Articles | Volume 8, issue 3
Solid Earth, 8, 671–682, 2017
Solid Earth, 8, 671–682, 2017

Research article 13 Jun 2017

Research article | 13 Jun 2017

Electric resistivity and seismic refraction tomography: a challenging joint underwater survey at Äspö Hard Rock Laboratory

Mathias Ronczka1, Kristofer Hellman1, Thomas Günther2, Roger Wisén1, and Torleif Dahlin1 Mathias Ronczka et al.
  • 1Engineering Geology, Lund University, Lund, Sweden
  • 2Leibniz Institute for Applied Geophysics, Hanover, Germany

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.

Short summary
Pre-investigation for tunnelling below water passages is a challenging task with the main objective of locating fracture zones that lead to low rock quality and thus reduced stability. An inversion approach was tested that combines different geophysical methods to improve the reliability of the results. A fracture zone and previously unknown sedimentary deposits were successfully detected. Synthetic studies pointed out the importance of 3-D effects and model resolution properties.