| 1. |
- Dias, Pedro, et al.
(författare)
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Geophysical investigation of the down-dip extension of the Lombador massive sulphide deposit, Neves-Corvo, Portugal
- 2021
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Ingår i: International journal of earth sciences. - : Springer Nature. - 1437-3254 .- 1437-3262. ; 110:3, s. 911-922
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Tidskriftsartikel (refereegranskat)abstract
- The 150 Mt Lombador massive sulphide deposit is one of the seven known deposits of the Neves-Corvo mine. The deposit dips approximately 30o-35o to the NE and is open down dip, with current exploitation reaching down at 1 km depth. To investigate the possible downwards continuation of the deposit, a 1D constrained inversion of time-domain electromagnetic (TEM) ground loop data was conducted, followed by 3D electromagnetic (EM) forward modeling and a constrained 3D gravimetric inversion over the same area. To perform the EM and gravity modeling/inversion, a 3D geologic model was built using a density database comprising of approximately 300 drill-holes, and an electrical conductivity database with measurements from resistivity surveys and 1D inversion of the TEM data. The EM modeling shows that the Neves Formation shales are a regional conductive layer extending down to approximately 1.6 km depth in the Lombador area. This layer, often topped by massive sulphides, has an average density of 2.83 g/cm(3), whereas stockwork and massive sulphide reach on average 3.1 g/cm(3) and 4.5 g/cm(3), respectively. The 3D constrained gravity inversion results do not support the hypothesis of the presence of massive sulphides located in the down-dip direction of the Lombador deposit in the immediate vicinity of the known deposit. The lack of spatial resolution of the gravity grid, the study area limited size and the lack of information from within the basement suggest further studies are required to confirm the presence and amount of stockwork mineralization down-dip the Lombador deposit inside the Neves-Formation or the Phyllite-Quartzite basement.
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| 2. |
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| 3. |
- Lundberg, Angela, et al.
(författare)
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Impulse Radar Snow Surveys – Influence of Snow Density
- 2000
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Ingår i: Nordic Hydrology. - : IWA Publishing. - 0029-1277 .- 1996-9694. ; 31:1, s. 1-14
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Tidskriftsartikel (refereegranskat)abstract
- Snow cover water equivalent (SWE) is of major importance for planning of e.g. hydropower production in areas where a large proportion of the annual precipitation falls as snow. Radar technique can be used to determine SWE from the two-way travel time (twt) of a radar-wave propagation through a snowpack. SWE is usually related to twt through an empirical relationship, SWE = -b+a twt, where the values of a and b are determined by linear regression from simultaneous measurements of SWE (with snow-courses) and twt (with radar technology). In this paper a theoretical relationship between twt and SWE is developed showing the need for introducing the density when relating twt to SWE. Use of different empirical relationships for the real dielectric constant showed that the a-value for dry snow with a density of 350 kg m-3 (a typical value at the end of the accumulation season in the Nordic countries) is 0.040 m ns-1 (twt given in nanoseconds). When the snow density deviates considerably from this value a corrected a-value has to be used. A density of 300 and 400 kg m-3 gives a = 0.036 and 0.045 m ns-1 respectively. The b-value should theoretically be zero for measurements at the snow surface, non-zero values are probably due to the use of the direct wave between transmitter and receiver antennas as reference. The theoretically derived equations were confirmed by laboratory and field measurements as well as by measurements taken from literature.
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| 4. |
- Lundberg, Angela, et al.
(författare)
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Impulse radar surveys - influence of liquid water : remote sensing and hydrology 2000
- 2001
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Ingår i: Remote Sensing and Hydrology 2000. - Wallingford, Oxfordshire : IAHS Press. - 1901502465 - 9781901502466 ; , s. 136-138
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Konferensbidrag (refereegranskat)abstract
- Snow water equivalent (SWE) of snow is of great importance for hydropower production in areas where a large proportion of the reservoir water comes from snow. Impulse radar to estimate the SWE is now in operational use in some Scandinavian basins. With radar technology the radar wave propagation time in the snow pack is converted into SWE with the help of a parameter usually termed the a value. Use of radar technology during late winter brings about risk for measurements of wet snow. The a value for dry snow cannot be used directly for wet snow. A liquid-water content of 5% (by volume) reduces the a value by » 20%.
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| 5. |
- Lundberg, Angela, et al.
