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Junno, N., Koivisto, E., Kukkonen, I., Malehmir, A., Wijns, C. & Montonen, M. (2020). Data mining of petrophysical and lithogeochemical borehole data to elucidate the origin of seismic reflectivity within the Kevitsa Ni-Cu-PGE bearing intrusion, northern Finland. Geophysical Prospecting, 68(1), 82-102
Open this publication in new window or tab >>Data mining of petrophysical and lithogeochemical borehole data to elucidate the origin of seismic reflectivity within the Kevitsa Ni-Cu-PGE bearing intrusion, northern Finland
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2020 (English)In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 68, no 1, p. 82-102Article in journal (Refereed) Published
Abstract [en]

The Kevitsa mafic-ultramafic intrusion, located within the Central Lapland Greenstone Belt in northern Finland, hosts a large, disseminated Ni-Cu-PGE sulphide deposit. A three-dimensional seismic reflection survey was conducted over the Kevitsa intrusion in 2010 primarily for open-pit mine planning and for deep mineral exploration purposes. In the Kevitsa three-dimensional seismic data, laterally continuous reflections are observed within a constrained region within the intrusion. In earlier studies, it has been suggested that this internal reflectivity mainly originates from contacts between the tops and more sulphide-rich bottoms of smaller scale, internally differentiated magma layers that represent a spectrum of olivine pyroxenites. However, this interpretation is not unequivocally supported by the borehole data. In this study, data mining, namely the Self-Organizing Map analysis, of extensive Kevitsa borehole data is used to investigate the possible causes for the observed internal reflectivity within the Kevitsa intrusion. Modelling of the effect of mineralization and alteration on the reflectivity properties of Kevitsa rock types, based on average modal compositions of the rock types, is presented to support the results of the Self-Organizing Map analysis. Based on the results, we suggest that the seismic reflectivity observed within the Kevitsa intrusion can possibly be attributed to alteration, and may also be linked to the presence of sulphide minerals.

Place, publisher, year, edition, pages
WILEY, 2020
Keywords
Borehole geophysics, Petrophysics, Seismics, Interpretation
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-406199 (URN)10.1111/1365-2478.12907 (DOI)000505379700006 ()
Available from: 2020-03-06 Created: 2020-03-06 Last updated: 2020-03-06Bibliographically approved
Malehmir, A., Manzi, M., Draganov, D., Weckmann, U. & Auken, E. (2020). Introduction to the special issue on "Cost-effective and innovative mineral exploration solutions". Geophysical Prospecting, 68(1), 3-6
Open this publication in new window or tab >>Introduction to the special issue on "Cost-effective and innovative mineral exploration solutions"
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2020 (English)In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 68, no 1, p. 3-6Article in journal, Editorial material (Other academic) Published
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-407172 (URN)10.1111/1365-2478.12915 (DOI)000505379700001 ()
Available from: 2020-03-23 Created: 2020-03-23 Last updated: 2020-03-23Bibliographically approved
Papadopoulou, M., Col, F., Mi, B., Backstrom, E., Marsden, P., Brodic, B., . . . Socco, L. V. (2020). Surface-wave analysis for static corrections in mineral exploration: A case study from central Sweden. Geophysical Prospecting, 68(1), 214-231
Open this publication in new window or tab >>Surface-wave analysis for static corrections in mineral exploration: A case study from central Sweden
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2020 (English)In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 68, no 1, p. 214-231Article in journal (Refereed) Published
Abstract [en]

In mineral exploration, increased interest towards deeper mineralizations makes seismic methods attractive. One of the critical steps in seismic processing workflows is the static correction, which is applied to correct the effect of the shallow, highly heterogeneous subsurface layers, and improve the imaging of deeper targets. We showed an effective approach to estimate the statics, based on the analysis of surface waves (groundroll) contained in the seismic reflection data, and we applied it to a legacy seismic line acquired at the iron-oxide mining site of Ludvika in Sweden. We applied surface-wave methods that were originally developed for hydrocarbon exploration, modified as a step-by-step workflow to suit the different geologic context of hardrock sites. The workflow starts with the detection of sharp lateral variations in the subsurface, the existence of which is common at hard-rock sites. Their location is subsequently used, to ensure that the dispersion curves extracted from the data are not affected by strong lateral variations of the subsurface properties. The dispersion curves are picked automatically, windowing the data and applying a wavefield transform. A pseudo-2D time-average S-wave velocity and time-average P-wave velocity profile are obtained directly from the dispersion curves, after inverting only a reference curve. The time-average P-wave velocity profile is then used for the direct estimation of the one-way traveltime, which provides the static corrections. The resulting P-wave statics from the field data were compared with statics computed through conventional P-wave tomography. Their difference was mostly negligible with more than 91% of the estimations being in agreement with the conventional statics, proving the effectiveness of the proposed workflow. The application of the statics obtained from surface waves provided a stacked section comparable with that obtained by applying tomostatics.

