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Publications (10 of 127) Show all publications
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
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
Brodic, B., Malehmir, A., Pugin, A. & Maries, G. (2018). Three-component seismic land streamer study of an esker architecture through S- and surface-wave imaging. Geophysics, 83(6), B339-B353
Open this publication in new window or tab >>Three-component seismic land streamer study of an esker architecture through S- and surface-wave imaging
2018 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 83, no 6, p. B339-B353Article in journal (Refereed) Published
Abstract [en]

We deployed a newly developed 3C microelectromechanical system-based seismic land streamer over porous glacial sediments to delineate water table and bedrock in Southwestern Finland. The seismic source used was a 500 kg vertical impact drop hammer. We analyzed the SH-wave component and interpreted it together with previously analyzed P-wave component data. In addition to this, we examined the land streamer’s potential for multichannel analysis of surface waves and delineated the site’s stratigraphy with surface-wave-derived S-wave velocities and VP∕VS ratios along the entire profile. These S-wave velocities and VP∕VS ratios complement the interpretation conducted previously on P-wave stacked section. Peculiarly, although the seismic source used is of a vertical-type nature, the data inspection indicated clear bedrock reflection on the horizontal components, particularly the transverse component. This observation led us to scrutinize the horizontal component data through side-by-side inspection of the shot records of all the three components and particle motion analysis to confirm the S-wave nature of the reflection. Using the apparent moveout velocity of the reflection, as well as the known depth to bedrock based on drilling, we used finite-difference synthetic modeling to further verify its nature. Compared with the P-wave seismic section, bedrock is relatively well delineated on the transverse component S-wave section. Some structures connected to the kettle holes and other stratigraphic units imaged on the P-wave results were also notable on the S-wave section, and particularly on the surface-wave derived S-wave velocity model and VP∕VS ratios. Our results indicate that P-, SV-, and SH-wave energy is generated simultaneously at the source location itself. This study demonstrates the potential of 3C seismic for characterization and delineation of the near-surface seismics.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-366908 (URN)10.1190/GEO2017-0747.1 (DOI)000457502000033 ()
Funder
Swedish Research Council Formas, 252-2012-1907
Available from: 2018-11-26 Created: 2018-11-26 Last updated: 2019-03-18Bibliographically approved
Malehmir, A., Tryggvason, A., Wijns, C., Koivisto, E., Lindqvist, T., Skyttä, P. & Montonen, M. (2018). Why 3D seismic data are an asset for exploration and mine planning?: Velocity tomography of weakness zones in the Kevitsa Ni-Cu-PGE mine, northern Finland. Geophysics, 83(2), B33-B46
Open this publication in new window or tab >>Why 3D seismic data are an asset for exploration and mine planning?: Velocity tomography of weakness zones in the Kevitsa Ni-Cu-PGE mine, northern Finland
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2018 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 83, no 2, p. B33-B46Article in journal (Refereed) Published
Abstract [en]

Kevitsa is a disseminated Ni-Cu-PGE (platinum group elements) ore body in northern Finland, hosted by an extremely high-velocity (6.5-8.5 km/s) ultramafic intrusion. It is currently being mined at a depth of approximately 100 m with open-pit mining. The estimated mine life is 20 years, with the final pit reaching a depth of 500-600 m. Based on a series of 2D seismic surveys and given the expected mine life, a high-resolution 3D seismic survey was justified and conducted in the winter of 2010. We evaluate earlier 3D reflection data processing results and complement that by the results of 3D first-arrival traveltime tomography. The combined results provide insights on the nature of some of the reflectors within the intrusion. In particular, a major discontinuity, a weakness zone, is delineated in the tomography results on the northern side of the planned pit. Supported by the reflection data, we estimate the discontinuity, likely a thrust sheet, to extend down approximately 600 m and laterally 1000 m. The weakness zone terminates prominent internal reflectivity of the Kevitsa intrusion, and it is associated with the extent of the economic mineralization. Together with other weakness zones, a couple of which are also revealed by the tomography study, the discontinuity forms a major wedge block that influences the mine bench stability on the northern side of the open pit and likely will cause more issues during the extraction of the ore in this part of the mine. We argue that 3D seismic data should routinely be acquired prior to commencement of mining activities to maximize exploration efficiency at depth and also to optimize mining as it continues toward depth. Three-dimensional seismic data over mineral exploration areas are valuable and can be revisited for different purposes but are difficult to impossible to acquire after mining has commenced.

Place, publisher, year, edition, pages
SOC EXPLORATION GEOPHYSICISTS, 2018
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-364483 (URN)10.1190/GEO2017-0225.1 (DOI)000443595400043 ()
Funder
Swedish Research Council Formas, 25220121907VINNOVA, 2014-06238
Available from: 2018-10-29 Created: 2018-10-29 Last updated: 2018-10-29Bibliographically approved
Brodic, B., Malehmir, A. & Juhlin, C. (2017). Bedrock and Fracture Zone Delineation UsingDifferent Near-surface Seismic Sources. In: : . Paper presented at Near Surface Geoscience 3-7 September 2017, Malmö, Sweden. Amsterdam, Netherlands: European Association of Geoscientists and Engineers (EAGE)
Open this publication in new window or tab >>Bedrock and Fracture Zone Delineation UsingDifferent Near-surface Seismic Sources
2017 (English)Conference paper, Published paper (Other academic)
Abstract [en]

