| 1. |
- Beckel, Ruth A.
(författare)
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Active and passive seismic methods for investigating the glacially-triggered Burträsk fault
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Glacially-triggered faults are of high scientific interest since their formation was likely accompanied by major earthquakes and they are still a centre of seismicity in northern Fennoscandia, today. Imaging their deeper structure mainly relies on reflection seismics since the method generally has the best resolving power at depth of all geophysical methods. This thesis uses data acquired at the glacially-triggered Burträsk fault to advance active and passive seismic imaging methods and improve the understanding of the area. Reprocessing of a vintage, crooked-line reflection dataset using a newly developed module for a local cross-dip correction improved the quality of the reflection image significantly and provided important 3D information for the interpretation of the fault and its surroundings. A fault segment to the southwest of the profile was imaged with a dip of approximately 50° but the segment intersecting the profile was not imaged, likely due to insufficient shot coverage close to the fault. Since the Burträsk area is seismically the most active area in Sweden, passive imaging using a dataset of local microearthquakes was attempted. As a first step, the earthquakes were re-located using a stacking-based location method. In spite of the poor azimuthal coverage of the array, the method yielded surprisingly good location results within 30–40 km from the array. The most important factors for success proved to be combined P- and S-wave location with down-weighted S-wave signals, and the use of a polarity-sensitive characteristic function. The distribution of the hypocentres confirmed that the trace of the Burträsk fault is the currently active fault plane. Disappointingly, passive seismic processing using reflection seismic interferometry (RSI) did not image the known reflections. To improve the planning of future studies, different aspects of imaging dipping faults with RSI were investigated using synthetic data. The results showed that reconstructing steeply dipping reflections requires sources in the hanging-wall and that the interpretation of the RSI sections is complicated due to strong artefacts caused by P-S converted arrivals. Thus, passive imaging needs careful planning and a good knowledge of the source positions for distinguishing between artefacts and subsurface structures. This knowledge can be obtained using the stacking-based location method.
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| 2. |
- Donoso, George A.
(författare)
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Seismic imaging of volcanogenic massive sulphide deposits : From legacy to innovative surveys
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The volcanogenic massive sulphide deposits at the Neves-Corvo mine area, part of the Iberian Pyrite Belt in southern Portugal, were studied using seismic methods. Three distinct reflection seismic datasets were used in this study: a 2D legacy data from 1996, an innovative 2019 surface and in-mine acquisition, and a 2011 3D seismic survey. The seismic data were processed with the main objective of improving the seismic signature of the volcanogenic massive sulphide deposits in the area by the use of modern and innovative seismic methods.The legacy dataset, acquired in 1996, was reprocess using today’s processing software. The Lombador massive sulphide was better imaged and a number of never-before-seen shallow but steeply dipping reflections were detected. In the 2019 seismic survey, 2D seismic profiles were acquired on the surface above the Lombador deposit, and four seismic profiles were deployed inside mine exploration tunnels at 650 m depth. Arguably, the first time a seismic acquisition survey of this size has been carried-out inside an active underground mine. The processing of the surface profiles was complemented by additional methods that indicate how the out-of-plane contribution of the Lombador deposit may not be detectable when employing 2D data only, showing the imaging potential of a small-scale survey implemented with innovative acquisition technologies. The 3D seismic data processing showed sensitivity to parameter selection for imaging. Processing results using pre-stack dip move-out and post-stack migration methods show moderate to steeply dipping reflections. Which can be correlated with known lithological contacts, some are interpreted to be originated from the Semblana and Lombador deposits. Despite the mixed signal-to-noise ratio the seismic volume reveals improved 3D seismic images of both shallow and deep structures, allowing to account for the deposit’s lateral extension beyond the capabilities of 2D seismic imaging alone. It was possible to distinguish strong diffraction patterns, interpreted as originating from faults and edges of the Lombador deposit, illustrating the usefulness of diffraction patterns for better interpretation of geological features in hard-rock environments. All of the aforesaid allows to conclude that an exploration programme solely based on 2D seismic data would have showed false-positive results in depth. This encourages the employment of 3D seismic methods, instead of 2D, in mineral exploration for accurate detection and targeting of VMS deposits.
