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

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.

Delineating fracture zones using surface‐tunnel‐surface seismic data, P‐S, and S‐P mode conversions

“…Parallel to our seismic experiment, an independent seismic survey was conducted on the sea above the tunnel, next to a small island in the central part of the site (Figure ; marine seismic experiment arrow) [see Ronczka et al ., ]. Data from four of these explosive sources were merged into our data set.…”

Data acquisition, processing and filtering for reliable 3D resistivity and time‐domain induced polarisation tomography in an urban area: field example of Vinsta, Stockholm

“…The inversion converged both for electrical resistivity and integral chargeability, obtaining a mean residual of the weighted squared error <1 in 15 iterations. The coverage, which is the overall integral sensitivity for all measurements in each model cell normalised by the cell volume, is a robust estimate of model resolution and, thus, reliability, as demonstrated by Ronczka et al (). Therefore, we restricted all images of the results to a minimum threshold so that only well‐resolved structures in the model are shown.…”

Joint inversion of lake-floor electrical resistivity tomography and boat-towed radio-magnetotelluric data illustrated on synthetic data and an application from the Äspö Hard Rock Laboratory site, Sweden

“…The cable outlets on land were connected to plate electrodes, which were buried where glacial sediments were present and attached on the granite bedrock with a conductive gel on outcrops. More details of the ERT data acquisition can be found in Ronczka et al (2017). ERT stations used for joint inversion are marked by green dots in Fig.…”