(författare)
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Radar snow surveys : influence of snow density
- 1998
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Ingår i: XX Nordic Hydrological Conference. - Helsinki : Nordic Association for Hydrology. - 9521103221 ; , s. 679-
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Konferensbidrag (refereegranskat)
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| 6. |
- Lundberg, Angela, et al.
(författare)
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Snow accumulation in forests from ground and remote sensing data
- 2004
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Ingår i: Hydrological Processes. - : Wiley. - 0885-6087 .- 1099-1085. ; 18:10, s. 1941-1955
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Tidskriftsartikel (refereegranskat)abstract
- Winter-forest processes affect global and local climates. The interception-sublimation fraction (F) of snowfall in forests is a substantial part of the winter water budget (up to 40%). Climate, weather-forecast and hydrological modellers incorporate increasingly realistic surface schemes into their models, and algorithms describing snow accumulation and snow-interception sublimation are now finding their way into these schemes. Spatially variable data for calibration and verification of wintertime dynamics therefore are needed for such modelling schemes. The value of F was determined from snow courses in open and forested areas in Hokkaido, Japan. The value of F was related to species and canopy-structure measures such as closure, sky-view fraction (SVF) and leaf-area index (LAI). Forest structure was deduced from fish-eye photographs. The value of F showed a strong linear correlation to structure: F = 0·44 - 0·6 × SVF for SVF < 0·72 and F = 0 for SVF > 0·72, and F = 0·11 LAI. These relationships seemed valid for evergreen conifers, larch trees, alder, birch and mixed deciduous stands. Forest snow accumulation (SF) could be estimated from snowfall in open fields (So) and to LAI according to SF = So (1 - 0·11 LAI) as well as from SVF according to SF = So (0·56 + 0·6 SVF) for SVF < 0·72. The value of SF was equal to So for SVF values above 0·72. The value of sky-view fraction was correlated to the normalized difference snow index (NDSI) using a Landsat-TM image for observation plots exceeding 1 ha. Variables F and SF were related to NDSI for these plots according to: F = -0·37NDSI + 0·29 and SF = So (0·81 + 0·37NDSI). These relationships are somewhat hypothetical because plot-size limitation only allowed one sparse-forest observation of NDSI to be used. There is, therefore, a need to confirm these relationships with further studies.
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| 7. |
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| 8. |
- Lundberg, Angela, et al.
(författare)
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Snow Wetness Influence on Impulse Radar Snow Surveys Theoretical and Laboratory Study
- 2000
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Ingår i: Nordic Hydrology. - : IWA Publishing. - 0029-1277 .- 1996-9694. ; 31:2, s. 89-106
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Tidskriftsartikel (refereegranskat)abstract
- The snow-water equivalent of late-winter snowpack is of utmost importance for hydropower production in areas where a large proportion of the reservoir water emanates from snowmelt. Impulse radar can be used to estimate the snow-water equivalent of the snowpack and thus the expected snowmelt discharge. Impulse radar is now in operational use in some Scandinavian basins. With radar technology the radar wave propagation time in the snowpack is converted into snow-water equivalent with help of a parameter usually termed the a-value. Use of radar technology during late winter brings about risk for measurements on wet snow. The a-value for dry snow cannot be used directly for wet snow. We have found that a liquid-water content of 5% (by volume) reduces the a-value by approximately 20%. In this paper an equation, based on snow density and snow liquid water content, for calculation of wet-snow a-value is presented.
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| 9. |
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| 10. |
- Mainali, Ganesh, et al.
(författare)
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Tailings dams monitoring in Swedish mines using self-potential and electrical resistivity methods
- 2015
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Ingår i: Electronic Journal of Geotechnical Engineering. - : Mete Öner. - 1089-3032. ; 20:13, s. 5859-5875
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Tidskriftsartikel (refereegranskat)abstract
- Tailings dam failures have been occurring in recent years. Many of these failures have caused human casualties, destruction of property, and damage to environment and huge economic loss to the mining industry. The monitoring of the dam is essential to know the existing state of the dams and to ensure the safety of the dam over its life time. The present study has been conducted to test the applicability of electrical resistivity and self-potential (SP), for detecting anomalous seepage through mine tailings dams in Sweden and monitoring the physical condition of the dam. This study has demonstrated the potential of using geoelectrical methods for monitoring the conditions of the tailing dams related to seepage.
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| 11. |
- Malehmir, Alireza, et al.
