| 1. |
- Markovic, Magdalena, et al.
(författare)
-
Diffraction denoising using self‐supervised learning
- 2023
-
Ingår i: Geophysical Prospecting. - : Wiley-Blackwell. - 0016-8025 .- 1365-2478. ; 71:7, s. 1215-1225
-
Tidskriftsartikel (refereegranskat)abstract
- Diffraction wavefield contains valuable information on subsurface composition through velocity extraction and sometimes anisotropy estimation. It can also be used for the delineation of geological features, such as faults, fractures and mineral deposits. Diffraction recognition is, therefore, crucial for improved interpretation of seismic data. To date, many workflows for diffraction denoising, including deep-learning applications, have been provided, however, with a major focus on sedimentary settings or for ground-penetrating radar data. In this study, we have developed a workflow for a self-supervised learning technique, an autoencoder, for diffraction denoising on synthetic seismic, ground-penetrating radar and hardrock seismic datasets. The autoencoder provides promising results especially for the ground-penetrating radar data. Depending on the target of the studies, diffraction signals can be tackled using the autoencoder both as the signal and/or noise when, for example, a reflection is a target. The real hardrock seismic data required additional pre- and post-autoencoder image processing steps to improve automatic delineation of the diffraction. Here, we also coupled the autoencoder with Hough transform and pixel edge detection filters. Along inlines and crosslines, diffraction signals have sometimes a similar character as the reflection and may spatially be correlated making the denoising workflow unsuccessful. Coupled with additional image processing steps, we successfully isolated diffraction that is generated from a known volcanogenic massive sulphide deposit. These encouraging results suggest that the self-supervised learning techniques such as the autoencoder can be used also for seismic mineral exploration purposes and are worthy to be implemented as additional tools for data processing and target detections.
|
|
| 2. |
|
|
| 3. |
- Markovic, Magdalena, et al.
(författare)
-
Diffraction pattern recognition using deep semantic segmentation
- 2022
-
Ingår i: Near Surface Geophysics. - : John Wiley & Sons. - 1569-4445 .- 1873-0604. ; 20:5, s. 507-518
-
Tidskriftsartikel (refereegranskat)abstract
- Diffraction imaging can help better understand small-scale geological structures. Due to their often-weak signal, in order to image them, it is necessary to separate diffraction signals from the rest of the wavefield. Many different methods have been developed for diffraction wavefield separation, and the newest trend includes the application of artificial neural networks and deep learning. Available case studies with a deep-learning approach for diffraction separation show good results when applied to synthetic and sedimentary setting datasets where diffraction signals are either strong or have pronounced characteristics. Examples, however, are missing from crystalline or hardrock geological settings where the signal-to-noise ratio is by far lower and diffraction signals are usually within a complex reflectivity medium, have steep tails and are usually incomplete. In this study, we showcase the application of a deep semantic segmentation model on synthetic seismic, real ground-penetrating radar, and hardrock seismic datasets. Synthetic seismic sections were generated using different random noise levels and coherent noise resembling a complex reflectivity pattern interfering with diffraction tails. For the real GPR dataset, diffraction signals were successfully delineated, although in some locations reflections were picked up because of their similar pixel values as the apex of the diffractions. As for the real seismic dataset, through a number of approaches, we were able to completely delineate a single diffraction within several inlines that was generated from a massive sulphide body. The algorithm also enabled us to recognize an incomplete diffraction, at the edge of the seismic cube, which was never labelled. This diffraction originated from outside of the seismic volume and may be a target for future mineral exploration programmes, thanks to the deep semantic segmentation algorithm providing this possibility.
|
|
| 4. |
- Adamczyk, Anna, et al.
