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Tredimensionell bergundersökning med geoelektriska och geologiska metoder

Jonsson, Peter LU orcid ; Johansson, Leif LU ; Johansson, Sara LU ; Olsson, Per-Ivar LU orcid and Dahlin, Torleif LU (2019) In BeFo Rapporter
Abstract
The aim of this project is to enhance the understanding of how geoelectrical investigations in a heterogeneous rock mass can image geological structures in order to develop and adapt future rock assessment methods.
A "good" rock quality forecast provides better opportunities for reduced risk level in the design and procurement phases of a construction project. Conversely, uncertainties in a forecast, in terms of rock quality, can entail large costs. The additional information obtained using non-destructive surveys will is therefore of large interest in this context. It has repeatedly (e.g. in connection with the construction of the Hallandsås tunnel) been shown that variations in, for example, the electrical properties, can be linked... (More)
The aim of this project is to enhance the understanding of how geoelectrical investigations in a heterogeneous rock mass can image geological structures in order to develop and adapt future rock assessment methods.
A "good" rock quality forecast provides better opportunities for reduced risk level in the design and procurement phases of a construction project. Conversely, uncertainties in a forecast, in terms of rock quality, can entail large costs. The additional information obtained using non-destructive surveys will is therefore of large interest in this context. It has repeatedly (e.g. in connection with the construction of the Hallandsås tunnel) been shown that variations in, for example, the electrical properties, can be linked to factors such as fracture zones, clay weathering or the presence of certain mineral types in the rock mass. .
The basic idea is to perform a three-dimensional measurement with simultaneous determination of DC resistivity and induced polarization, (DCIP), in a rock volume in a quarry. The examined rock volume is then removed by bench blasting as part of the normal quarry activities. The vertical benches that occur after each blast are documented with photogrammetric methods, geological sampling and by detailed studies with a sweep electron microscope (SEM). Furthermore, photographic methods such as panoramic photography and 3D computer models created with the help of unmanned aerial vehicles, drones, have been used.
The result is another three-dimensional model, with detailed geological information. This creates an opportunity to compare results and interpretations from the geoelectrical methods with geological information throughout the examined volume. For example, the three-dimensional distribution of fracture zones or dolerite dikes can be identified in the geological model and compared to the geophysical.
The measuring object in the project was a quarry in Dalby, 10 km East of Lund, Sweden and operated by Sydsten. The site is well investigated from both geological and geophysical viewpoints. The rock consists mainly of three different types: Granitic gneiss, dolerite and amphibolite, but smaller units of other rocks occur. The structures are complex with folding and formation of lenses, mainly in the amphibolite. Due to large-scale tectonic processes, the rock has been subjected to extensive deformation on several occasions. Brecciated and crushed zones occur as well as clay alteration zones.
The geophysical method used in the project is resistivity measurement with simultaneous measurement of induced polarization, DCIP. The resistivity method is based on the basic assumption that properties in the ground such as porosity, the actual rock matrix and the conductivity of the pore fluid are reflected in changes in the conductivity. The IP effects rely heavily on the internal composition of the geo-materials, filling in the pores, and structures in micro-scale and upwards.
The report initially describes the background and purpose, then the measurements and methodologies in the sections 2 and 3. In section 4 the collected measurement results and input data are described, including recommendations and experiences for panorama- and UAV photography, as well as the results of the geological mapping (visual inspection and SEM) and the geophysics.
One of the main objectives of project was to investigate how well geoelectrical measurements in a heterogeneous bedrock can depict geological structures. The ability to document the rock mass in the Dalby Quarry has given an opportunity to compare geological reality with results from DCIP measurements.
A comparison in section 5 between the geological and geophysical measurements confirms that the DCIP in the test environment can be used to indicate clay weathering zones, weakness zones and crushed rock. This can be used to distinguish rock mass with zones of clay weathering with potentially high fine material content from other rock, providing an opportunity to assess the quality before the fragmentation of the rock.
Further, it is noted that the ability to depict geological structures depends on the design of the geophysical investigation, the inversion process, and the obtained data quality. The data quality can to some extent be affected at the time of measurement, i.e. already during the planning of the assessment, while other factors cannot be affected using available measurement methodology.
One example is that dipping geological structures do not show up as clearly as vertical in the geophysical results. The reason for this is unclear. One explanation may be that the petrophysical contrast between, for example, gneiss and amphibolite is too small to be detected by geoelectrical methods, another that the numerical inversion process has difficulties representing these structures correctly.
