<p>Reflection seismic and seismic tomography methods can be used to image the upper kilometer of hard bedrock and the loose unconsolidated sediments covering it. Developments of these two methods and their application, as well as identifying issues concerning their usage, are the main focus of the thesis. Data used for this development were acquired at three different sites in Sweden, in Forsmark 140 km north of Stockholm, in the Oskarshamn area in southern Sweden, and in the northern part of the Siljan Ring impact crater area.</p><p>The reflection seismic data were acquired with long source-receiver offsets relative to some of the targeted depths to be imaged. In the initial processing standard steps were applied, but the uppermost part of the sections were not always clear. The longer offsets imply that pre-stack migration is necessary in order to image the uppermost bedrock as clearly as possible. Careful choice of filters and velocity functions improve the pre-stack migrated image, allowing better correlation with near-surface geological information.</p><p>The seismic tomography method has been enhanced to calculate, simultaneously with the velocity inversion, optimal corrections to the picked first break travel times in order to compensate for the delays due to the seismic waves passing through the loose sediments covering the bedrock.</p><p>The reflection seismic processing used in this thesis has produced high-quality images of the upper kilometers, and in one example from the Forsmark site, the image of the uppermost 250 meters of the bedrock has been improved. The three-dimensional orientation of reflections has been determined at the Oskarshamn site. Correlation with borehole data shows that many of these reflections originate from fracture zones. </p><p>The developed seismic tomography method produces high-detail velocity models for the site in the Siljan impact area and for the Forsmark site. In Forsmark, detailed estimates of the bedrock topography were calculated with the use of the developed tomography method.</p>

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In this work the magnetic and magnetotelluric (MT) geophysical methods have been used to produce two 3D models for the Laver area in southern Norrbotten. A new discovery of a porphyry copper deposit located at the Lill-Laverberget was reported by Boliden in 2012. The goal with the models is to form a better understanding of the setting for the porphyry deposit and to see if any of the geophysical methods are suited to be used in exploration for it. For starters an edge detector (Beiki, 2010) was used on magnetic data to identify structures and petrological contacts in the area. The magnetic data was modelled using the SimPEG (Cocket, et al., 2015) inversion software to create a 3D model over the area. The Magnetic method quite clearly managed to resolve the volcanic units in the Laver area. MT data was collected during fieldwork in September of 2018 and modelled using the MOD3DEM (Egbert &amp; Kelbert, 2012) inversion software. The MT model did not show any clear anomalies that could be related to the deposit. / I detta arbete så har de magnetiska och magnetotelluriska (MT) geofysiska metoderna används för att skapa två 3D modeller över Laverområdet i södra Norrbotten. I området finns en porfyrkopparfyndighet som Boliden rapporterade in 2012. Målet med båda modellerna är att fördjupa förståelsen för bildningsmiljön för fyndigheten och för att se hur lämpade de geofysiska metoderna är för prospektering. Det första steget var att använda befintliga magnetiska data och processa den med ED funktionen (Beiki, 2010) för att identifiera strukturer och bergartskontakter i området. Den magnetiska data användes för att producera en 3D modell med inversionsprogrammet SimPEG (Cocket, et al., 2015). Denna modell kunde skilja mellan vulkaniska och övriga bergarter i området. Magnetotelluriska data samlades in i september 2018 och modellerades med MOD3DEM (Egbert &amp; Kelbert, 2012), denna modell hade dock problem att finna anomalier i området.

A surface characterisation of the Arnager Greensand in south Sweden, a deep saline aquifer found to be suitable for geological storage of CO2 or energy storage, was undertaken in this thesis. Vintage seismic reflection data only available as scanned tiff images of the final stacked sections were digitized and processed using modern interpretation software to provide new insights into the morphology of the Arnager Greensand and to analyse the reservoir’s potential as an energy storage unit. The primary energy storage method discussed and evaluated was Compressed Air Energy Storage (CAES). This is a modern energy storage method developed as a tool for regulating inherently intermittent renewable energy sources. Such methods are key to the growth of the renewable sector and for providing a competitive alternative to fossil fuels. Moreover, in comparison with other energy storage methods such as battery storage, CAES is known to have strong potential to deliver highperformance energy storage at large scales for relatively low costs compared with any other solution. Previous studies conducted in the 1980’s by Swedegas produced a 2D isochrone surface map of the Arnager Greensand by hand interpolation methods utilizing analogue data collected by Oljeprospektering AB (OPAB, currently Svenska Petroleum). The Geological Survey of Sweden (SGU) has now transferred a vast amount of the historical OPAB dataset to modern digital format. This thesis contributes to those efforts and seeks to find new interpretations from the vintage data. A more comprehensive 3D model of the top of the Arnager Greensand employing the application of modern interpretation software was produced in this study. Strong similarities between morphology and dip-trend have been observed between the surface model generated in this report and the historical Swedegas isochrone surface map. Reservoir properties such as thickness, porosity and permeability gleaned from the earlier reports show the Arnager Greensand to exhibit excellent potential as a storage unit. Preliminary effective capacity estimates by Nordic CCS Competence centre show the Aranger Greensand to be one of the top three storage aquifers in Sweden. These positive appraisals highlight the need for a better characterization of the Aranger Greensand through data digitization and modern interpretation means. This thesis contributes to that endeavour.