Place, publisher, year, edition, pages
WILEY, 2020
Keywords
Surface waves, Statics, Mining
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-406181 (URN)10.1111/1365-2478.12895 (DOI)000505379700013 ()
Available from: 2020-03-06 Created: 2020-03-06 Last updated: 2020-03-06Bibliographically approved
Bastani, M., Sadeghi, M., Malehmir, A., Luth, S. & Marsden, P. (2019). 3D magnetic susceptibility model of a deep iron-oxide apatite-bearing orebody incorporating borehole data in Blötberget, Sweden. In: : . Paper presented at SAGA Biennial Conference & Exhibition, Durban, 6-9 October, 2019..
Open this publication in new window or tab >>3D magnetic susceptibility model of a deep iron-oxide apatite-bearing orebody incorporating borehole data in Blötberget, Sweden
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2019 (English)Conference paper, Oral presentation with published abstract (Refereed)
Abstract [en]

We have processed and modelled in 3D available airborne magnetic data flown over the Blötberget iron-oxide deposit in central Sweden. 3D magnetic susceptibility models were then compared to the existing borehole information, downhole logging data and ground geological observations. Use of such information in the 3D inversion of the magnetic field data in the form of a reference model has considerably improved the 3D model in resolving the deeper parts of the iron-oxide orebody as well as the geometry of nearby geological structures. Unconstrained inversion results, however, do not resolve the true dip and depth extension of the iron-oxide orebody.

National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-399962 (URN)
Conference
SAGA Biennial Conference & Exhibition, Durban, 6-9 October, 2019.
Available from: 2019-12-17 Created: 2019-12-17 Last updated: 2020-01-23Bibliographically approved
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
Kammann, J., Malehmir, A., Brodic, B., Tagliavento, M., Stemmerik, L., Normark, E., . . . Nielsen, L. (2019). Deep onshore reflection seismic imaging of the chalk group strata using a 45 kg accelerated weight-drop and combined recording systems with dense receiver spacing. Geophysics, 84(4), B259-B268
Open this publication in new window or tab >>Deep onshore reflection seismic imaging of the chalk group strata using a 45 kg accelerated weight-drop and combined recording systems with dense receiver spacing
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2019 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 84, no 4, p. B259-B268Article in journal (Refereed) Published
Abstract [en]

The Chalk Group forms important hydrocarbon reservoirs offshore and water aquifers onshore Denmark. Within a day of fieldwork, a 450 m long reflection seismic profile was acquired onshore in an area in southeast Denmark, where the Chalk Group extends almost to the surface and is approximately 900 m thick. The main objective of the study was to image the complete Chalk Group in high resolution and to study the origin of reflectivity within the different chalk units. A 45 kg accelerated weight- drop source, in combination with dense receiver spacing using microelectromechanical sensors mounted on a streamer and 48 planted geophones, was used for data acquisition. The profile runs subparallel to the cliffs of Stevns, and the recorded signal reaches the base of the Chalk Group at approximately 600 ms. The fully cored 443 m-deep Stevns-1 borehole, which is located at the recorded seismic line, provides excellent control on lithologic and facies changes. Comparison with the borehole data demonstrates that our seismic data set provides a high-resolution image of the internal layering of the Chalk Group. We find that the internal reflection coefficients of the Chalk Group are, in general, small based on wireline-log data. However, the reflected amplitudes are just big enough to be recorded with the receiver setup used, even from the pure chalk beds of the Chalk Group. The reflectivity seen on the high-resolution seismic profile is influenced by occurrences of clay-enriched chalk layers. Flint bands consisting of numerous flint nodules are a characteristic of the uppermost part of the Chalk Group at Stevns. The flint nodules appear to produce significant scattering of the seismic signals, and flint-rich layers appear with diffuse internal reflectivity characteristics. Outcrop-scale mound structures in Danian and Upper Cretaceous outcrops are for the first time seismically resolved.