(författare)
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The Paleoproterozoic Kristineberg mining area, northern Sweden : Results from integrated 3D geophysical and geologic modeling, and implications for targeting ore deposits
- 2009
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Ingår i: Geophysics. - : Society of Exploration Geophysicists. - 0016-8033 .- 1942-2156. ; 74:1, s. B9-B22
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Tidskriftsartikel (refereegranskat)abstract
- The Kristineberg mining area in the western part of the Paleoproterozoic Skellefte Ore District, northern Sweden, is well known for its base-metal and recent gold discoveries. A pilot 3D geologic model has been constructed on a crustal scale, covering an area of 30×30 km to depths of 10 km. Constrained 3D inverse and forward gravity modeling have been performed to confirm and refine previous modeling along seismic profiles using mainly 2.5D techniques. The 3D inverse gravity modeling was geared to generating isodensity surfaces that enclose regions within the model of anomalous density contrast. The 3D forward gravity modeling was conducted to include faulting and folding systems that are difficult to include in the inversion. The 3D geologic model supports many previous interpretations but also reveals new features of the regional geology that are important for future targeting of base-metal and gold deposits. The margins of a thick granite in the south dip steeply inward, suggesting the possibility of room to accommodate another large base-metal deposit if the granitic rocks are juxtaposed with volcanic rocks at depth. Gravity modeling also suggests the observed Bouguer gravity high within the western metasediments can be explained by a large mafic intrusion that has dioritic to tonalitic composition and no significant magnetic signature. Because mafic-ultramafic intrusions within metasediments can indicate gold, this interpretation suggests the western metasediments have a high gold potential.
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| 12. |
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| 13. |
- Nisca, Dan H., et al.
(författare)
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The lycksele structure, a huge ring formation in Northern Sweden : result of an impact?.
- 2000
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Ingår i: S T A R (Online). - 1548-8837. ; 38
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The current database of impact structures in Fennoscandia reveals 28 proven impact craters of various ages and sizes. Recently, we started to search for large and old impact structures, which may show traces of the impact in their shape or in their rocks and minerals and which can be diagnostically identified through the masking effects of postimpact deformations. Since we are dealing with strongly eroded and often deformed structures, the classical criteria for proving an impact origin are not tenable. Through a combined analysis of topography, drainage, gravity, magnetic, and petrophysical data, a circular structure has been identified in northern Sweden. The structure is characterized by a circular system of faults, arc-shaped contacts between rocks, and a circular distribution of granitic intrusions. The fault zone at the edges is defined from gradients in the gravity data, from magnetic data by arc-shaped anomalies, and from topographic data by an arc-shaped relief, which is also reflected in the drainage pattern. The zone is characterized by vertical faults that also cut the granitic intrusions at the edge of the formation, and from interpretation of gravity data an uplift of high density rocks (about 2850 kg/m(exp 3)) is indicated in the central part. The age of granites and pegmatites ranges from 1.82 to 1.78 Ga. Therefore, based on age data and on paleomagnetic results of various generations of dikes that cut and are truncated by the structure, the age of the formation lies within 1.80-1.26 Ga. The rocks have not been studied petrologically to see if any traces of shock metamorphism can be found. At present, four models for the origin of the structure are possible: (1) basement doming, (2) meteorite impact, (3) large buried pluton, or (4) fault-bounded block. An impact origin, however, seems most plausible.
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| 14. |
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| 15. |
- Tavakoli, Saman, et al.
(författare)
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Geophysical modelling of the central Skellefte district, Northern Sweden; an integrated model based on the electrical, potential field and petrophysical data
- 2012
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 82, s. 84-100
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Tidskriftsartikel (refereegranskat)abstract
- The central Skellefte district (CSD) is a part of a major ore-bearing district in northern Sweden. Studying the depth and patterns of the contact relationship between the two major stratigraphic units of the CSD, the Skellefte Group and the Vargfors Group, is a key issue to understand the geometry and structure of the area and to guide exploration of base metals. In this study, we interpret geoelectrical data collected along two profiles and magnetic and gravity data obtained from the database of the Swedish Geological Survey (SGU) and Boliden Mineral, to reveal contact relationship and depth extension of the major geological structures. Petrophysical analyses of the different lithologies were conducted on samples from the database of the SGU. Electric resistivity, induced polarization (IP), magnetic susceptibility and density were determined on 154 core samples representing the different lithologies of the area. The resistivity/IP data were acquired to define structural relations down to a maximum depth of ~ 430 m. The major contact between sediments of the Vargfors basin and volcanic rocks of the Skellefte Group were outlined from the inversion of the resistivity/IP sections, suggesting a synform boundary between the Vargfors Group and Skellefte Group. The contact relationship between the felsic and mafic volcanic rocks of the Skellefte Group is also understood with the help of the resistivity/IP data. The resistivity models were tested using the magnetic data and magnetic susceptibility inferred on the resistivity bodies. The result suggests a good correlation between the initial resistivity model and the magnetic and gravity field calculated from that model. The integration and interpretation of geological and geophysical data improved the basic understanding of the geometry of CSD. Based on previous geological investigations, the potential ore deposits are believed to be found along the volcano-sedimentary contact. The result from this study can thus be used for the base metal exploration, finding the locations of potential sulphide deposits and give a better understanding about spatial relationship between different geology structures in the CSD.
