| 1. |
- Danielsen, Berit Ensted, et al.
(författare)
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Model study of the resolution of resistivity tomography with different electrode arrays
- 2005
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Konferensbidrag (refereegranskat)abstract
- In connection to a large tunnel project in Sweden a model study of the resolution of resistivity tomography with different electrode arrays has been carried out. Resistivity tomography will be used within the tunnel project for in site measurements in boreholes in front of a TBM and possibly for monitoring the freezing of a weak zone.
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| 2. |
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| 3. |
- Danielsen, Berit Ensted, et al.
(författare)
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Comparison of geoelectrical imaging and tunnel documentation at the Hallandsas Tunnel, Sweden
- 2009
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Ingår i: Engineering Geology. - : Elsevier BV. - 0013-7952. ; 107:3-4, s. 118-129
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Tidskriftsartikel (refereegranskat)abstract
- For construction in rock a thorough pre-investigation is important in order to avoid unforeseen conditions which may delay the work. It is crucial to remember the results from this investigation in the further work, and use the experience from the construction to update the geological prognosis and reduce the uncertainties. Different geophysical methods have proved valuable tools in such investigations. In this work the electrical imaging is evaluated with regards to the method's applicability. The evaluation is done qualitatively by comparing the electrical imaging with tunnel documentation from a tunnel in Southern Sweden. By evaluating the result continuously when making the tunnel a more detailed geological prognosis can be compiled and used in the continued work with the tunnel. The parameters used for the comparison are lithology, Q, RQD, weathering and water leakage. The result was that virtually every change in electrical resistivity image coincides with a change in rock conditions. The general trend was that high resistivity corresponded with good quality gneiss whereas low resistivity corresponds to poor quality rock e.g., high weathering, low RQD, low Q and/or several lithological contacts. The intermediate resistivity is often amphibolites or rock with water bearing fractures. The results were supported by in-situ resistivity measurements inside the tunnel and resistivity logging in a core drilling. Geoelectrical imaging proved to give valuable information for a detailed geological model, which could be compiled for a section where the tunnel had not yet been drilled as a help for planning of the continued tunnel work. As is the case other geophysical methods it is clear that for the interpretation of data a priori information about the geological setting is necessary. (C) 2009 Elsevier B.V. All rights reserved.
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| 4. |
- Danielsen, Berit Ensted, et al.
(författare)
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Geoelectrical and IP Imaging Used for Pre-investigation at a Tunnel Project
- 2008
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Ingår i: - : European Association of Geoscientists & Engineers. ; , s. 44-44
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Konferensbidrag (refereegranskat)abstract
- At a large tunnel project in Southern Sweden a number of geoelectrical and IP imaging were carried out. The tunnel is drilled through a horst with a complex geology which includes fractures, high water content and weathered zones. The measurements where carried out to compile a geologic model to use in the further planning of the project. The resistivity and IP complements each other very well since they focus on different specific characteristics. IP proved to be useful at locating dolerites and resistivity was useful for the general distribution of fractures. An advantage is that with the seven-channel Lund Imaging system both types of data can be measured in a fast and cost efficient way. Magnetic measurements are a good supplement to pinpoint the exact position of the dolerite. Based on the general geologic information, resistivity and IP measurements recommendations can be given on where to drill in order to improve the geological model.
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| 5. |
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| 6. |
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| 7. |
- Danielsen, Berit Ensted, et al.