Place, publisher, year, edition, pages
SOC EXPLORATION GEOPHYSICISTS, 2019
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-393752 (URN)10.1190/GEO2018-0755.1 (DOI)000480678900077 ()
Funder
Swedish Research Council Formas, 252-2012-1907
Available from: 2019-11-11 Created: 2019-11-11 Last updated: 2019-11-11Bibliographically 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
Lahivaara, T., Malehmir, A., Pasanen, A., Karkkainen, L., Huttunen, J. M. J. & Hesthaven, J. S. (2019). Estimation of groundwater storage from seismic data using deep learning. Geophysical Prospecting, 67(8), 2115-2126
Open this publication in new window or tab >>Estimation of groundwater storage from seismic data using deep learning
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2019 (English)In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 67, no 8, p. 2115-2126Article in journal (Refereed) Published
Abstract [en]

Convolutional neural networks can provide a potential framework to characterize groundwater storage from seismic data. Estimation of key components, such as the amount of groundwater stored in an aquifer and delineate water table level, from active-source seismic data are performed in this study. The data to train, validate and test the neural networks are obtained by solving wave propagation in a coupled poroviscoelastic-elastic media. A discontinuous Galerkin method is applied to model wave propagation, whereas a deep convolutional neural network is used for the parameter estimation problem. In the numerical experiment, the primary unknowns estimated are the amount of stored groundwater and water table level, while the remaining parameters, assumed to be of less of interest, are marginalized in the convolutional neural network-based solution. Results, obtained through synthetic data, illustrate the potential of deep learning methods to extract additional aquifer information from seismic data, which otherwise would be impossible based on a set of reflection seismic sections or velocity tomograms.

Place, publisher, year, edition, pages
WILEY, 2019
Keywords
Modelling, Wave, Monitoring, Inverse problem
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-396134 (URN)10.1111/1365-2478.12831 (DOI)000477189800001 ()
Available from: 2019-11-04 Created: 2019-11-04 Last updated: 2019-11-04Bibliographically approved
Sadeghi, M., Bastani, M., Luth, S., Malehmir, A., Bäckström, E. & Marsden, P. (2019). GIS-based mineral system approach for prospectivity mapping of iron-oxide apatite-bearing mineralisation in Bergslagen, Sweden. In: LIFE WITH ORE DEPOSITS ON EARTH, PROCEEDINGS OF THE 15TH SGA BIENNIAL MEETING, 2019, VOLS 1-4: . Paper presented at 15th SGA Biennial Meeting on Life with Ore Deposits on Earth, AUG 27-30, 2019, Univ Glasgow, Glasgow, SCOTLAND (pp. 1216-1219). SOC GEOLOGY APPLIED MINERAL DEPOSITS-SGA, 3
Open this publication in new window or tab >>GIS-based mineral system approach for prospectivity mapping of iron-oxide apatite-bearing mineralisation in Bergslagen, Sweden
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2019 (English)In: LIFE WITH ORE DEPOSITS ON EARTH, PROCEEDINGS OF THE 15TH SGA BIENNIAL MEETING, 2019, VOLS 1-4, SOC GEOLOGY APPLIED MINERAL DEPOSITS-SGA , 2019, Vol. 3, p. 1216-1219Conference paper, Published paper (Refereed)
Abstract [en]

Bergslagen is one of the richest mineral districts in Sweden for base and precious/critical minerals and metals. In this work a mineral system approach for targeting of iron-oxide apatite-bearing mineralisation has been developed. GIS-based mapping of prospectivity for this type of mineralisation has been carried out with a focus on Ludvika mining area from Blotberget to Haksberg with known and high-quality iron-oxide deposits. According to spatial analysis on mappable criteria's, strong positive airborne magnetic anomalies, density of structures and the contact between felsic volcanic rocks and granites are crucial for this type of mineralisation in the study area This GIS-based model will also be used in targeting of iron-oxide deposits at depth in the Blotberget area. However, the mineral systems approach considers the origin of deposits in the framework of lithospheric-scale processes from the time-honored aspects of the source, fluids, transport and physical and/or thermo-dynamical traps. Applied to exploration strategy, this approach allows for more predictive models. Rather than matching patterns, knowledge of the underlying geological processes and tectonic-structural setting can be used for identifying areas with higher probability of finding deposits of interest. Furthermore, this method can broaden the scope of prospectivity indicators and allows for earlier and more efficient fertility assessments.

Place, publisher, year, edition, pages
SOC GEOLOGY APPLIED MINERAL DEPOSITS-SGA, 2019
National Category
Geology
Identifiers
urn:nbn:se:uu:diva-403032 (URN)000503111600319 ()9780852619643 (ISBN)
Conference
15th SGA Biennial Meeting on Life with Ore Deposits on Earth, AUG 27-30, 2019, Univ Glasgow, Glasgow, SCOTLAND
Funder
EU, Horizon 2020, 775971
Available from: 2020-01-23 Created: 2020-01-23 Last updated: 2020-01-23Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-1241-2988

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