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| 16. |
- Tavakoli, Saman, et al.
(författare)
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Regional-scale geometry of the central Skellefte district, northern Sweden : results from 2.5D potential field modeling along three previously acquired seismic profiles
- 2012
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 85, s. 43-58
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Tidskriftsartikel (refereegranskat)abstract
- The Skellefte district in northern Sweden is one of the most important mining districts in Europe hosting approximately 80 volcanic massive sulfide (VMS) deposits. Due to its economical importance, geological and geophysical studies were carried out in order to create an image of the geometry of the upper crustal structure and integral geological elements and to evaluate their relationship to mineral deposits. Consequently, seismic reflection data along three sub-parallel profiles were acquired during 2009–2010 to map the spatial relationships between the geological structures down to a depth of ~4.5 km. Although these seismic studies helped researchers understand the regional relationship between geologic units in the central Skellefte district (CSD), the seismic reflection data did not succeed entirely in mapping the lithological contacts in the area. In this study, themodel derived fromthe seismic reflection datawas examined by using 2.5D modeling of potential field data (down to a 5 km depth) constrained by physical properties of the rocks and surface geology.Moreover, we modeled gravity and magnetic data along the non-reflective or poorly reflective parts of the seismic profiles to identify major lithological contacts and shear zones in the CSD, which could not be modeled on the basis of the seismic reflection data. Gravity and magnetic data helped reveal the spatial relationship between the Skellefte volcanic rocks, Vargfors groupmeta-sedimentary rocks and two metaintrusive complexes.Results suggest amaximum depth extent of 2.1 kmfor the tectonic contact at the southern border of the Jörn granitoid. Furthermore, this north-dipping Skellefte–Jörn contact coincides closely with magnetic lows and gravity highs, which implies that the Jörn intrusive rocks have a greater thickness than the underlying basalt. Further to the NW, gravity and magnetic data suggest a depth extent of 2 km for the Gallejaur complex, which coincides with a set of gently dipping reflectors. In addition, this study supports previous concepts of fault geometries and fault patterns as a result of upper-crustal extension and subsequent inversion during crustal shortening. In the final model interpretations of the IP data were included, thus relating indications of mineralization to the geological structures.
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| 17. |
- Tavakoli, Saman, et al.
(författare)
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Skellefte mining district in 3D : results from integrated interpretation of potential field, resistivity/IP and reflection-seismic data
- 2012
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Konferensbidrag (refereegranskat)abstract
- Located in northern Sweden, the Skellefte mining district has been subject to several geological and geophysical investigations, as it is hosting abundant volcanic-hosted massive sulfide deposits. The importance of mineral exploration at greater depths in the Skellefte District has been increased since most of mineralization at shallow depths are already discovered and exploited. Therefore, geophysical methods become particularly important as they can improve our knowledge about spatial relationship between geological features at the depth. In the first part (local-scale) of this study, we used resistivity/IP data to map the subsurface geometry down to 430m. Furthermore, the results of the resistivity/IP studies were constrained with potential field data down to 1.5 km depth. In the second part (Regional-scale), potential field data were used to constrain the interpretation of the reflection-seismic data down to 5 km depth. The result from the first part indicated a good correlation between the initial resistivity model and the magnetic and gravity field calculated from that model. In Part II, the gravity and magnetic data were investigated to better understand the contact between the Skellefte Group,volcanic rocks and the Bothnian Basin sedimentary rocks. Furthermore the method was used to constrain the geometry of late-orogenic gabbro-diorite and granite intrusions which occur along inferred shear zones that are only poorly indicated, or not visible at all on the reflection-seismic profiles. As the main outcome, the proposed integrated 3D model of the central Skellefte District (CSD) revealed crucial information about the spatial relationship between key lithologies which will be further used to understand the evolution of CSD in the 4th dimension, time.
