Technischer Bericht NTB 90-34

Sondierbohrung SiblingenUntersuchungsbericht

In 1980, Nagra (the National Cooperative for the Disposal of Radioactive Waste) launched an extensive geological investigation programme covering an area of some 1'200 m2 in Northern Switzerland. The aim of the programme was to collect the geological information necessary for assessing the suitability of the crystalline basement as a disposal medium for high-level radioactive waste. The wide range of investigations performed can be divided logically into a deep drilling programme, geophysical reconnaissance of the rock formations and structures in the region, a hydrogeological programme for investigation of water flow-paths deep in the bedrock and a neotectonic programme for identification of active crustal movements in the area.

The Siblingen borehole was the seventh in the deep drilling programme. The drill site (coordinates 680.090/286.693, 574.35 m a.s.l.) lies within the community of Siblingen in Canton Schaffhausen, approximately 1.5 km north-west of the Siblingen village boundary. The final depth of the borehole was 1'522 m.

The borehole penetrated through the sedimentary overburden and more than 1,000 m into the crystalline basement. A wide range of investigations was carried out in the borehole itself, as well as on drillcores and water samples. State-­of-the-art methods were used to draw a detailed picture of the geology of the basement and the deep groundwater regime.

Drilling began on 1st September 1988 and continued, with regular interruptions for scientific investigations, until 2nd April 1989. During the whole of April 1989, further tests and investigations were carried out. Because of prevailing hydraulic conditions (numerous interconnected disturbed zones), it was decided not to install a multipacker system for long-term observations and, at the beginning of June, an additional fluid logging run with a newly developed heat pulse packer-flowmeter was performed. An estimate of the horizontal regional gradient using return flow of tracer-marked drilling fluid to the borehole, investigations of the influence of earth tides on water levels and several temperature/conductivity logs completed the programme. Periodical water measurements have been carried out since 17th May 1989 and are still continuing. The planned investigation programme was carried out more or less in full, and with a considerable degree of success.

The drilling and investigation programmes were planned and managed by Nagra employees with the cooperation of external advisers. More than 40 technical colleges, contractors and service companies from Switzerland, Germany, France, Great Britain, the Netherlands, Canada and the USA, representing a total manpower of over 150 scientists and technicians, were involved in the investigations.

This report summarises the most significant results obtained during the course of the programme. The series of investigation reports for the deep boreholes (Böttstein, Weiach, Riniken, Leuggern, Schafisheim, Kaisten and Siblingen) is structured in such a way that cross comparisons can easily be made between the results and datasets for the different locations. However, the conclusions to be drawn from the data will be reserved for a final synthesis report. Detailed documentation of the individual experiments is provided in Nagra's Technical Report series (see NTB list at the end of this report).