| 1. |
- Bazin, S., et al.
(author)
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Seismic and Electrical Resistivity Investigations for the Planning of a Tunnel in Oslo Outskirts
- 2016
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Conference paper (peer-reviewed)abstract
- Oslo municipality is presently planning bus and car tunnels to facilitate its accessibility and increase traffic efficiency. Urban environment is usually a challenge for geophysical pre-investigations because of the various sources of noise, vibrations and restriction both in time and space. These technical challenges were overcome with the use of a newly developed seismic streamer specifically designed for noisy urban areas, from an industry-academia partnership. A total of 3.5 km long seismic data along 14 profiles were acquired for the tunnels pre-investigation with the main goals of (1) obtaining information about depth to bedrock, (2) detecting potential weakness zones, and (3) optimizing the number of drillings and their locations for a follow-up study. In addition, six electrical resistivity tomography profiles were acquired near the planned tunnel alignments. Inversion of first breaks and electrical resistivity data provides a seamless depth to bedrock interface that is in most places in good agreement with the nearby geotechnical soundings. In addition, the geophysical sections reveal the bedrock undulation character and provide some indication of weakness zones. This case study also illustrates that if the pre-investigation had been based only on boreholes, it would have overseen a potential difficulty during excavation.
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| 2. |
- Kashubin, A., et al.
(author)
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A footprint of rainfall on land seismic data repeatability at the CO2 storage pilot site, Ketzin, Germany
- 2011
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Other publication (other academic/artistic)abstract
- Two vintages of land time-lapse seismic data were acquired in 2005 and in 2009 at the Ketzin CO2 storage site in Germany. The datasets showed some differences in frequency content, signal-to-noise ratio and refraction statics despite that they were acquired with the same equipment and during the same seasons. The spatial variations in the data appeared to show good correlation with the difference in precipitation during the campaigns. These observations provide a ground for estimation of spatially varying operators that may be used in pre-stack or post-stack cross-equalization corrections of the time-lapse datasets. © 2011 Society of Exploration Geophysicists.
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| 3. |
- Alcalde, J., et al.
(author)
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3-D reflection seismic imaging of the Hontomin structure in the Basque-Cantabrian Basin (Spain)
- 2013
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In: Solid Earth. - : Copernicus GmbH. - 1869-9510 .- 1869-9529. ; 4:2, s. 481-496
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Journal article (peer-reviewed)abstract
- The Basque-Cantabrian Basin of the northern Iberia Peninsula constitutes a unique example of a major deformation system, featuring a dome structure developed by extensional tectonics followed by compressional reactivation. The occurrence of natural resources in the area and the possibility of establishing a geological storage site for carbon dioxide motivated the acquisition of a 3-D seismic reflection survey in 2010, centered on the Jurassic Hontomin dome. The objectives of this survey were to obtain a geological model of the overall structure and to establish a baseline model for a possible geological CO2 storage site. The 36 km(2) survey included approximately 5000 mixed (Vibroseis and explosives) source points recorded with a 25 m inline source and receiver spacing. The target reservoir is a saline aquifer, at approximately 1450 m depth, encased and sealed by carbonate formations. Acquisition and processing parameters were influenced by the rough topography and relatively complex geology. A strong near-surface velocity inversion is evident in the data, affecting the quality of the data. The resulting 3-D image provides constraints on the key features of the geologic model. The Hontom n structure is interpreted to consist of an approximately 10(7) m(2) large elongated dome with two major (W-E and NW-SE) striking faults bounding it. Preliminary capacity estimates indicate that about 1.2 Gt of CO2 can be stored in the target reservoir.
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| 4. |
- Buntin, Sebastian, et al.
(author)
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Emplacement and 3D geometry of crustal-scale saucer-shaped intrusions in the Fennoscandian Shield
- 2019
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In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 9
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Journal article (peer-reviewed)abstract
- 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.
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| 5. |
- Malehmir, Alireza, et al.
(author)
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A Review of Reflection Seismic Investigations in Three Major Metallogenic Regions : The Kevitsa Ni-Cu-PGE district (Finland), Witwatersrand Goldfields (South Africa), and the Bathurst Mining Camp (Canada)
- 2014
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In: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 56, s. 423-441
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Research review (peer-reviewed)abstract
- Effective exploration for mineral deposits depends on a sound understanding of the processes and geological structures that contributed to their formation. The reflection seismic method has proven to be a powerful tool that provides a high-resolution image of the subsurface and information about structural and lithological relationships that control mineral deposits. The method has also become an attractive geophysical tool for deep exploration and mine planning. In this paper, we review the use of reflection seismic methods to obtain a better understanding of the architecture and ore-forming processes of three diverse mineral regions: the Kevitsa Ni-Cu-PGE district in Finland, the goldfields of the Witwatersrand Basin South Africa, and the Bathurst Mining Camp, Canada. Seismic data, both 2D and 3D, from the Kevitsa deposit clearly image the 3D geometry of the ore-bearing intrusion and provide information about its relationship to the host rock units and nearby intrusions within a larger tectonic framework. 3D seismic data from the Witwatersrand Basin not only provide clear images of major structures, including a distinct reflection that acts as a marker horizon for the gold-bearing reef, but also provide information that may be useful in resolving a long-standing controversy regarding the origin of the gold in the Basin. For example, it might be possible to show that dykes formed impermeable barriers, thereby falsifying the epigenetic hydrothermal models. 2D and 3D seismic data from the Brunswick No. 6 area in the Bathurst Mining Camp suggest that the Brunswick horizon (which contains the bulk of the massive sulfide and associated iron deposits) occurs within a reflective package that extends down to at least 6-7 km depth.