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| 18. |
- Thunehed, Hans
(författare)
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A new fast method to measure azimuthal resistivity anisotropy
- 2001
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Ingår i: Proceedings of SAGEEP, the Symposium on the Application of Geophysics to Engineering and Environmental Problems, Environmental and Engineering Geophysical Society annual meeting. - Denver.
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Konferensbidrag (refereegranskat)
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| 19. |
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| 20. |
- Thunehed, Hans, et al.
(författare)
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Lycksele structure in northern Sweden: Result of an impact?
- 2000
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Ingår i: Large meteorite impacts and planetary evolution II. - Boulder, Colo : Geological Society of America. - 0813723396 ; , s. 125-30
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The Lycksele structure in northern Sweden is a large circular structure with a diameter of approximately 130 km. The structure has been defined from a combined analysis of topography, gravity, and magnetic data and is characterized by a circular system of faults, arc-shaped contacts between rocks, a quasi-circular system of downfaulted lowdensity granitic rocks, and an uplift of high-density rocks in the center. On the basis of ages of granite intrusions and a thrust zone that is cut by faults of the ring, together with paleomagnetic data, the age of the structure is between 1.80 and 1.26 Ga. The Bouguer gravity anomaly is similar to that associated with other known large impact structures on Earth. The rim of the structure, defined from the gravity gradients, magnetic data, and topography, is not restricted to specific rock types but cuts regional geologic structures as well as smaller intrusions, which may be expected for an impact structure in Precambrian deformed target rocks. Therefore, an impact origin seems most plausible for this structure, but to confirm this idea we need identifications of shock metamorphic features in the rocks.
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| 21. |
- Thunehed, Hans
(författare)
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Mapping and characterisation of Swedish bedrock by DC resistivity and transient-field electromagnetic measurement
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- DC electrical and electromagnetic methods have been used for a long time to detect and characterise fracture zones in site investigations and other engineering projects. However, the methods have not been used very much in basic geological mapping of crystalline rock on a regional to semi-regional scale. One reason for this is that measurements with traditional instrument set-ups used, for example, in ore and water prospecting are quite time- consuming. Three alternative methods for DC resistivity measurements are presented in this thesis. Vertical soundings can be performed efficiently with a modification of the traditional Schlumberger-array and large areas can be covered quickly with gradient array measurements. Resistivity anisotropy can be estimated with the new so called x-configuration. Nine electrodes, arranged in such a way that they form the letter x, are connected to an instrument in the centre. The apparent resistivity and apparent anisotropy can be found by least-squares fitting using different combinations of current and potential electrodes. Resistivity measurements using the new procedures have been performed in a number of areas. Systematic differences in resistivity were observed between different rock types indicating different intensities of fracturing. Marine volcanics for example have lower resistivity than terrestrial volcanics in the Arvidsjaur area. This means that it is possible to use resistivity measurements to characterise rock types and to detect boundaries between rocks of different character. The anisotropy was found to coincide with the foliation or bedding direction of the rock. Deep electrical soundings were performed in four areas in northern Sweden. The depth of investigation was several kilometres. The electric resistivity of the near-surface rocks was determined and their depth extent was estimated in three of the areas. A large lateral anomaly probably caused by marine volcanics was found in the fourth area. Transient-field electromagnetic (TEM) sounding is a method that has been used in mapping of sedimentary basins, geothermal exploration etc. The fundamental theory of TEM soundings and the processing of TEM sounding data are described in the thesis followed by examples of TEM soundings on crystalline bedrock. It is necessary to use large transmitter moments when measurements are made on highly resistive rock due to the weak secondary field. Bulk resistivity estimates made with DC measurements and TEM soundings were consistent in the Revsund granite area. TEM soundings offer a possibility to make bulk resistivity determinations to large depths in crystalline bedrock without the need for large transmitter-receiver separations. Fracture density and pore-water salinity are two important factors in the choice of a suitable location for a repository for highly active nuclear waste. A case history is presented where DC resistivity measurements were used in combination with TEM soundings in order to gain knowledge about the above mentioned factors. Two different types of granite were investigated and the resistivity of the younger Götemar granite was found to be around five times higher than the resistivity of the Småland granite, indicating a considerably lower fracture density in the former. The depth to saline groundwater was determined primarily with the TEM soundings and good agreement was found with data from deep drillholes.
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