It is also clear that visual geological attributes are not fully sufficient to explain all anomalies appearing in the geophysical model, in particular regarding the IP results. More detailed studies aimed at quantifying these complex effects are needed to understand these complex phenomena. The resistivity anomalies are better explained by the visual observations made. This is because resistivity to a greater extent depends on the composition of the rock mass and macro structures such as fractures, but also here a need to quantify and study correlations in laboratory scale exists.
The spatial resolution can be improved by modifying the measurement procedure. One way forward is to install electrodes in a borehole, in addition to the surface electrodes used today. This implies practical difficulties but has a great development potential for the future. Although modern instruments have been used in the project, instruments can be developed towards even more effective measurements, for example by using more channels for the potential measure-ment, dynamic measurement protocols, and adaptive current transmission that adjusts the measurement to the actual conditions on the site. (Less)
Abstract (Swedish)
Målet med denna studie är att ge ökad insikt i hur väl geoelektriska under-sökningar i en heterogen berggrund kan avbilda geologiska strukturer för att kunna vidare¬utveckla och anpassa framtida bergundersökningar.
En ”bra” prognos av bergkvalitet ger bättre möjligheter till minskad risknivå vid projektering och upphandling. Omvänt gäller att osäkerheter i bergtekniska prognoser vad gäller bergets kvalitet kan medföra mycket stora kostnadsökningar. Den tilläggs¬information som erhålls med hjälp av icke-förstörande undersökningar blir därför av stort intresse. Det har vid upprepade tillfällen (t.ex. i samband med byggandet av Hallandsåstunneln) visats att variationer i till exempel de elektriska egenskaperna, kan kopplas till faktorer... (More)
Målet med denna studie är att ge ökad insikt i hur väl geoelektriska under-sökningar i en heterogen berggrund kan avbilda geologiska strukturer för att kunna vidare¬utveckla och anpassa framtida bergundersökningar.
En ”bra” prognos av bergkvalitet ger bättre möjligheter till minskad risknivå vid projektering och upphandling. Omvänt gäller att osäkerheter i bergtekniska prognoser vad gäller bergets kvalitet kan medföra mycket stora kostnadsökningar. Den tilläggs¬information som erhålls med hjälp av icke-förstörande undersökningar blir därför av stort intresse. Det har vid upprepade tillfällen (t.ex. i samband med byggandet av Hallandsåstunneln) visats att variationer i till exempel de elektriska egenskaperna, kan kopplas till faktorer som sprickzoner, lervittring och vissa mineraltyper i bergmassan.
Projektets grundidé är att i en bergvolym i en bergtäkt utföra en tredimensionell undersökning med samtidig bestämning av likströmsresistivitet och inducerad polarisation (DCIP). Den undersökta bergvolymen sprängs sedan bort med pallsprängning. De vertikala bergskärningar som uppstår efter varje losshållning dokumenteras med fotodokumentation, drönarfotografering , provtagning och detaljstudier med svepelektron¬mikroskop, vilket efter hand skapar en tredimensionell modell med detaljerad geologisk information. På så sätt skapas en möjlighet att jämföra resultat och tolkningar från de geo¬elektriska metoderna med geologisk information i hela den undersökta volymen. Till exempel kan den tredimensionella utbredningen av sprickzoner, diabasgångar etc. följas i den geologiska modellen och jämföras med den geofysiska.
Som mätobjekt i projektet användes Sydstens bergtäkt i Dalby, Dalby stenbrott, öster om Lund. Detta är sedan tidigare väl undersökt både ur geologisk synvinkel samt med tvådimensionell resistivitets¬mätning. Berggrunden här består i huvudsak av tre olika bergarter: granitisk gnejs, diabas och amfibolit, men mindre partier av andra bergarter förekommer. Strukturerna är komplexa med veckning och bildning av linser, främst i amfiboliterna. Bergarterna har på grund av storskaliga tektoniska processer utsatts för omfattande deformation vid flera tillfällen. Breccior och krosszoner förekommer liksom leromvandlingszoner.
Den geofysiska metod som använts i projektet är resistivitetsmätning med samtidig mätning av inducerad polarisation, DCIP. Resistivitetsmätning bygger på det grundläggande antagandet att egenskaper i marken såsom porositet, själva bergarten (matrisen) och porvätskans ledningsförmåga avspeglas i förändringar i markens ledningsförmåga. IP-effekterna är starkt beroende av geomaterialens inre sammansättning, porfyllnad och struktur i mikroskala och uppåt.