(författare)
-
Application of first-arrival tomography to characterize a quick clay landslide site in Southwest Sweden
- 2013
-
Ingår i: Acta Geophysica. - : Springer Science and Business Media LLC. - 1895-6572 .- 1895-7455. ; 61:5, s. 1057-1073
-
Tidskriftsartikel (refereegranskat)abstract
- First-arrival traveltime tomography was applied to high-resolution seismic data acquired over a known quick-clay landslide scar near the Gota River in southwest Sweden in order to reveal the geometry and physical properties of clay-related normally consolidated sediments. Investigated area proved to be a challenging environment for tomographic imaging because of large P-wave velocity variations, ranging from 500 to 6000 m/s, and relatively steeply-dipping bedrock. Despite these challenges, P-wave velocity models were obtained down to ca. 150 m for two key 2D seismic profiles (each about 500-m long) intersecting over the landslide scar. The models portrait the sandwich-like structure of marine clays and coarse-grained consolidated sediments, but the estimated resolution (20 m) is too small to distinguish thin layers within this structure. Modelled velocity structures match well the results of reflection seismic processing and resistivity tomography available along the same profiles.
|
|
| 5. |
- Adamczyk, Anna, et al.
(författare)
-
Delineating shallow quick-clay structures using acoustic full-waveform inversion – case studyfrom southwest Sweden
- 2013
-
Konferensbidrag (refereegranskat)abstract
- Full waveform inversion (FWI) was applied to imageshallow structures of marine-clay sediments and to provideinsight on the mechanism of a quick-clay landslide. Thedata was acquired in a high-resolution seismic surveyconducted over a known landslide scar near the Göta riverin southwest Sweden. Inversion proved to be challengingbecause of contrasted P-wave velocity structure – thevelocities ranged from 500 m/s in weathered top layer to6000 m/s in the shallow granitic bedrock (up to 30 m belowthe surface). FWI applied to 3 profiles provided highresolution2D P-wave velocity models revealing theintercalating layers of clays and coarse-grain material andthe shape of the bedrock. The multiscale approach was usedto mitigate the strong nonlinearity of the inverse problem.The models were used in pre-stack depth migration andproved significant improvement in reflector flattening andfocusing over the starting first-arrival traveltimetomography models.
|
|
| 6. |
- Adamczyk, A., et al.
(författare)
-
High-resolution near-surface velocity model building using full-waveform inversion-a case study from southwest Sweden
- 2014
-
Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 197:3, s. 1693-1704
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 7. |
- Ahmadi, Omid, et al.
(författare)
-
3D Seismic Interpretation and Forward Modeling : an approach to providing reliable results from 2D seismic data
- 2013
-
Ingår i: Proceedings of the 12th Biennial Meeting. ; , s. 50-53
-
Konferensbidrag (refereegranskat)abstract
- Accurate 3D interpretations is challenging when only 2D seismic reflection data are available. This can be compensated for by using additional data. Here we present two case studies where 2D seismic reflection data have been used in combination with geological/geophysical data to create and verify 3D interpretations of specific structures targeted for scientific deep drilling and mining. In the first case, a surface geological map and high resolution 2D seismic reflection data were used to create a 3D lithological model of the subsurface structures in an area around a scientific deep drilling site. This model was also compared to results from constrained 3D inverse modeling of gravity data. In the second case, seismic forward ray-trace modeling was used to delineate a massive sulfide ore body by using high resolution 2D seismic reflection data. By comparison of the generated synthetic data with the real data, it was found that the top of the ore body was detected.
|
|
| 8. |
- Ahmadi, Omid, et al.
(författare)
-
3D Seismic Waveform Modeling of an Ore Body within a Stochastic Heterogeneous Medium
- 2016
-
Konferensbidrag (refereegranskat)abstract
- Shallow mineral deposits of giant sizes are rapidly mined out and thus to sustain mining and help the economic growth, there is a tendency to explore deeper deposits. Most economic size mineral deposits are hosted within a complex and heterogeneous medium affected by various stages of deformation and metamorphism. Therefore, to understand their seismic responses, 3D heterogeneous modeling of various scale lengths should be considered. Here we present an algorithm that allows to build a model with various degrees of heterogeneity and structural anisotropy for the medium and use that to study a 6 Mt massive sulfide deposit at about 1 km depth. The seismic response was simulated using a 3D acoustic finite-difference method. Wavefield records through the model show imaging of the ore body in the presence of a high-degree of structural anisotropy/heterogeneity is difficult, but the associated amplitude anomaly appeared as diffraction can be detected within the 3D recorded wavefields and likely possible to be imaged using high-fold seismic data. The recorded wavefield however suggests some asymmetric pattern for the diffraction due to the high-degree of structural anisotropy introduced and hence care must be taken when processing and locating these deposits within highly preferentially-oriented heterogeneous medium.