To delineate the bedrock surface and a fracture zone intersected by a well at c. 50 m depth, a seismic survey wasconducted using four different near-surface seismic sources. These were a 5-kg sledgehammer, a metal I-beamhit laterally, an accelerated weight drop and a prototype source tested for the first time called Udarnik. TheUdarnik source has two hammers whose impacts are initiated by an electromagnetic force of the stable coilexciting its inner moving part. Two hammers separated by a distance of approximately 50 cm successively hittwo contact plates mounted on the bottom of the source. The sweep length is adjustable and maximum 18 hitscan be made per second. In this study, we compare the performance of every source used and present reflectionseismic sections and tomography results from the high-fold (star-type acquisition was used) combinedlandstreamer and wireless recorder survey. Preliminary results indicate that bedrock was well delineated both ontomography results and stacked sections for all sources and some weak reflectivity is observed where thefracture zone is expected with most of the sources used showing the potential of the seismic methods forfracture zone imaging and near-surface characterization

Place, publisher, year, edition, pages
Amsterdam, Netherlands: European Association of Geoscientists and Engineers (EAGE), 2017
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-327950 (URN)10.3997/2214-4609.201702068 (DOI)
Conference
Near Surface Geoscience 3-7 September 2017, Malmö, Sweden
Projects
TRUST Geoinfra
Available from: 2017-08-14 Created: 2017-08-14 Last updated: 2017-08-22Bibliographically approved
Wang, S., Bastani, M., Kalscheuer, T., Malehmir, A. & Dynesius, L. (2017). Controlled Source Boat-towed Radio-magnetotellurics for Site Investigation at Äspö Hard Rock Laboratory, Southeastern Sweden. In: : . Paper presented at 79th EAGE Conference and Exhibition, 12–15 June 2017, Paris, France (pp. 1-5).
Open this publication in new window or tab >>Controlled Source Boat-towed Radio-magnetotellurics for Site Investigation at Äspö Hard Rock Laboratory, Southeastern Sweden
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2017 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The radio-magnetotelluric (RMT) method has traditionally been used for land investigations. However, with the development of the boat-towed RMT system, this method is used on shallow water. The lowest frequency of the RMT method is about 14 kHz and in addition water resistivity is quite low in some cases, therefore controlled source measurements is naturally considered for data acquisition. In order to resolve a fracture zone under a brackish water body, the controlled source boat-towed RMT (CSRMT) approach was tested. CSRMT and RMT one-dimensional inversions were carried out separately to analyze galvanic distortions and source effects in our dataset. Serious distortions observed in both inversions as well as the two-dimensional (2D) structure observed in our previous study made us consider 2D inversion for modeling the data. Due to the sufficiently large distance between transmitter and receivers, the CSRMT data were inverted using a 2D inversion code originally designed for plane-wave RMT data. Occam and damped Occam schemes were used in our 2D inversions for CSRMT and RMT data. The results show that CSRMT can better resolve the fracture zone than RMT. This study further illustrates the use of the boat-towed RMT system and particularly when combined with controlled source.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-325421 (URN)10.3997/2214-4609.201700565 (DOI)
Conference
79th EAGE Conference and Exhibition, 12–15 June 2017, Paris, France
Available from: 2017-06-23 Created: 2017-06-23 Last updated: 2018-06-04Bibliographically approved
Mehta, S., Bastani, M., Malehmir, A. & Pedersen, L. B. (2017). CSRMT Survey on Frozen Lake - A New Technique with an Example from the Stockholm Bypass Tunnel. In: : . Paper presented at 23rd European Meeting of Environmental and Engineering Geophysics, 3-7 September 2017, Malmö, Sweden.
Open this publication in new window or tab >>CSRMT Survey on Frozen Lake - A New Technique with an Example from the Stockholm Bypass Tunnel
2017 (English)Conference paper, Oral presentation only (Refereed)
Abstract [en]

More than 7% of the Scandinavian landmass is covered with fresh-water bodies in the form of lakes and rivers. This poses a unique challenge to carry out electromagnetic survey on shallow-water bodies for various purposes for example geotechnical investigations. Recently boat-towed RMT (radio-magnetotelluric) technique was introduced and used for measurements over the Lake Mälaren in Stockholm, Sweden. The RMT covers a wide range of frequencies (10-250 kHz) and provides good resolution for shallow subsurface studies although it lacks resolution at greater depths. Using controlled-source frequencies in the range of 1-10 kHz sufficient penetration depths can be achieved for most of the near surface targets. In this study, we present the results from the combined use of controlled-source and RMT (CSRMT) data that were obtained over frozen Lake Mälaren. The objective of this study was to map bedrock surface and fractures in the middle of the profile where using only RMT data these were not adequate. We demonstrate a new technique where CSRMT surveys were carried out over frozen-shallow-water bodies and we expect the idea to be used in the near future for other applications where moderately-resistive water bodies are present. 

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-328045 (URN)10.3997/2214-4609.201702056 (DOI)
Conference
23rd European Meeting of Environmental and Engineering Geophysics, 3-7 September 2017, Malmö, Sweden
Funder
Swedish Research Council Formas, 25220121907
Available from: 2017-08-16 Created: 2017-08-16 Last updated: 2017-08-22Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-1241-2988

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