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Wang, S., Bastani, M., Constable, S., Kalscheuer, T. & Malehmir, A. (2019). Boat-towed radio-magnetotelluric and controlled source audio-magnetotelluric study to resolve fracture zones at Äspö Hard Rock Laboratory site, Sweden. Geophysical Journal International, 218(2), 1008-1031
Open this publication in new window or tab >>Boat-towed radio-magnetotelluric and controlled source audio-magnetotelluric study to resolve fracture zones at Äspö Hard Rock Laboratory site, Sweden
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2019 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 218, no 2, p. 1008-1031Article in journal (Refereed) Published
Abstract [en]

Boat-towed radio-magnetotelluric (RMT) measurements using signals between 14 and 250 kHz have attracted increasing attention in the near-surface applications for shallow water and archipelago areas. A few large-scale underground infrastructure projects, such as the Stockholm bypass in Sweden, are planned to pass underneath such water zones. However, in cases with high water salinity, RMT signals have a penetration depth of a few metres and do not reach the geological structures of interest in the underlying sediments and bedrock. To overcome this problem, controlled source signals at lower frequencies of 1.25 to 12.5 kHz can be utilized to improve the penetration depth and to enhance the resolution for modelling deeper underwater structures. Joint utilization of boat-towed RMT and controlled source audio-magnetotellurics (CSAMT) was tested for the first time at the Aspo Hard Rock Laboratory (HRL) site in south-eastern Sweden to demonstrate acquisition efficiency and improved resolution to model fracture zones along a 600-m long profile. Pronounced galvanic distortion effects observed in 1-D inversion models of the CSAMT data as well as the predominantly 2-D geological structures at this site motivated usage of 2-D inversion. Two standard academic inversion codes, EMILIA and MARE2DEM, were used to invert the RMT and CSAMT data. EMILIA, an object-oriented Gauss-Newton inversion code with modules for 2-D finite difference and 1-D semi-analytical solutions, was used to invert the RMT and CSAMT data separately and jointly under the plane-wave approximation for 2-D models. MARE2DEM, a Gauss-Newton inversion code for controlled source electromagnetic 2.5-D finite element solution, was modified to allow for inversions of RMT and CSAMT data accounting for source effects. Results of EMILIA and MARE2DEM reveal the previously known fracture zones in the models. The 2-D joint inversions of RMT and CSAMT data carried out with EMILIA and MARE2DEM show clear improvement compared with 2-D single inversions, especially in imaging uncertain fracture zones analysed in a previous study. Our results show that boat-towed RMT and CSAMT data acquisition systems can be utilized for detailed 2-D or 3-D surveys to characterize near-surface structures underneath shallow water areas. Potential future applications may include geo-engineering, geohazard investigations and mineral exploration.

Place, publisher, year, edition, pages
OXFORD UNIV PRESS, 2019
Keywords
Marine electromagnetics, Joint inversion, Fractures, faults and high strain deformation zones, Magnetotellurics, Electrical properties
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-390504 (URN)10.1093/gji/ggz162 (DOI)000474771100020 ()
Funder
Swedish Research Council Formas, 25220121907
Available from: 2019-08-12 Created: 2019-08-12 Last updated: 2019-08-12Bibliographically approved
Place, J., Draganov, D., Malehmir, A., Juhlin, C. & Wijns, C. (2019). Crosscoherence-based interferometry for the retrieval of first arrivals and subsequent tomographic imaging of differential weathering. Geophysics, 84(4), Q37-Q48
Open this publication in new window or tab >>Crosscoherence-based interferometry for the retrieval of first arrivals and subsequent tomographic imaging of differential weathering
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2019 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 84, no 4, p. Q37-Q48Article in journal (Refereed) Published
Abstract [en]