|
|
| 9. |
- Ahmadi, Omid, 1980-
(författare)
-
Application of the Seismic Reflection Method in Mineral Exploration and Crustal Imaging : Contributions to Hardrock Seismic Imaging
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The seismic reflection method has been used extensively in mineral exploration and for imaging crustal structures within hardrock environments. In this research the seismic reflection method has been used and studied to address problems associated with hardrock settings. Papers I and II, address delineating and imaging a sulfide ore body and its surrounding rocks and structures in Garpenberg, central Sweden, at an active mine. 3D ray-tracing and finite-difference modeling were performed and the results suggest that although the detection of the ore body by the seismic reflection method is possible in the area, the presence of backfilled stopes in the mine makes seismic imaging of it difficult. In paper III the deeper structures of the Pärvie fault system in northern Sweden were revealed down to about 8 km through 2D seismic reflection profiling. The resulting images were interpreted using microearthquake data as a constraint. Based on the interpretation, some locations were suggested for future scientific deep drilling into the fault system. In paper IV, the seismic signature of complex geological structures of the Cue-Weld Range area in Western Australia was studied using a portion of a deep 2D seismic reflection profile. The pronounced reflections on the seismic images were correlated to their corresponding rock units on an available surface geological map of the study area. 3D constant velocity ray-tracing was performed to constrain the interpretation. Furthermore, the proposed structural model was tested using a 2D acoustic finite-difference seismic modeling method. Based on this study, a new 3D structural model was proposed for the subsurface of the area. These studies have investigated the capability of the seismic reflection method for imaging crustal structures within challenging hardrock and complex geological settings and show some its potential, but also its limitations.
|
|
| 10. |
- Ahmadi, Omid, et al.
(författare)
-
High-resolution 2D seismic imaging and forward modeling of a polymetallic sulfide deposit at Garpenberg, central Sweden
- 2013
-
Ingår i: Geophysics. - : Society of Exploration Geophysicists. - 0016-8033 .- 1942-2156. ; 78:6, s. B339-B350
-
Tidskriftsartikel (refereegranskat)abstract
- We acquired a high-resolution 2D seismic profile to test the capability of the seismic method in imaging a sulfide ore body at Garpenberg, central Sweden. Delineation of the geologic structures, which surround and host the ore body, is another goal of the survey. Due to the 3D geology of the structures, a cross-dip correction performed to image out-of-the-plane reflections, resulting in a clear high amplitude anomaly at a time and location to that to be expected from near the top of the ore body. Furthermore, DMO processing and migration are applied to the data, providing images of four main reflection groups. The reflections have been interpreted as corresponding to geologic rock units in the area that partly interfere with the potential ore body signal. To further investigate the seismic response of the ore body, forward modeling by ray-tracing is applied using the ore body geometry as mapped by drilling. We use two ray-tracing approaches: standard 3D ray-tracing and an exploding reflector approach. Seven representative samples from the mine area are used to determine P-wave velocities. The measurements show a considerable contrast between the ore body and host rock. By comparing the modeled and observed data, we find that the high amplitude signal in the real seismic section most likely emanates from near the top of one concentrated ore which lies inside the larger mapped ore body that has been modeled as a resource. The base of the ore body is only observed on the synthetic data whereas a signal penetration analysis suggests that the seismic signal penetrated efficiently along the entire survey line. Presence of disseminated ore and lower fold toward the northern end of the profile could be combined reasons that make imaging the base of the ore body difficult.
|
|
| 11. |
- Ahmadi, Pouya, et al.