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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.

The geological and geophysical knowledge about the northernmost part of Sweden has recently increased due to the Barents project, which includes acquisition of modern geophysical and geological information on behalf of the Swedish Geological Survey (SGU). During August 2013, a helicopter-borne versatile time domain electromagnetic (VTEM) survey was performed by Geotech Ltd, in the Soppero area northeast of Kiruna. From the VTEM measurements, a number of TEM anomalous zones have been identified and two of them are located south and southeast of the Lannavaara village. The main conductive features in the Lannavaara area can be explained by the presence of graphitic schist, which is spatially associated with a number of sulphide and iron oxide mineralisation occurrences. In this project, Maxwell thin sheet modelling and EM Flow conductivity-depth-imaging (CDI) software have been applied to selected anomalies in the Lannavaara area, for the purpose of extracting geometrical parameters of conductive features. This information has been used in order to confirm the structural framework of the area and evaluate the utility of VTEM measurements in this geological environment. In general, Maxwell thin sheet models of anomalies with small amplitudes show a better correlation with existing drill holes than models of anomalies with large amplitudes. The use of small amplitudes managed to confirm the structural model in the central part of the investigated area, which is an anticline. However, the use of different models and their distribution across the area is limited. Compared with Maxwell, CDIs from EM Flow provided a better way of confirming the general structural model in the area, although they include artefacts due to strong lateral gradients in conductivity. The Lannavaara area has also been investigated by VLF, Slingram and magnetic measurements and based on these data, multivariate analysis in SiroSOM reveals a strong correlation between VTEM and Slingram data, while VLF data appears to have much less or more complicated correlation with the other data sets. In summary, the results from the various software raise a question about the geological complexity in parts of the Lannavaara area, which may include multiple layers of graphitic schist, possibly expressed as smooth transitions in conductivity when represented by data from electromagnetic methods.

Boat-towed high-frequency, short source and receiver spacing, reflection seismic data, 16 profiles and in total 3884 m long, were acquired in 2008 for the planning of the Stockholm Bypass multilane (3 lanes in each direction in two different tubes) underground motorway tunnel designed to ease the ever increasing car traffic on the city and neighbouring regions. The planned bypass will be about 21 km long of which 18 km will be in the form of bedrock tunnel. The planned tunnel will intersect three water passages at where the tunnel will be at around 40-50 m depth. In this study, the seismic data along two of three water passages have been processed and interpreted. Due to the relatively shallow water depth (&lt; 10 m), the main challenge for the reflection data processing was the interferences of strong multiples from lake sediments and bedrock. After a number of tests, it was found that conventional processing methods could not attenuate multiples effectively. Therefore, an optimized workflow based on predictive deconvolution de-multiple method was developed. The new workflow proved to be effective at suppressing multiple reflections, while primary reflections as well as diffraction signals could be well preserved. After carefully attenuating the multiples in the pre-stack and post-stack domains, processing continued with time-to-depth conversion for data interpretations. To reduce uncertainty with time-to-depth conversion errors, bathymetry data available from the study area were used to match the water-sediment interface that also generated a clear reflection in the data. Bedrock surface shows strong undulations, which is typical for the Scandinavian geology from steep valleys to sometimes sub-horizontal at some parts of the water passages. Nevertheless, a dominant bedrock valley-type direction can be recognized striking in the same direction as the water passages. The planned tunnel at the nearest point is estimated to locate approximately 19 m below bedrock surface, which is important factor for the excavation planning of the tunnel and its reinforcements. The steep valley-shaped bedrock may also imply a zone of weakness, fault and/or fracture zone, where the water passages were formed and the reflection seismic data clearly depict this shape under the overlying saucer-shaped unconsolidated sediments.

The Main Ethiopian Rift is a part of the East African Rift, from where the African plate is being teared apart and separated from the Indian and the Arabian plate. Even though earthquakes in this area are relatively less frequent, the subsurface structure is a subject of big research interest, since information about the subsurface layers has considerable relevance when it comes to site amplication related to earthquakes. The aim of this project is to map and compare the subsurface structures of two sites located in the Ethiopian Rift, using seismic refraction technique. By looking at the first arrivals of artificial seismic waves on a designated site, the velocities as well as the thicknessof the subsurface layers can be obtained. The result showed that the both sites contained a low velocity structure which contained weathered material.