För att skapa den geologiska modellen användes förutom traditionell geologisk okulär¬besiktning och provtagning även fotografiska metoder: panoramafotografering och framställning av tredimensio¬nella modeller med hjälp av drönarfotografering samt visualisering i dator.
I rapporten beskrivs inledningsvis bakgrund och syfte, därefter mätningar och metodik i avsnitten 2 och 3. I avsnitt 4 beskrivs insamlade mätresultat och dataunderlag, bland annat rekommendationer och erfarenheter för panorama-fotografering och drönarfotografering samt resultatet av den geologiska karteringen (okulärbesiktning samt elektronmikroskopering) och geofysik
Ett av huvudmålen med projektet var att undersöka hur väl geoelektriska undersökningar i en heterogen berggrund kan avbilda geologiska strukturer. Möjligheten att dokumentera berguttaget i Dalby stenbrott har givit en möjlighet att jämföra den geologiska verkligheten med resultat från resistivitets- och IP-mätningar.
En jämförelse i avsnitt 5 mellan de geologiska och de geofysiska mätningarna bekräftar att DCIP i den undersökta miljön kan användas för att indikera ler-vittrade zoner, svaghetsstrukturer och uppkrossade zoner. Detta kan användas till att särskilja bergmassa med zoner av lervittring (eller med potentiellt högt finmaterialinnehåll efter krossning) från annat berg, vilket ger en möjlighet till att bedöma kvaliteten innan losshållning skett.
Vidare noteras att förmågan att avbilda geologiska strukturer beror på under-sökningens utformning, den använda inversionsprocessen samt erhållen datakvalitet. Datakvaliteten kan i viss mån påverkas vid mättillfället, så redan under planerings¬stadiet av en DCIP-undersökning finns flera faktorer som kan påverka resultaten och mätningarnas upplösning, medan andra faktorer inte kan påverkas med dagens mätmetodik.
Ett exempel är att lutande geologiska strukturer inte återspeglades lika väl som vertikala i de geofysiska resultaten. Anledningen till detta är oklar. Antingen är de fysiska kontrasterna mellan till exempel gnejs och amfibolit för små för att detekteras med geoelektriska metoder, eller så misslyckas den numeriska inversions¬processen med att representera lutande strukturer korrekt.
Det står också klart att geologiska egenskaper grundade på visuella attribut inte fullt ut räcker för att förklara alla anomalier i den geofysiska modellen, speciellt för IP-resultaten. Fler och mer detaljerade studier med syfte att kvantifiera dessa komplexa effekter behövs för att tillföra kunskap om dessa komplexa fenomen.. Resistivitetsanomalierna förklaras dock bättre av de visuella observationer som gjorts. Detta beror på att resistiviteten i högre utsträckning beror på bergmassans sammansättning och makrostrukturer som sprickor, men även här finns ett behov av att kvantifiera och studera samband i laboratorieskala.
Den spatiala upplösningen kan förbättras genom att förändra mätförfarandet. En metod är att installera elektroder i borrhål, utöver de ytförlagda elektroderna. Detta medför praktiska svårigheter, men finns en stor utvecklingspotential inför framtiden. Även om moderna instrument har använts i projektet, kan instru-menten utvecklas mot ännu effektivare mätningar, till exempel genom att använda fler kanaler för potentialmätningen, dynamiska mätprotokoll och adaptiv strömsändning som anpassar mätningen till de faktiska förhållandena på platsen (Less)
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in
BeFo Rapporter
pages
84 pages
publisher
Stiftelsen bergteknisk forskning
report number
185
ISSN
1104-1773
project
Geophysical and geological survey of rock quality in Dalby quarry for detailed comparison in 3D
language
Swedish
LU publication?
