Abstract
Abstract. Tunnelling below water passages is a challenging task in terms of planning, pre-investigation and construction. Fracture zones in the underlying bedrock lead to low rock quality and thus reduced stability. For natural reasons, they tend to be more frequent at water passages. Ground investigations that provide information on the subsurface are necessary prior to the construction phase, but these can be logistically difficult. Geophysics can help close the gaps between local point information by producing subsurface images. An approach that combines seismic refraction tomography and electrical resistivity tomography has been tested at the Äspö Hard Rock Laboratory (HRL). The aim was to detect fracture zones in a well-known but logistically challenging area from a measuring perspective. The presented surveys cover a water passage along part of a tunnel that connects surface facilities with an underground test laboratory. The tunnel is approximately 100 m below and 20 m east of the survey line and gives evidence for one major and several minor fracture zones. The geological and general test site conditions, e.g. with strong power line noise from the nearby nuclear power plant, are challenging for geophysical measurements. Co-located positions for seismic and ERT sensors and source positions are used on the 450 m underwater section of the 700 m profile. Because of a large transition zone that appeared in the ERT result and the missing coverage of the seismic data, fracture zones at the southern and northern parts of the underwater passage cannot be detected by separated inversion. Synthetic studies show that significant three-dimensional (3-D) artefacts occur in the ERT model that even exceed the positioning errors of underwater electrodes. The model coverage is closely connected to the resolution and can be used to display the model uncertainty by introducing thresholds to fade-out regions of medium and low resolution. A structural coupling cooperative inversion approach is able to image the northern fracture zone successfully. In addition, previously unknown sedimentary deposits with a significantly large thickness are detected in the otherwise unusually well-documented geological environment. The results significantly improve the imaging of some geologic features, which would have been undetected or misinterpreted otherwise, and combines the images by means of cluster analysis into a conceptual subsurface model.
Highlights
Underground structures have become an increasingly important part of modern infrastructure, and the possibilities to improve construction approaches have attracted much attention
Hydrophone positions were determined by a differential GNSS, while the topography of the seabed was mapped with a multibeam echo sounder (MBES)
We used the open-source electrical resistivity tomography (ERT) software package BERT (Boundless Electrical Resistivity Tomography) for ERT inversion (Günther et al, 2006b) using irregular triangle meshes to accurately take into account both the surface and submarine topography (Rücker et al, 2006)
Summary
Underground structures have become an increasingly important part of modern infrastructure, and the possibilities to improve construction approaches have attracted much attention. Except for southwestern Scania and the islands Gotland and Öland, crystalline bedrock is the dominant material for underground infrastructure construction in Sweden For these geologic conditions, weakness zones that are im-. The Swedish transportation authority has provided funding for research in an increasing number of projects with the aim of developing site investigations based on additional geophysical measurements for mapping the structure and quality of the rock mass. The final velocity and resistivity models were consistent with deformed sedimentary units Another multidisciplinary geophysical approach for mapping a fault zone is given by Malehmir et al (2016). The methodical approach of a structurally coupled joint inversion presented in this study shows how results can be improved such that an easier and more unique interpretation of the underground models is possible. After describing the site conditions and the numerical background, we show a synthetic study on the 3-D effects and the influence of the seabed topography on ERT data before the analysis and interpretation of the field data is presented
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