Exhumation of crust exposes rocks to weathering agents that weaken the rocks' mechanical strength. Weakened rocks will have lower seismic velocity than intact rocks and can therefore be mapped using seismic methods. However, if the rocks are heavily weathered, they will attenuate controlled-source seismic waves to such a degree that the recorded wavefield would become dominated by ambient noise and/or surface waves. Therefore, we have examined the structure of differential weathering by first-break traveltime tomography over a seismic profile extending approximately 3.5 km and acquired at a mining site in Zambia using explosive sources and a source based on the swept-impact seismic technique (SIST). Seismic interferometry has been tested for the retrieval of supervirtual first arrivals masked by uncorrelated noise. However, use of crosscorrelation in the retrieval process makes the method vulnerable to changes in the source signal (explosives and SIST). Thus, we have developed a crosscoherence-based seismic-interferometry method to tackle this shortcoming. We investigate the method's efficiency in retrieving first arrivals and, simultaneously, correctly handling variations in the source signal. Our results illustrate the superiority of the cross-coherence-over cross-correlation-based method for retrieval of the first arrivals, especially in alleviating spurious ringyness and in terms of the signal-to-noise ratio. These benefits are observable in the greater penetration depth and the improved resolution of the tomography sections. The tomographic images indicate isolated bodies of higher velocities, which may be interpreted as fresh rocks embedded into a heavily weathered regolith, providing a conspicuous example of differential weathering. Our study advances the potential of seismic methods for providing better images of the near surface (the critical zone).

Place, publisher, year, edition, pages
SOC EXPLORATION GEOPHYSICISTS, 2019
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-393753 (URN)10.1190/GEO2018-0405.1 (DOI)000480678900034 ()
Funder
EU, European Research Council, 775971
Available from: 2019-10-16 Created: 2019-10-16 Last updated: 2019-10-16Bibliographically approved
Buntin, S., Malehmir, A., Koyi, H., Högdahl, K., Malinowski, M., Larsson, S. A., . . . Gorszczyk, A. (2019). Emplacement and 3D geometry of crustal-scale saucer-shaped intrusions in the Fennoscandian Shield. Scientific Reports, 9, Article ID 10498.
Open this publication in new window or tab >>Emplacement and 3D geometry of crustal-scale saucer-shaped intrusions in the Fennoscandian Shield
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2019 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 9, article id 10498Article in journal (Refereed) Published
Abstract [en]

Saucer-shaped intrusions of tens of meters to tens of kilometres across have been observed both from surface geological mapping and geophysical observations. However, there is only one location where they have been reported to extend c. 100 km laterally, and emplaced both in a sedimentary basin and the crystalline basement down to 12 km depth. The legacy BABEL offshore seismic data, acquired over the central Fennoscandian Shield in 1989, have been recovered and reprocessed with the main goal of focusing on this series of globally unique crustal-scale saucer-shaped intrusions present onshore and offshore below the Bothnian Sea. The intrusions (c. 1.25 Ga), emplaced in an extensional setting, are observed within both sedimentary rocks (<1.5 Ga) and in the crystalline basement (>1.5 Ga). They have oval shapes with diameters ranging 30-100 km. The reprocessed seismic data provide evidence of up-doming of the lower crust (representing the melt reservoir) below the intrusions that, in turn, are observed at different depths in addition to a steep seismically transparent zone interpreted to be a discordant feeder dyke system. Relative age constraints and correlation with onshore saucer-shaped intrusions of different size suggest that they are internally connected and fed by each other from deeper to shallower levels. We argue for a nested emplacement mechanism and against a controlling role by the overlying sedimentary basin as the saucer-shaped intrusions are emplaced in both the sedimentary rocks as well as in the underlying crystalline basement. The interplay between magma pressure and overburden pressure, as well as the, at the time, ambient stress regime, are responsible for their extensive extent and rather constant thicknesses (c. 100-300 m). Saucer-shaped intrusions may therefore be present elsewhere in the crystalline basement to the same extent as observed in this study some of which are a significant source of raw materials.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2019
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-391379 (URN)10.1038/s41598-019-46837-x (DOI)000476468700026 ()31324841 (PubMedID)
Funder
Swedish Research Council, 2015-05177
Available from: 2019-09-03 Created: 2019-09-03 Last updated: 2019-09-03Bibliographically approved
Almqvist, B., Björk, A., Mattsson, H. B., Hedlund, D., Gunnarsson, K., Malehmir, A., . . . Marsden, P. (2019). Magnetic characterisation of magnetite and hematite from the Blötberget apatite-iron-oxide deposits (Bergslagen), south-central Sweden. Canadian journal of earth sciences (Print)
Open this publication in new window or tab >>Magnetic characterisation of magnetite and hematite from the Blötberget apatite-iron-oxide deposits (Bergslagen), south-central Sweden
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2019 (English)In: Canadian journal of earth sciences (Print), ISSN 0008-4077, E-ISSN 1480-3313Article in journal (Refereed) Epub ahead of print
Abstract [en]