(författare)
-
Elastic Anisotropy of Deformation Zones : From Lab Measurements to Real Seismic Data, an Example from Eastern Sweden
- 2013
-
Konferensbidrag (refereegranskat)abstract
- Estimation of elastic anisotropy, which is usually caused by rock fabrics and mineral orientation, has animportant role in exploration seismics and better understanding of crustal seismic reflections. If notproperly taken care of during processing steps, it may lead to wrong interpretation or distorted seismicimage. In this paper, a state-of-the-art under development Laser Doppler Interferometer (LDI) device isused to measure anisotropy of rock samples from a major deformation zone in the Bergslagen region ineastern Sweden. Results are then used to correct for the anisotropy effects using a non-hyperbolic moveoutequation. The resulting stacked section shows partial improvement of the deformation zone compared withthe isotropic processing section. This suggests that rock anisotropy may also contribute to generation ofreflections from the deformation zones in the study area but requires further investigations.
|
|
| 12. |
- Ahmadi, Pouya, et al.
(författare)
-
Laser Doppler Interferometry (LDI) to obtain full stiffness tensor: A case study on a deformation zone in Sweden
- 2015
-
Konferensbidrag (refereegranskat)abstract
- Estimation of elastic anisotropy, which is usually caused by rock fabrics and mineral orientations, has an important role in exploration seismology and a better understanding of crustal seismic reflections. If not properly taken care of during data processing steps, it leads to wrong interpretation and/or distorted seismic image. In this work, a state-of-the-art under the development Laser Doppler Interferometer (LDI) device is used to measure phase velocities on the surface of rock samples from a major poly-phase crustal scale deformation zone (Österbybruk Deformation Zone) in the Bergslagen region of eastern Sweden. Then, a general inversion code is deployed to invert the measured phase velocities to obtain full elastic stiffness tensors of two samples from the deformation zone. At the end, results are used to correct for the anisotropy effects using three dimensionless Tsvankin's parameters and a non-hyperbolic moveout equation. The resulting stacked section shows partial reflection improvement of the deformation zone compared with the traditional isotropic processing approach. This illustrates that rock anisotropy contributes to the generation of the reflections from the deformation zones in the study area although they do not show significant density contrast with their surrounding rocks.
|
|
| 13. |
- Ahokangas, Elina Marita, et al.
(författare)
-
Geophysical characterization of late-Quaternary glaciofluvial complex and glacial stratigraphy in the Satakunta sandstone area, Köyliö, southwest Finland
- 2021
-
Ingår i: Quaternary Research. - : Cambridge University Press. - 0033-5894 .- 1096-0287. ; 100, s. 135-153
-
Tidskriftsartikel (refereegranskat)abstract
- We acquired high-resolution reflection seismic data using a broadband digital-based landstreamer system to characterize a depression related to the Mesoproterozoic (Jothnian) Satakunta sandstone basin in the Köyliö study area, southwest Finland. This ca. 800-m-wide depression is infilled with up to 100-m-thick (late) Quaternary interlobate glaciofluvial complex sediments. The seismic images clearly reveal details of the glaciofluvial complex, sandstone depression topography, and brittle structures related to the formation of the sandstone basin by oblique transtension. Additionally, we identified the setting of a diabase laccolith within the sandstone and the geometry and position of the steep sandstone contact. The esker core does not follow the sandstone-Svecofennian basement rock contact or lean to it. The esker core is at a depth of 50–60 m on the flank of the depression. The seismic data image the esker core and other architectural elements of the esker. We highlight the potential of the digital-based landstreamer in the research of complex Quaternary sediments in major bedrock depressions including the characterization of the underlying bedrock properties. We also discuss the possible tunnel valley origin of the sandstone depression.
|
|
| 14. |
- Ahokangas, E., et al.
