Tredimensionell bergundersökning med geoelektriska och geologiska metoder
Summary, in English
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.
- Engineering Geology
- Lithosphere and Biosphere Science
Stiftelsen bergteknisk forskning
- Geotechnical Engineering
- Geophysical and geological survey of rock quality in Dalby quarry for detailed comparison in 3D
- ISSN: 1104-1773