Rock magnetic measurements were carried out on drill core material and hand specimens from the Blötberget apatite-iron oxide deposit in the Bergslagen ore province, south-central Sweden, to characterise their magnetic properties. Measurements included several kinds of magnetic susceptibility and hysteresis parameters. Petrographic and scanning electron microscopy (SEM) were used to independently identify and quantify the amount and type of magnetite and hematite. Two hematite-rich samples were studied with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to quantify the trace element chemistry in hematite and investigate the potential influence of trace elements on magnetic properties. Three aspects of this study are noteworthy. 1) Hematite-rich samples display strong anisotropy of magnetic susceptibility, which is likely to affect the appearance and modelling of magnetic anomalies. 2) The magnitude-drop in susceptibility across Curie and Néel temperature transitions show significant correlation with the respective weight percent (wt%) of magnetite and hematite. Temperature dependent magnetic susceptibility measurements can therefore be used to infer the amounts of both magnetite and hematite. 3) observations of a strongly depressed Morin transition at ca -60 to -70 C (200 to 210 K) are made during low-temperature susceptibility measurements. This anomalous Morin transition is most likely related to trace amounts of V and Ti that substitute for Fe in the hematite. When taken together, these magnetic observations improve the understanding of the magnetic anomaly signature of the Blötberget apatite-iron oxide deposits and may potentially be utilised in a broader context when assessing similar (Paleoproterozoic) apatite-iron oxide systems.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-374225 (URN)10.1139/cjes-2018-0183 (DOI)
Available from: 2019-01-18 Created: 2019-01-18 Last updated: 2019-04-17Bibliographically approved
Bellefleur, G., Cheraghi, S. & Malehmir, A. (2019). Reprocessing legacy three-dimensional seismic data from the Halfmile Lake and Brunswick No. 6 volcanogenic massive sulphide deposits, New Brunswick, Canada. Canadian journal of earth sciences (Print), 56(5), 569-583
Open this publication in new window or tab >>Reprocessing legacy three-dimensional seismic data from the Halfmile Lake and Brunswick No. 6 volcanogenic massive sulphide deposits, New Brunswick, Canada
2019 (English)In: Canadian journal of earth sciences (Print), ISSN 0008-4077, E-ISSN 1480-3313, Vol. 56, no 5, p. 569-583Article in journal (Refereed) Published
Abstract [en]

We reprocessed legacy three-dimensional (3D) seismic data from the Halfmile Lake and Brunswick areas, both of which were acquired for mineral exploration in the Bathurst Mining Camp, New Brunswick. Each 3D seismic survey was acquired over known volcanogenic massive sulphide deposits and covered areas with strong mineral potential. Most improvements resulted from a reduction of coherent and random noise on prestack gathers and from an improved velocity model, combined with re-imaging with dip moveout corrections and poststack migration or prestack time migration. At Halfmile Lake, the new imaging results show the Deep zone and a possible extension of the sulphide mineralization at greater depth. True amplitude processing has shown that this anomaly has strong amplitudes and is offset from the Deep zone by a shallowly dipping fault (<15 degrees). With the clearer geological context provided by our results, this anomaly, which appears as a stand-alone anomaly on an original image obtained by Noranda Exploration Ltd., becomes a defendable exploration target. Nonorthogonal acquisition geometry and receiver patches of the Brunswick No. 6 3D seismic survey generated artefacts after dip moveout processing that reduced the overall quality of the seismic volumes. By using a filtering approach based on the application of a weighted Laplacian-Gaussian filter in the Kx-Ky domain, we reduced the noise and improved the continuity of reflections. We also imaged the short and flat reflections observed previously only in the shallow part of prestack time migrated data. These short reflections appear as diffractions on the filtered stacked section with dip moveout corrections, indicating that they originate from small geological bodies or discontinuities in the subsurface.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-387296 (URN)10.1139/cjes-2018-0103 (DOI)000467696400011 ()
Available from: 2019-06-24 Created: 2019-06-24 Last updated: 2019-06-24Bibliographically approved
Mattsson, H. B., Högdahl, K., Carlsson, M. & Malehmir, A. (2019). The role of mafic dykes in the petrogenesis of the Archean Siilinjärvi carbonatite complex, east-Central Finland. Lithos, 342-343, 468-479
Open this publication in new window or tab >>The role of mafic dykes in the petrogenesis of the Archean Siilinjärvi carbonatite complex, east-Central Finland
2019 (English)In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 342-343, p. 468-479Article in journal (Refereed) Published
Abstract [en]