(författare)
-
Seismic Imaging of Esker Sediments within the Satakunta Sandstone Depression in Köyliö, SW Finland
- 2016
-
Konferensbidrag (refereegranskat)abstract
- The Satakunta sandstone depression infilled by the Pori-Koski interlobate esker sediments hosts a major high-quality groundwater reservoir in Köyliö, SW Finland. These up to 100 m thick sediments were delineated for the first time down to bedrock level by high-resolution reflection seismic method using a newly developed landstreamer consisting of 80-3C MEMs (micro electro mechanical) broadband sensors together with 50 wireless recorders connected to 10 Hz geophones to obtain greater depth penetrations. The 5-day survey resulted in about 5 km long seismic data (2-4 m receiver and shot spacing) and two profiles. Indications of crystalline basement are lacking in the tomography sections, implying that the (fractured) Rapakivi granite area extends further southeast than expected. The sandstone contact position was also ca. 500 m further to the east than expected. The sandstone depression and infilling esker sediments and the bedrock level were shown with good accuracy in both tomographic model and the reflection section. The hydraulically conductive esker core does not follow the sandstone contact and is underlain by older sediments. This case study illustrates the capability of high-resolution seismic surveys with the parameters used in this study for hydrogeological investigations and in particular in thick glacial sediments.
|
|
| 15. |
- Alcalde, J., et al.
(författare)
-
3-D reflection seismic imaging of the Hontomin structure in the Basque-Cantabrian Basin (Spain)
- 2013
-
Ingår i: Solid Earth. - : Copernicus GmbH. - 1869-9510 .- 1869-9529. ; 4:2, s. 481-496
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 16. |
- Almqvist, Bjarne, et al.
(författare)
-
Magnetic characterisation of magnetite and hematite from the Blötberget apatite-iron-oxide deposits (Bergslagen), south-central Sweden
- 2019
-
Ingår i: Canadian journal of earth sciences (Print). - : Canadian Science Publishing. - 0008-4077 .- 1480-3313. ; 56:9, s. 948-957
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 17. |
- Andersson, Magnus, 1977-
(författare)
-
3D Structure and Emplacement of the Alnö Alkaline and Carbonatite Complex, Sweden : Integrated Geophysical and Physical Property Investigations
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbonatites are carbonate-rich magmatic rocks that are rare and of great relevance for our understanding of crustal and mantle processes. Although found on all continents and in settings ranging from Archaean to present-day, their deeper plumbing system is still poorly understood. Therefore, the main goal of this thesis is to broaden the existing knowledge of carbonatite systems, often limited to surface geological observations, by providing depth constraints using a number of geophysical methods and petrophysical measurements. The Alnö alkaline and carbonatite complex in central Sweden was chosen for this purpose. Data from three reflection seismic lines, ground gravity and magnetic measurements are presented. These data are complemented by a series of petrophysical measurements, including ultrasonic velocities, density, magnetic bulk susceptibility, anisotropy of magnetic susceptibility (AMS), and magnetic remanence, to aid in the interpretation of the geophysical data. The reflection seismic data indicate a solidified saucer-shaped fossil magma chamber at about 3 km depth. Caldera-style volcanism, constrained by surface geological observations, provides a plausible scenario to explain the emplacement of the complex, suggesting that carbonatite magmas have been stored, transported and erupted in a similar manner to known emplacement mechanisms for silicic calderas, although these are compositionally different. The AMS data from most of the carbonatite sheets in Alnö show a strong degree of anisotropy and oblate-shaped susceptibility ellipsoids. A set of syn- and post-emplacement processes that may control the AMS signature is evaluated based on the dataset. Overprinting of the primary flow patterns by processes related to sheet closure at the terminal stage of magma transport may explain the AMS observations. A complementary study using 3D inversion of ground gravity and aeromagnetic data was then carried out to better delineate the 3D internal architecture of the complex. Resulting models indicate a depth extent of the complex to about 3-4 km, consistent with the interpretation of the reflection seismic data. The modelling results of a ring-shaped magnetic anomaly observed in the Klingefjärden bay adjacent to Alnö Island further suggest that the complex may extend laterally about 3 km towards the north.
|
|
| 18. |
- Andersson, Magnus, et al.