yes
id
8ad97d07-363c-4e48-ab34-3c61c0b491d1
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http://www.befoonline.org/publikationer/r-185__1544
date added to LUP
2019-02-14 13:41:18
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@techreport{8ad97d07-363c-4e48-ab34-3c61c0b491d1,
  abstract     = {{The aim of this project is to enhance the understanding of how geoelectrical investigations in a heterogeneous rock mass can image geological structures in order to develop and adapt future rock assessment methods.<br/>A "good" rock quality forecast provides better opportunities for reduced risk level in the design and procurement phases of a construction project. Conversely, uncertainties in a forecast, in terms of rock quality, can entail large costs. The additional information obtained using non-destructive surveys will is therefore of large interest in this context. It has repeatedly (e.g. in connection with the construction of the Hallandsås tunnel) been shown that variations in, for example, the electrical properties, can be linked to factors such as fracture zones, clay weathering or the presence of certain mineral types in the rock mass. .<br/>The basic idea is to perform a three-dimensional measurement with simultaneous determination of DC resistivity and induced polarization, (DCIP), in a rock volume in a quarry. The examined rock volume is then removed by bench blasting as part of the normal quarry activities. The vertical benches that occur after each blast are documented with photogrammetric methods, geological sampling and by detailed studies with a sweep electron microscope (SEM). Furthermore, photographic methods such as panoramic photography and 3D computer models created with the help of unmanned aerial vehicles, drones, have been used.<br/>The result is another three-dimensional model, with detailed geological information. This creates an opportunity to compare results and interpretations from the geoelectrical methods with geological information throughout the examined volume. For example, the three-dimensional distribution of fracture zones or dolerite dikes can be identified in the geological model and compared to the geophysical.<br/>The measuring object in the project was a quarry in Dalby, 10 km East of Lund, Sweden and operated by Sydsten. The site is well investigated from both geological and geophysical viewpoints. The rock consists mainly of three different types: Granitic gneiss, dolerite and amphibolite, but smaller units of other rocks occur. The structures are complex with folding and formation of lenses, mainly in the amphibolite. Due to large-scale tectonic processes, the rock has been subjected to extensive deformation on several occasions. Brecciated and crushed zones occur as well as clay alteration zones.<br/>The geophysical method used in the project is resistivity measurement with simultaneous measurement of induced polarization, DCIP. The resistivity method is based on the basic assumption that properties in the ground such as porosity, the actual rock matrix and the conductivity of the pore fluid are reflected in changes in the conductivity. The IP effects rely heavily on the internal composition of the geo-materials, filling in the pores, and structures in micro-scale and upwards.<br/>The report initially describes the background and purpose, then the measurements and methodologies in the sections 2 and 3. In section 4 the collected measurement results and input data are described, including recommendations and experiences for panorama- and UAV photography, as well as the results of the geological mapping (visual inspection and SEM) and the geophysics.<br/>One of the main objectives of project was to investigate how well geoelectrical measurements in a heterogeneous bedrock can depict geological structures. The ability to document the rock mass in the Dalby Quarry has given an opportunity to compare geological reality with results from DCIP measurements.<br/>A comparison in section 5 between the geological and geophysical measurements confirms that the DCIP in the test environment can be used to indicate clay weathering zones, weakness zones and crushed rock. This can be used to distinguish rock mass with zones of clay weathering with potentially high fine material content from other rock, providing an opportunity to assess the quality before the fragmentation of the rock.<br/>Further, it is noted that the ability to depict geological structures depends on the design of the geophysical investigation, the inversion process, and the obtained data quality. The data quality can to some extent be affected at the time of measurement, i.e. already during the planning of the assessment, while other factors cannot be affected using available measurement methodology.<br/>One example is that dipping geological structures do not show up as clearly as vertical in the geophysical results. The reason for this is unclear. One explanation may be that the petrophysical contrast between, for example, gneiss and amphibolite is too small to be detected by geoelectrical methods, another that the numerical inversion process has difficulties representing these structures correctly.<br/>It is also clear that visual geological attributes are not fully sufficient to explain all anomalies appearing in the geophysical model, in particular regarding the IP results. More detailed studies aimed at quantifying these complex effects are needed to understand these complex phenomena. The resistivity anomalies are better explained by the visual observations made. This is because resistivity to a greater extent depends on the composition of the rock mass and macro structures such as fractures, but also here a need to quantify and study correlations in laboratory scale exists.<br/>The spatial resolution can be improved by modifying the measurement procedure. One way forward is to install electrodes in a borehole, in addition to the surface electrodes used today. This implies practical difficulties but has a great development potential for the future. Although modern instruments have been used in the project, instruments can be developed towards even more effective measurements, for example by using more channels for the potential measure-ment, dynamic measurement protocols, and adaptive current transmission that adjusts the measurement to the actual conditions on the site.}},
  author       = {{Jonsson, Peter and Johansson, Leif and Johansson, Sara and Olsson, Per-Ivar and Dahlin, Torleif}},
  institution  = {{Stiftelsen bergteknisk forskning}},
  issn         = {{1104-1773}},
  language     = {{swe}},
  number       = {{185}},
  series       = {{BeFo Rapporter}},
  title        = {{Tredimensionell bergundersökning med geoelektriska och geologiska metoder}},
  url          = {{http://www.befoonline.org/publikationer/r-185__1544}},
  year         = {{2019}},
}