The Archean (~2.6 Ga) Siilinjärvi carbonatite complex in east-central Finland is crosscut by a few ultramafic lamprophyre dykes, together with a broad array of more evolved mafic dykes that range in composition from foidites to various types of alkali basalts. A possible genetic link between the primitive lamprophyres and the carbonatite complex has previously been hypothesised, but their exact relations have been unclear due to the regional metamorphic overprint (i.e., greenschist facies). Here we focus on the petrology and petrography of the mafic dykes, and integrate the data to present a coherent model that can explain the genesis of the Siilinjärvi carbonatite complex. Field-relations, in combination with petrography and geochemistry, indicate that there are at least three generations of mafic dykes present. The oldest dykes (Generation I) are strongly deformed, and inferred to have been emplaced shortly after the formation of the complex itself. These dykes can be divided into two groups (i.e., ultramafic lamprophyres and Group A), where Group A comprises foidites characterised by low SiO2 (41.4–51.5 wt%) and high alkali (>10 wt% K2O) content. We interpret the foiditic magmas to have evolved from primitive ultramafic lamprophyres by fractionating a clinopyroxene-olivine dominated mineral assemblage that was devoid of feldspar. This fractionation path forced alkali-enrichment in the magmas belonging to Group A, which pushed them into the miscibility gap, and resulted in liquid immiscibility that produced moderately alkaline conjugate carbonatite(s). Subsequent fractionation of the conjugate carbonatite by predominantly calcite and apatite produced the mineralogically homogeneous carbonatite cumulate that is exposed at Siilinjärvi. Younger, less deformed, mafic dykes (belonging to Generations II and III) exhibit trace element characteristics, broadly similar to basaltic dyke swarms in the region. The younger dykes are characterised by the presence of large plagioclase crystals in thin sections. Crystallisation of a feldspar-bearing mineral assemblage resulted in only moderate enrichment of alkalis with increased fractionation, which caused the younger dykes to evolve along the more common basalt-to-trachyte series. Thus, the magmas belonging to Generations II and III at Siilinjärvi never fulfilled the conditions required to produce carbonatites by liquid immiscibility.

Keywords
Siilinjarvi, Carbonatite, Ultramafic lamprohyre, Archean, Fractional crystallisation, Liquid immiscibility
National Category
Geochemistry
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-385377 (URN)10.1016/j.lithos.2019.06.011 (DOI)000477091500030 ()
Funder
Vinnova, 2014–06238Swedish Research Council, 2018-03702The Geological Survey of Sweden (SGU), 311-1485/2017
Available from: 2019-06-13 Created: 2019-06-13 Last updated: 2019-09-20Bibliographically approved
Muhamad, H., Juhlin, C., Malehmir, A. & Sopher, D. (2018). Integrated interpretation of geophysical data of the Paleozoic structure in the northwestern part of the Siljan Ring impact crater, central Sweden. Journal of Applied Geophysics, 148, 201-215
Open this publication in new window or tab >>Integrated interpretation of geophysical data of the Paleozoic structure in the northwestern part of the Siljan Ring impact crater, central Sweden
2018 (English)In: Journal of Applied Geophysics, ISSN 0926-9851, E-ISSN 1879-1859, Vol. 148, p. 201-215Article in journal (Refereed) Published
Abstract [en]

The Siljan Ring impact structure is the largest known impact structure in Europe and is Late Devonian in age. It contains a central uplift that is about 20-30 km in diameter and is surrounded by a ring-shaped depression. The Siljan area is one of the few areas in Sweden where the Paleozoic sequence has not been completely eroded, making it an important location for investigation of the geological and tectonic history of Baltica during the Paleozoic. The Paleozoic strata in this area also provide insight into the complex deformation processes associated with the impact. In this study we focus on the northwestern part of the Siljan Ring, close to the town of Orsa, with the main objective of characterizing the subsurface Paleozoic succession and uppermost Precambrian crystalline rocks along a series of seismic reflection profiles, some of which have not previously been published. We combine these seismic data with gravity and magnetic data and seismic traveltime tomography results to produce an integrated interpretation of the subsurface in the area. Our interpretation shows that the Paleozoic sequence in this area is of a relatively constant thickness, with a total thickness typically between 300 and 500 m. Faulting appears to be predominantly extensional, which we interpret to have occurred during the modification stage of the impact. Furthermore, based on the geophysical data in this area, we interpret that the impact related deformation to differ in magnitude and style from other parts of the Siljan Ring.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-311546 (URN)10.1016/j.jappgeo.2017.10.001 (DOI)000424171900019 ()
Available from: 2016-12-28 Created: 2016-12-28 Last updated: 2018-03-28Bibliographically approved
Andersson, M. & Malehmir, A. (2018). Internal architecture of the Alnö alkaline and carbonatite complex (central Sweden) revealed using 3D models of gravity and magnetic data. Tectonophysics, 740-741, 53-71
Open this publication in new window or tab >>Internal architecture of the Alnö alkaline and carbonatite complex (central Sweden) revealed using 3D models of gravity and magnetic data
2018 (English)In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 740-741, p. 53-71Article in journal (Refereed) Published
Abstract [en]