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| 3. |
- Maries, Georgiana
(författare)
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Seismic investigations and physical property studies of natural resources in Finland and Sweden : Efficient exploration of groundwater and mineral resources
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Natural resources, such as mineral deposits and groundwater in particular, are crucial for our society, as the world prepares itself for a smooth transition towards green technologies and decarbonization. Apart from extraction and use, innovative mineral exploration solutions are needed to complete the full value chain and to achieve the sustainable development goals. The application of seismic methods for both near-surface environmental and deep mineral exploration investigations is known, but high costs are associated with data acquisition and processing. In order to illustrate the potential of the seismic methods for efficient exploration of groundwater and mineral resources, cost-effective seismic surveys were acquired within two locations in Finland and Sweden, for aquifer delineation and imaging of iron-oxide mineralization in a hardrock environment, respectively. Physical properties, obtained from geophysical downhole logging and laboratory measurements, were analyzed for a complete characterization of the mineralization and its host rocks. 3D ray-tracing and 2D finite-difference forward modeling were carried out for better assessing the seismic response of the mineralization.The effectiveness of these seismic surveys was revealed by the quality seismic data acquired using a low-cost, easily operated seismic source and different sensors, including a broadband seismic landstreamer. In particular, the seismic source provided adequate penetration in two different and challenging environments, namely soft glacial sediments at Virttaankangas, southwest Finland, and swampy glacial cover at Blötberget, south central Sweden. The large-scale units of the Virttaankangas aquifer were successfully delineated and integrated with the hydrogeological units of the groundwater flow model. The mineralization at Blötberget was interpreted to further extend 300-400 m downdip, below the currently known depth from borehole observations. 3D processing of the 2D seismic profiles revealed a lateral extent of least 300 m, providing encouraging results for improved assessments of the mineral resources. The reflection pattern validated through forward modeling, suggested a possible new mineralized horizon below the known deposits. Physical property studies helped characterize the mineralization and its host rocks in terms of seismic attenuation and rock quality. Fracture zones detected through sonic full-waveform logging were associated with high seismic attenuation, suggesting low mechanical competence of the mineralized rocks despite good rock quality designation, providing thus important information for mine planning and exploration. The studies presented in this thesis illustrate the potential of seismic methods and physical property studies for efficient natural resources exploration in crystalline rocks and in overlying glacial sediments.
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| 4. |
- Pertuz, Tatiana, 1990-
(författare)
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Innovative seismic imaging solutions : From hardrock mineral exploration to quick-clay site characterization
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Seismic methods are an efficient and powerful tool to explore the subsurface. It principally helps to gain information on the subsurface structure, bedrock topography, and the nature of the host rock. This thesis showcases two sites in Sweden, where their geological setting, rock structure and geological environment are different, but seismic still shows its potential in soil investigation.The first case application was in Blötberget mine, where there is an iron-oxide deposit of high-quality. It was tested the validity of an electromagnetic vibrator (E-Vib) as a seismic source for a hardrock environment, and applied a tailored processing workflow to the seismic data to image the iron-oxide deposit.The second case study was in Lilla Edet city, where there is the presence of quick-clays in the region. From this seismic dataset, it was retrieved a S-wave reflection section from a vertical-impact source recorded on vertical-component. In the same study area, it was also imaged the P-P and S-S wavefield reflections from a 9C dataset. This provided insight on the benefit of 9C seismic data acquisition for quick-clay landslide studies.Findings of this work prove that the E-Vib test at the Blötberget mine (Sweden) validated its effectiveness and quality for the hardrock environment. The broadband source combined with a tailored processing workflow allowed imaging of the iron-oxide deposits in improved resolution compared to the earlier seismic data. The seismic dataset in the second case study, especially the S-wavefield reflections, presented detailed information on the structures and physical conditions of the sediments. P-wavefield seismic sections in quick-clay environments proved useful to locate the bedrock level and layering above it, but with low resolution. However, S-wavefield seismic section presented more advantages for this geological setting presenting higher resolution images.
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