(författare)
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Numerical modelling of resolution and sensitivity of ERT in horizontal boreholes
- 2010
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851. ; 70:3, s. 245-254
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Tidskriftsartikel (refereegranskat)abstract
- Resistivity in horizontal boreholes can give useful detailed information about the geological conditions for construction in rock, i.e. in front of a tunnel bore machine. This paper is an attempt to identify a suitable methodology for an effective measuring routine for this type of geophysical measurements under actual construction site conditions. Prior to any measurements numerical modelling was done in order to evaluate the resolution of different electrode arrays. Four different arrays were tested; dipole-pole, cross-hole dipole-dipole, cross-hole pole-tripole and multiple gradient array. Additionally the resolution of a combination of cross-hole dipole-dipole and multiple gradient was assessed. The 2D sensitivity patterns for various arrangements of the cross-hole dipole-dipole and multiple gradient array were examined. The sensitivity towards inaccurate borehole geometry and the influence of water in the boreholes were also investigated. Based on the model study the cross-hole dipole-dipole array, multiple gradient array and a combination of these were found to give the best result and therefore were used for test measurements in horizontal boreholes. The boreholes were 28.5 m long and drilled 6.5 m apart. Prototypes of semi-rigid borehole cables made it possible to insert multi electrode cables in an efficient way, allowing fast measurement routines. These measurements were then studied to determine their accuracy and applicability. The results showed a high resistivity rock mass at the site. A transition from high resistivity to slightly lower resistivity coincides well with a change in lithology from gneiss-granite to gneiss. It is likely that the shotcrete on the tunnel wall is seen as a low resistivity zone. The measurements are a valuable tool, but further development of the cables and streamlining of measuring routines have to be performed before the resistivity tomography can be used routinely in pilot holes during construction in rocks. (C) 2010 Elsevier B.V. All rights reserved.
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| 8. |
- Danielsen, Berit Ensted
(författare)
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The Applicability of Geoelectrical Methods in Pre-Investigation for Construction in Rock
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Construction in rock is associated with risks because knowledge of the geology and ground conditions is limited. Unforeseen rock conditions involve a large risk to the project and can in the end entail delays and extra costs. To minimize the risks, an optimized pre-investigation program has to be conducted where essential information is gathered in order to make the best decisions throughout the construction project. In this Ph.D. thesis the main focus has been on the applicability of geoelectrical methods as a tool for predicting geological and rock mass conditions. The application of the geoelectrical methods at different scales has been proved to provide useful information at different stages of rock tunnel construction. In the geological setting at the Hallandsås Horst the method can indicate fractured, water bearing rock, weathered rock and to some extent lithology changes in crystalline bedrock. Large scale geoelectrical imaging is useful in the design/production planning stage and in the construction stage. Geoelectrical measurements are performed at a more detailed scale between two horizontal boreholes mainly in the construction stage. At even more detailed scale, geoelectrical methods may be combined with other geophysical methods in borehole logging and be applied late in the design/production planning stage. Additionally, borehole geophysics is important for in situ correlation/verification of the large-scale geoelectrical data. In an attempt to demonstrate the applicability of geoelectrical imaging in pre-investigations for rock tunnel construction, a framework for Value of Information Analysis (VOIA) has been developed. The VOIA is used for choosing the pre-investigation program best suited for a specific geological environment. VOIA is based on Bayesian statistics and cost-benefit analysis and is suitable for problems where different alternatives are evaluated and compared. In VOIA the cost for new information is compared with the reduced risk for taking an economically unfavourable decision. New information is only interesting when it can change the outcome of the decision and thus is of value for the decision-maker. The cost of an investigation or measurement should be less than what is expected to be saved; otherwise the investigation should not be made. The VOIA of geophysical methods used in pre-investigation showed indisputably that the value of performing geoelectrical imaging and ground based magnetic measurements prior to drillings has a higher value than only drilling. This result is only valid for this particular geological setting and is site specific. Nevertheless the framework can help designing the best measurement program for a specific geological setting if the VOIA is used to decide between different geophysical methods, e.g. geoelectrical imaging, seismic, magnetic or a combination. The framework developed has the potential to become an integral part of any pre-investigation. With an optimized pre-investigation with well integrated results, the uncertainty in the engineering geological prognosis is reduced and the risk that something unexpected happens is reduced. Geoelectrical imaging and borehole geophysics contributes to reduce the uncertainties and should therefore be considered as a prospective part of all pre-investigations as well as the production stage.