(författare)
-
Carbonatite ring-complexes explained by caldera-style volcanism
- 2013
-
Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 3
-
Tidskriftsartikel (refereegranskat)abstract
- Carbonatites are rare, carbonate-rich magmatic rocks that make up a minute portion of the crust only, yet they are of great relevance for our understanding of crustal and mantle processes. Although they occur in all continents and from Archaean to present, the deeper plumbing system of carbonatite ring-complexes is usually poorly constrained. Here, we show that carbonatite ring-complexes can be explained by caldera-style volcanism. Our geophysical investigation of the Alnö carbonatite ring-complex in central Sweden identifies a solidified saucer-shaped magma chamber at ∼3 km depth that links to surface exposures through a ring fault system. Caldera subsidence during final stages of activity caused carbonatite eruptions north of the main complex, providing the crucial element to connect plutonic and eruptive features of carbonatite magmatism. The way carbonatite magmas are stored, transported and erupt at the surface is thus comparable to known emplacement styles from silicic calderas.
|
|
| 19. |
- Andersson, Magnus, 1977-, et al.
(författare)
-
Internal architecture of the Alnö alkaline and carbonatite complex (central Sweden) revealed using 3D models of gravity and magnetic data
- 2018
-
Ingår i: Tectonophysics. - : ELSEVIER SCIENCE BV. - 0040-1951 .- 1879-3266. ; 740-741, s. 53-71
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 20. |
- Andersson, Magnus, et al.
(författare)
-
Magma transport in sheet intrusions of the Alnö carbonatite complex, central Sweden
- 2016
-
Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
-
Tidskriftsartikel (refereegranskat)abstract
- Magma transport through the Earth's crust occurs dominantly via sheet intrusions, such as dykes and cone-sheets, and is fundamental to crustal evolution, volcanic eruptions and geochemical element cycling. However, reliable methods to reconstruct flow direction in solidified sheet intrusions have proved elusive. Anisotropy of magnetic susceptibility (AMS) in magmatic sheets is often interpreted as primary magma flow, but magnetic fabrics can be modified by post-emplacement processes, making interpretation of AMS data ambiguous. Here we present AMS data from cone-sheets in the Alno carbonatite complex, central Sweden. We discuss six scenarios of syn- and post-emplacement processes that can modify AMS fabrics and offer a conceptual framework for systematic interpretation of magma movements in sheet intrusions. The AMS fabrics in the Alno cone-sheets are dominantly oblate with magnetic foliations parallel to sheet orientations. These fabrics may result from primary lateral flow or from sheet closure at the terminal stage of magma transport. As the cone-sheets are discontinuous along their strike direction, sheet closure is the most probable process to explain the observed AMS fabrics. We argue that these fabrics may be common to cone-sheets and an integrated geology, petrology and AMS approach can be used to distinguish them from primary flow fabrics.
|
|
| 21. |
- Andersson, Magnus, et al.
(författare)
-
Reflection seismic and potential-field investigations of the Alnö alkaline and Carbonatite igneous complex
- 2014
-
Ingår i: 74th EAGE conference and exhibition incorporating SPE EUROPEC 2012. - Red Hook, NY : Curran Associates, Inc.. - 9781629937908 ; , s. 5393-5395
-
Konferensbidrag (refereegranskat)abstract
- Alnö Island on the east coast of Sweden is well-known for its alkaline and carbonatite igneous rocks. Detailed surface geological mapping studies provide a good knowledge of the surface geology, from which geological structures and their depth extent have been inferred. The aim of this research is to constrain geological structures at depth and to improve our understanding of the intrusion mechanism(s) and the geometry of the Alnö complex as well as similar intrusions elsewhere. Three high-resolution reflection seismic profiles were acquired during 2011 across the complex. The seismic profiles suggest that the intrusion is highly reflective down to about 3 km at where the reflectivity terminates. Densely sampled surface gravity and magnetic data have also been acquired along the seismic profiles and in combination with petrophysical measurements and the seismic data will allow to construct a detailed 3D geological model of the Alnö complex.
|
|
| 22. |
- Balestrini, Florencia, et al.
