concept of CSF is implemented to reconcile 2D NMR interpretations with multi-layer static and dynamic petrophysical models. It is found that quantifying the exact radial zone of response and corresponding fluid saturations can only be accomplished with studies of mud-filtrate invasion that honor available resistivity and nuclear logs. This thesis indicates that the two interpretation methods complement each other and when applied in conjunction, improve and refine the overall petrophysical understanding of permeable rock formations. Examples of successful application include field data acquired in thinly-bedded gas formations invaded with water-base mud, where bed-boundary effects are significant and residual hydrocarbon saturation is relatively high. In such cases, numerical simulation of mud-filtrate invasion and well logs acquired after invasion enables reliable interpretations of petrophysical and fluid properties. The interpretation procedure introduced in this thesis also provides an explicit way to determine the uncertainty of petrophysical and fluid interpretations. / text