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| 9. |
- Danielsen, Berit Ensted, et al.
(författare)
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The applicability of geoelectrical methods in pre-investigation for construction in rock
- 2012
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Konferensbidrag (refereegranskat)abstract
- Constructing in rock will always involve unforeseen rock conditions. This involves a large risk to a project and can in the end entail delays and extra costs. To minimize the risks, an optimized pre-investigation program has to be conducted where essential information is gathered in order to make the best decisions throughout the construction project. In this work the main focus has been on the applicability of geoelectrical methods as a tool for predicting geological and rock mass conditions. The application of the geoelectrical methods at different scales has been used to provide useful information at different stages of rock tunnel construction. In the geological setting at the Hallandsås Horst, Sweden, the method indicate fractured, water bearing rock, weathered rock and to some extent lithology changes in crystalline bedrock. Large scale geoelectrical imaging is useful in the design/production planning stage and in the construction stage. Geoelectrical methods may be combined with other geophysical methods in borehole logging and be applied late in the design/production planning stage. Additionally, borehole geophysics is important for in situ correlation/verification of the large-scale geoelectrical data.
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| 10. |
- Davydycheva, Sofia, et al.
(författare)
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3D Electromagnetic Modeling and Quality Control of Ultradeep Borehole Azimuthal Resistivity Measurements
- 2023
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Konferensbidrag (refereegranskat)abstract
- Reliable interpretation of borehole electromagnetic (EM) measurements acquired in horizontal and high-angle wells requires fast, robust, and versatile solutions of forward and inverse problems of Maxwell's system in spatially complex 3D anisotropic formations. Based on recent advances in numerical simulation methods, we implement new 3D anisotropic EM modeling and inversion software and algorithms to simulate ultra-deep azimuthal resistivity (UDAR) measurements and to perform their quality control (QC). The combination of fast modeling and inversion under complex and anisotropic 3D earth-model conditions enables us to accurately quantify the limits of resolution and uncertainty of UDAR measurements.Our software and algorithms allow fast and robust modeling based on the finite-volume homogenization technique together with a special reduced-order gridding procedure. This modeling strategy enables the use of model-independent finite-volume grids in tool coordinates combined with a global-model grid accepting inputs from commonly used 3D earth-model rendering formats. While the tool moves along the well trajectory, the formation determined on the 3D global grid shifts and rotates in tool coordinates. Furthermore, we implement several fast direct and iterative solvers in the modeling/inversion workflow, all of which yield practically identical results. Parallel computing also allows real-time modeling.Our modeling approach is effective for the multi-dimensional inversion of UDAR profiling/logging measurements acquired along arbitrary well trajectories. Benchmarks and examples of UDAR simulations on operator's 3D subsurface models confirm the efficacy of our simulation method. We describe benchmark examples including 3D simulation of commercial UDAR measurements acquired across a spatially complex formation model with two faults. Numerical simulation times for 3,000 couplings of logging points and tool configurations are less than 8 CPU hours on a typical laptop and less than 20 seconds on a supercomputer. The benchmark was also verified against an independent 3D EM modeling method. Our 3D fully anisotropic modeling software can be used for real-time inversion and QC of commercial UDAR tool measurements. A 3D simulation based on a 2D model of the well curtain section (obtained as stitched-together 1D models, i.e., results obtained from 1D inversion of commercial measurements) and comparison of this simulation to actual tool measurements identifies the sections of the well trajectory where 2D-3D inversion is needed to decrease the data misfit to acceptable values (i.e., measurement noise levels).Future endeavors include fast fully anisotropic 2D-3D measurement simulation using adaptive upscaling of 3D models and novel 2D-3D inversion algorithms specifically designed for UDAR measurement conditions. Our goal is to develop real-time 2D and 3D inversion of UDAR measurements for well geosteering and refined 3D subsurface model rendering as additional measurements and geometrical constraints are included into the inversion by asset teams.
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