(författare)
-
Improved target illumination at Ludvika mines of Sweden through seismic-interferometric surface-wave suppression
- 2020
-
Ingår i: Geophysical Prospecting. - : Wiley. - 0016-8025 .- 1365-2478. ; 68:1, s. 200-213
-
Tidskriftsartikel (refereegranskat)abstract
- In mineral exploration, new methods to improve the delineation of ore deposits at depth are in demand. For this purpose, increasing the signal-to-noise ratio through suitable data processing is an important requirement. Seismic reflection methods have proven to be useful to image mineral deposits. However, in most hard rock environments, surface waves constitute the most undesirable source-generated or ambient noise in the data that, especially given their typical broadband nature, often mask the events of interest like body-wave reflections and diffractions. In this study, we show the efficacy of a two-step procedure to suppress surface waves in an active-source reflection seismic dataset acquired in the Ludvika mining area of Sweden. First, we use seismic interferometry to estimate the surface-wave energy between receivers, given that they are the most energetic arrivals in the dataset. Second, we adaptively subtract the retrieved surface waves from the original shot gathers, checking the quality of the unveiled reflections. We see that several reflections, judged to be from the mineralization zone, are enhanced and better visualized after this two-step procedure. Our comparison with results from frequency-wavenumber filtering verifies the effectiveness of our scheme, since the presence of linear artefacts is reduced. The results are encouraging, as they open up new possibilities for denoising hard rock seismic data and, in particular, for imaging of deep mineral deposits using seismic reflections. This approach is purely data driven and does not require significant judgment on the dip and frequency content of present surface waves, which often vary from place to place.
|
|
| 23. |
- Bastani, Mehrdad, 1960-, et al.
(författare)
-
3D magnetic susceptibility model of a deep iron-oxide apatite-bearing orebody incorporating borehole data in Blötberget, Sweden
- 2019
-
Konferensbidrag (refereegranskat)abstract
- 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.
|
|
| 24. |
- Bastani, Mehrdad, et al.
(författare)
-
Boat-towed radio-magnetotellurics : A new technique and case study from the city of Stockholm
- 2015
-
Ingår i: Geophysics. - 0016-8033 .- 1942-2156. ; 80:6, s. B193-B202
-
Tidskriftsartikel (refereegranskat)abstract
- We have developed a new data acquisition system and technique to measure the radio magnetotelluric (RMT) signals from distant radio transmitters with the objective of mapping and modeling electric resistivity structures below a river or lake. The acquisition system is towed by a boat; therefore, we call the technique boat-towed RMT. The data acquisition is fast with a production rate of approximately 1 km/hr using a nominal sampling spacing of 10–15 m. Given the ample number of radio transmitters available in most parts of the world, the method can be used for near-surface studies of various targets. We have developed boat-towed RMT measurements on Lake Mälaren near the city of Stockholm in Sweden to determine the feasibility of the method. Approximately 15 km of RMT data were collected during three days above a planned 60-m-deep bypass tunnel with the goal of providing information on the bedrock depth and possible weak zones within the bedrock. The measured resistivity and phase data were of high quality with errors on the order of a few percent. The resistivity models from 2D inversion of the data showed a good correlation with available geologic data in resolving bedrock depth and also resistivity layering within the lake. Resistivity maps derived from the dense 2D models suggested a northeast–southwest-striking low-resistivity zone at less than a 30-m depth. The zone likely represents fractured crystalline bedrock. The boat-towed RMT technique is well suited for water bodies with moderate electric resistivity such as in brackish and freshwater environments.
|
|
| 25. |
|
|