The Alnö Complex in central Sweden is one of the largest alkaline and carbonatite ring-shaped intrusions in the world. Presented here is the 3D models of ground gravity and aeromagnetic data that confirm some of the previous ideas about the 3D geometry of the complex but also suggest that the complex may continue laterally further to north than previously expected. The data show the complex as (i) a strong positive Bouguer anomaly, around 20 mGal, and (ii) a strong positive magnetic anomaly, exceeding 2000 nT. Magnetic structures are clearly discernible within the complex and surrounding area. Both gravity and magnetic inversion models suggest that dense (> 2850 kg/m(3)) and magnetic ( > 0.05 SI) rocks extend down to about 3.5-4 km depth. Previous studies have suggested a solidified magma reservoir at this approximate depth. The inversion models further suggest that two apparently separate regions within the complex are likely connected at depth, starting from 800 to 1000 m, implying a common source for the rocks observed in these two regions. Modelling of the aeromagnetic data indicates that a > 3 km wide ring-shaped magnetic high situated in the sea north of Alnö Island may be a part of the complex. This could link a smaller satellite intrusion in Soraker on mainland to the larger intrusion on Alnö Island. While the rim of the ring must consist of highly magnetic rocks to support the anomaly, the centre has relatively low magnetisation and is probably made up of low-magnetic wall-rocks or metasomatised wall-rocks down to about 2 km depth. Below this depth the 3D susceptibility model suggests higher magnetic susceptibility values. Worldwide alkaline and carbonatite complexes are the main resources for rare earth elements (REEs), and owing to the size of the Alnö Complex, it can be highly prospective for REEs at depth.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2018
Keywords
Carbonatite, Volcanism, Gravity, Magnetic, 3D modelling, Rare earth elements
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-359865 (URN)10.1016/j.tecto.2018.05.008 (DOI)000436218900005 ()
Funder
Swedish Research Council, 621-2009-4439
Available from: 2018-09-07 Created: 2018-09-07 Last updated: 2018-09-07Bibliographically approved
Wang, S., Kalscheuer, T., Bastani, M., Malehmir, A., Pedersen, L. B., Dahlin, T. & Naser, M. (2018). Joint inversion of lake-floor electrical resistivity tomography and boat-towed radio-magnetotelluric data illustrated on synthetic data and an application from the Äspö Hard Rock Laboratory site, Sweden. Geophysical Journal International, 213(1), 511-533
Open this publication in new window or tab >>Joint inversion of lake-floor electrical resistivity tomography and boat-towed radio-magnetotelluric data illustrated on synthetic data and an application from the Äspö Hard Rock Laboratory site, Sweden
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2018 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 213, no 1, p. 511-533Article in journal (Refereed) Published
Abstract [en]