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| 6. |
- Malehmir, Alireza, et al.
(author)
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Near-Surface Geophysical Characterization of Areas Prone to Natural Hazards : A Review of the Current and Perspective on the Future
- 2016
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In: Advances in Geophysics. - : Elsevier. - 0065-2687 .- 2162-7622. ; 57, s. 51-146
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Journal article (peer-reviewed)abstract
- Natural hazards such as landslides, floods, rockfalls, earthquakes, volcanic eruptions, sinkholes, and snow avalanches represent potential risks to our infrastructures, properties, and lives. That potential will continue to escalate with current and continued human encroachment into risk areas. With the help of geophysical techniques many of those risks can be better understood and quantified, thereby minimized and at least partly mitigated through accurate, site-specific, and proper planning and engineering. On occasions these hazards simply cannot be avoided, but better characterization and therefore understanding of the subsurface geology and natural processes responsible for the threats is possible through integration of various cost-effective geophysical methods with relevant geotechnical, geomechanical, and hydrogeological methods. With the enhanced characterization possible when geophysics is incorporated into natural hazard analysis, potential risks can be better quantified and remediation plans tuned to minimize the threat most natural hazards present to civilizations. In this article we will first review common geophysical methods that can be and have been utilized in studying natural hazard prone areas, then we provide selected case studies and approaches using predominantly our own examples, and finally a look into the future detailing how these methods and technologies can be better implemented and thereby more time- and cost-effective and provide improved results.
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| 7. |
- Malehmir, A., et al.
(author)
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Seismic imaging in the Skellefte ore district, northern Sweden
- 2005
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In: 20th World Mining Congress, November 7-11, 2005, Tehran, Iran. - Teheran : Geological Survey of Iran. ; , s. 399-404
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Conference paper (peer-reviewed)abstract
- In the western part of the Skellefte ore district, which is the most important metallogenic zone in Northern Sweden, several world class mines, e.g., the Kristineberg VMS mine (20.1mt, Cu-Zn-Pb-Ag-Au) are situated. In order to understand why these deposits occur where they do, it is important to understand the crustal architecture of the region. One way to understand the contact relationships between the ore bearing volcanic formations and the surrounding rocks is to develop a detailed 3-D geological model of the region. To establish the structural geologic framework at depth, new seismic reflection data were acquired along two profiles in the Kristineberg area in late August and early September 2003. Data along the two seismic profiles (Profile 1 and Profile 5), each about 25 km long and running in parallel, were collected with the purpose of obtaining high resolution images of the top 10 km of the crust. Although the structural geology is very complex, preliminary stacked sections of the data have revealed numerous reflections which can be correlated with surface geology. Results along Profile 1, which passes on top of the Kristineberg mine show the mine to be located in a major synform extending down to about 2.5-3 km depth. The structure and stratigraphy of the Kristineberg area have been debated for many years. Our seismic results suggest that the deposits occur on the northern limb of a regional syncline. The results help to identify new prospective areas, both down-plunge from known ores, and on the southern limb of the ore-bearing syncline. The results for Profile 5 show that the Revsund granite can have a thickness about 3-3.5 km but not more. Ultramafic rocks are imaged clearly. Diffraction patterns can be interpreted as orienting from either a mafic-ultramafic intrusion or a mineralization zone. A detailed study has to be done in order to determine the source of this reflectivity. In this study seismic reflection profiling has been particularly effective for imaging the major structures around the ore body, demonstrating that the seismic-reflection technique can be used for delineating complex structures that are significant in mineral exploration.
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| 8. |
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| 9. |
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| 10. |
- Adamczyk, A., et al.
(author)
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High-resolution near-surface velocity model building using full-waveform inversion-a case study from southwest Sweden
- 2014
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In: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 197:3, s. 1693-1704
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Journal article (peer-reviewed)abstract
- Full-waveform inversion (FWI) is an iterative optimization technique that provides high-resolution models of subsurface properties. Frequency-domain, acoustic FWI was applied to seismic data acquired over a known quick-clay landslide scar in southwest Sweden. We inverted data from three 2-D seismic profiles, 261-572 m long, two of them shot with small charges of dynamite and one with a sledgehammer. To our best knowledge this is the first published application of FWI to sledgehammer data. Both sources provided data suitable for waveform inversion, the sledgehammer data containing even wider frequency spectrum. Inversion was performed for frequency groups between 27.5 and 43.1 Hz for the explosive data and 27.5-51.0 Hz for the sledgehammer. The lowest inverted frequency was limited by the resonance frequency of the standard 28-Hz geophones used in the survey. High-velocity granitic bedrock in the area is undulated and very shallow (15-100 m below the surface), and exhibits a large P-wave velocity contrast to the overlying normally consolidated sediments. In order to mitigate the non-linearity of the inverse problem we designed a multiscale layer-stripping inversion strategy. Obtained P-wave velocity models allowed to delineate the top of the bedrock and revealed distinct layers within the overlying sediments of clays and coarse-grained materials. Models were verified in an extensive set of validating procedures and used for pre-stack depth migration, which confirmed their robustness.
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