Technischer Bericht NTB 00-03

Nagra is investigating the Opalinus Clay in north-east Switzerland as a potential host rock for a deep geological repository for high-level and long-lived intermediate-level waste. In 1991/92, a campaign of 2D reflection seismic profile measurements was carried out, to investigate the Mesozoic sedimentary rocks in north-east Switzerland. The results of this survey were used as a basis for identifying an area of around 50 km² with almost undisturbed stratification in the north of Canton Zürich for further research. With the three-dimensional reflection seismic survey (3D seismics) described here, the structural geology and lithostratigraphy particularly of the Opalinus Clay over the depth range of interest (400 to 900 m below ground surface) was further investigated. Together with the information from the Benken borehole, which was drilled in the survey area in 1998/99, the results of the 3D seismic measurements provide a sound basis for evaluation of the investigation area.

The 3D seismic method allows the structure and spatial orientation of rock strata to be completely investigated and imaged from the earth's surface. Structural geological elements and lateral changes in the character of the rock can be recorded on a large-scale. The results from the 3D seismic survey make a significant contribution to improving the overall understanding of the geological history and allow a structural model to be derived, which can be used as a basis for hydrogeological modelling, for performance assessment and as input to planning of engineering projects.

The 3D seismic field measurements carried out between January and March 1997 placed tough requirements on the team performing the survey, in the sense that the reflection signals had to be recorded in a relatively densely populated and intensively farmed region. Almost 9'000 seismic source points distributed over the survey area were used, each recorded with 480 geophone stations divided over a strip of eight receiver lines. The field parameters were selected with the aim of efficient implementation in the field, while at the same time ensuring the best possible data quality. Symmetrical receiver line spacings of 180 m and station intervals of 30 m allowed the underground to be scanned on a regular grid of 15 by 15 m. Thanks to the relatively high multiple coverage of each underground area, it was usually possible to balance out the perturbing effect on data quality of near-surface water-saturated strata and deep Quaternary channel deposits. The 3D DMO stacking and 3D migration procedures applied during data processing contributed the most to the marked improvement in the signal to noise ratio of the recorded field data. The relatively wide usable frequency spectrum of about 10 to 85 Hz at the target horizon resulted in high quality imaging of the rock strata.

There is an excellent correlation between the 3D reflection seismic data measured from the surface and the geophysical data recorded in the Benken borehole. The synthetic seismogram derived from sonic and density logging in the borehole allows the reflection data to be calibrated accurately. In addition, the walkaway VSP helps to bridge local variations in quality in the 3D seismic dataset in the vicinity of the borehole and thus enables the high resolution measurements from the borehole to be extrapolated over the entire survey area. The values for the parameters thickness, density and seismic wave propagation velocity determined in the Benken borehole can be used as input to a modelling procedure which allows the interpreted reflection seismic data also to be analysed in terms of lithostratigraphy.

The high spatial resolution of the 3D seismics allows detailed analysis of geological structures down to a metre scale. The structural analysis was carried out using conventional amplitude displays in profile sections, from which faults and flexures with a vertical displacement of down to 10 m can be recognised directly. Coherence data and seismic attribute maps can be used to identify even smaller structures. These structures, which have been made visible for the first time, throw new light on the interpretation of the local structural geology.

All the structural elements intersecting the Opalinus Clay host rock interval, including the Murchisonae Beds, which fulfil the criteria set for minimum lateral and vertical extent were illustrated and described in a catalogue. All faults, which displace the Opalinus Clay for at least 10 m and show lateral extent along strike of at least 150 m, have been quantified and mapped. The analysis confirms the almost undisturbed stratification of the host rock in the investigation area. It can generally be said that the degree of deformation is small down to the level of the Opalinus Clay layers and shows a general increase towards the Base Mesozoic. As a first approximation, three different tectono-stratigraphic units can be distinguished:

• Upper Mesozoic – Molasse: Faults with mappable displacement of all reflection horizons over larger distances can be identified only in the north-east of the survey area, in association with the Neuhausen fault.
• Lower Mesozoic: The base of the Mesozoic and the Triassic layers show small-scale deformation particularly in the south-west of the survey area. Compression structures are an indication of inversion tectonics and overprinting of basement structures in the period extending from the Middle to Late Triassic. Some of these inversion structures appear to have been slightly reactivated by extensional tectonics during the Liassic and lower Dogger. Local differences in thickness within the host rock are recognised and can be interpreted as a result of differential subsidence at the beginning of the deposition of the Opalinus Clay.
• Sub-Mesozoic: Interpretation of the Permo-Carboniferous sequence using the 3D reflection seismic image and the regional seismic profiles from 1991/92 allows a distinction to be drawn between the crystalline high zone penetrated in the Benken borehole and the WSW-ENE-striking Permo-Carboniferous trough (Weiach trough) in the southern part of the 3D survey area.

The main structures in the 3D survey area are the high zone of Benken, with its bounding elements of the Rafz – Marthalen flexure in the south and the Wildensbuch flexure in the north. In the north-east, the survey area is bordered by the Neuhausen fault already known from earlier regional investigations, in the east by the Trüllikon anticline and in the west by the Niderholz structural zone. The W-E striking Wildensbuch flexure is the most prominant structure in the central part of the survey area. This feature is made up of a flexure zone and individual en échelon fault elements; it runs around 700 m to the north of the Benken borehole and merges with the Neuhausen fault in the east. The fault elements associated with the Wildensbuch flexure have a maximum vertical displacement of around 17 m at the level of the Opalinus Clay.

As part of the seismo-stratigraphic analysis, an initial step involved calculating different attributes in order to visualise local changes in seismic character in various depth intervals. Based on the results from the Benken borehole, the lithostratigraphic conditions were then modelled at four selected points in the 3D survey area, from the top of the Effingen Beds to the lower Gipskeuper. This provided qualitative information on seismo-stratigraphic changes in the vicinity of the host rock and the surrounding confining units ("Rahmengesteine").

An existing regional model of seismic wave propagation velocities, supplemented with data from the Benken borehole, was used to calculate depth and thickness maps from the reflection times of the evaluated seismic horizons. The thickness map for the Opalinus Clay interval, including the Murchisonae Beds, indicates a general thickness of the strata between 105 and 125 m, with a gradual decrease towards the south-east. Further small-scale thickness variations within the range 90 to 140 m are associated with local tectonic structures.