The electrical resistivity tomography (ERT) method provides moderately good constraints for both conductive and resistive structures, while the radio-magnetotelluric (RMT) method is well suited to constrain conductive structures. Additionally, RMT and ERT data may have different target coverage and are differently affected by various types of noise. Hence, joint inversion of RMT and ERT data sets may provide a better constrained model as compared to individual inversions. In this study, joint inversion of boat-towed RMT and lake-floor ERT data has for the first time been formulated and implemented. The implementation was tested on both synthetic and field data sets incorporating RMT transverse electrical mode and ERT data. Results from synthetic data demonstrate that the joint inversion yields models with better resolution compared with individual inversions. A case study from an area adjacent to the Aspo Hard Rock Laboratory (HRL) in southeastern Sweden was used to demonstrate the implementation of the method. A 790-m-long profile comprising lake-floor ERT and boat-towed RMT data combined with partial land data was used for this purpose. Joint inversions with and without weighting (applied to different data sets, vertical and horizontal model smoothness) as well as constrained joint inversions incorporating bathymetry data and water resistivity measurements were performed. The resulting models delineate subsurface structures such as a major northeasterly directed fracture system, which is observed in the HRL facility underground and confirmed by boreholes. A previously uncertain weakness zone, likely a fracture system in the northern part of the profile, is inferred in this study. The fractures are highly saturated with saline water, which make them good targets of resistivity-based geophysical methods. Nevertheless, conductive sediments overlain by the lake water add further difficulty to resolve these deep fracture zones. Therefore, the joint inversion of RMT and ERT data particularly helps to improve the resolution of the resistivity models in areas where the profile traverses shallow water and land sections. Our modification of the joint inversion of RMT and ERT data improves the study of geological units underneath shallow water bodies where underground infrastructures are planned. Thus, it allows better planning and mitigating the risks and costs associated with conductive weakness zones.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-327094 (URN)10.1093/gji/ggx414 (DOI)000448715000035 ()
Funder
Swedish Research Council Formas, 25220121907
Available from: 2017-08-02 Created: 2017-08-02 Last updated: 2019-01-18Bibliographically approved
Malehmir, A., Bergman, B., Andersson, B., Sturk, R. & Johansson, M. (2018). Seismic imaging of dyke swarms within the Sorgenfrei-Tornquist Zone (Sweden) and implications for thermal energy storage. Solid Earth, 9(6), 1469-1485
Open this publication in new window or tab >>Seismic imaging of dyke swarms within the Sorgenfrei-Tornquist Zone (Sweden) and implications for thermal energy storage
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2018 (English)In: Solid Earth, ISSN 1869-9510, E-ISSN 1869-9529, Vol. 9, no 6, p. 1469-1485Article in journal (Refereed) Published
Abstract [en]

There is a great interest and demand for green-type energy storage in Sweden for both short-and longterm (hours, days, weeks and seasons) periods. While there are a number of approaches proposed (e.g., compressed air, geothermal and thermal), only a few have commercially been demonstrated through upscaling projects. Among these, the thermal energy storage (TES) that stores energy (excess heat or cold) in fluids is particularly interesting. The excess energy can be stored underground in excavated caverns and used for large district heating and cooling purposes as well as for balancing and regulating electrical energy in power grids. For an upscaling underground TES project within the Tornquist suture zone of Scania in the southwest of Sweden, three high-resolution seismic profiles, each approximately 1 km long, were acquired. Geologically, the site sits within the southern margin of the Romeleasen fault zone in the Sorgenfrei-Tornquist Zone (STZ), where dolerite dyke swarms of Carboniferous-Permian age are observed striking in the SE-NW direction for hundreds of kilometers both on land and in offshore seismic and magnetic data (from Scania to Midland Valley in the UK). These dykes, 10-50 m thick, in the nearby quarries (within both Precambrian gneiss and quartzite) express themselves mostly in a subvertical manner. They can therefore act as a good water/fluid barrier, which can be an important geological factor for any TES site. For the data acquisition, combined cabled and wireless recorders were used to provide continuity on both sides of a major road running in the middle of the study area. Bedrock depressions are clearly depicted in the tomograms, suggesting the possibility of zones of weaknesses, highly fractured and/or weathered, in the bedrock and confirmed in several places by follow-up boreholes. Several steeply dipping (60-65 degrees) reflections were imaged down to 400 m depth and interpreted to originate from dolerite dykes. This interpretation is based on their orientations, strong amplitudes, regular occurrences and correlation with downhole logging data. In addition, groundwater flow measurements within the unconsolidated sediments and in bedrock suggest steeply dipping structures are the dominant factor in directing water mainly along a SE-NW trend, which is consistent with the strike of the dyke swarm within the STZ. To provide further insight on the origin of the reflections, even the historical crustalscale offshore BABEL (Baltic and Bothnian Echoes from the Lithosphere) lines (A-AA-AB) were revisited. Clear multiphase faults and signs of intrusions or melt source in the lower crust are observed, as well as a Moho step across the Tornquist zone. Overall, we favor that the reflections are of dolerite origin and their dip component (i.e., not subvertical) may imply a Precambrian basement (and dykes) tilting, block rotation, towards the NE as a result of the Romeleasen reverse faulting. In terms of thermal storage, these dykes then may be encountered during the excavation of the site and can complicate underground water flow should they be used as a fluid barrier in case of leakage.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-372927 (URN)10.5194/se-9-1469-2018 (DOI)000453320000001 ()
Funder
Swedish Research Council Formas, 2012-1907Swedish Research Council, 2015-05177
Available from: 2019-01-09 Created: 2019-01-09 Last updated: 2019-01-09Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-1241-2988

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