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.

Electrical resistivity tomography (ERT) and refraction seismics are among the most frequently used geophysical methods for site-investigations and the combined results can be very helpful to fill in the gaps between the point measurements made by traditional geotechnical methods such as Cone Penetration Test (CPT), core-drilling and geophysical borehole logging. The interpretation of the results from a geophysical investigation constituting a single method often yields ambiguous results. Hence, an approach utilizing multiple techniques is often necessary. To facilitate interpretation of such a combined dataset, we propose a more controlled and objective approach and present a method for a structurally coupled inversion of 2D electrical resistivity and refraction seismic data using unstructured meshes. Mean shift clustering is used to combine the two images and to compare the separate and coupled inversion methodologies. Two synthetic examples are used to demonstrate the method, and a field-data example is included as a proof of concept. In all cases a significant improvement by the coupling is visible. The methodology can be used as a tool for improved data interpretation and for obtaining a more comprehensive and complete picture of the subsurface by combining geophysical methods.

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 of the subsurface are necessary prior to the construction phase, but can be logistically difficult. Geophysics can help close the gaps between local point information and produce 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 and, from a measuring perspective, challenging area. The presented surveys cover a water passage along a 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 powerline 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 long underwater section of the 700 m long 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 part of the underwater passage cannot be detected by separated inversion. A simple synthetic study shows significant three dimensional artefacts corrupting the ERT model that have to be taken into account while interpreting the results. A structural coupling cooperative inversion approach is able to image the northern fracture zone successfully. In addition, previously unknown sedimentary deposits with a significant large thickness are detected in the otherwise unusually well documented geological environment. The results significantly improve imaging of some geologic features, which would have been not detected or misinterpreted otherwise, and combines the images by means of cluster analysis to a conceptual subsurface model.

No abstract

SUMMARYCollated DC resistivity and seismic profiling has become a popular method when doing pre-investigations for infrastructural projects such as roads and tunnels. The motivation behind the usage of two methods is that combined usage of both of these methods can decrease the ambiguity inherent to the individual use of these methods. Several software packages for inversion exist, but possibilities to combine several geophysical methods are limited; research algorithms that have this functionality exists and are evaluated herein. Joint inversion and fuzzy c-means cluster analysis are potentially ways to get the most of the collated data; thus enabling the methods to enforce each other is such a way that interpretation could be improved. These methods were applied on a field dataset from a pre-investigation of a new road. The joint inversion did not result in any major changes in the inverted models for this example, probably due to the very complex geology. The cluster analysis provides an additional tool for interpretation; highlighting for example a dolerite dyke, which can have strong influence on the construction and planning process.