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Arbeitsbericht NAB 13-40

Gravity Data in Northern Switzerland ans Southern Germany

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The primary goals of the "Gravity Data in Northern Switzerland and Southern Germany" project were to (i) compile a new Bouguer gravity map approximately centred about Nagra's five siting regions in Northern Switzerland and (ii) derive a suite of residual and filtered gravity maps that can be used to delineate tectonic features, geological contacts, buried Quaternary valleys and other structures. Initially, gravity data and digital terrain models from multiple sources were evaluated. Based on this evaluation, gravity data from one source in Switzerland and three sources in Germany were eventually included in the gravity compilation, and 25 × 25 m digital terrain models for Northern Switzerland and Southern Germany and a 60 × 60 m digital terrain model for neighbouring regions were combined to form a composite digital terrain model. For calibration purposes and to fill in gaps, new gravity data were acquired. Standard processing of the compiled gravity data set that was based on the composite digital terrain model produced a complete Bouguer gravity map for Northern Switzerland and Southern Germany in Swiss coordinates. The initial part of the project was concerned with the entire complete Bouguer gravity map. Upward continuation proved to be an appropriate method for defining the regional component of the Bouguer gravity field and associated residual gravity field. Before applying various filters to the Bouguer gravity data, it was necessary to suppress the effects of numerous very short wavelength anomalies (many of these low amplitude anomalies were defined by only one or two data values) by upward continuing the data 200 - 1000 m. Filters that produced useful maps included the analytic signal, tilt derivative, maximum horizontal gradient and second vertical derivative. The suite of maps was displayed using colour schemes based on linear or histogram-equalised distributions. To enhance subtle but potentially important structures, histogram-equalised shaded relief was added to the residual gravity maps.

In addition to the images based on the entire complete regional Bouguer gravity map for Northern Switzerland and Southern Germany, a variety of large-scale residual and filtered gravity maps were derived for six investigation sites in order to study the gravity signatures of geological features in the shallow subsurface. Features of specific interest were Permo-Carboniferous troughs and buried Quaternary valleys. Four sites encompassed the proposed siting regions for high- and low-level radioactive waste disposal in northern Switzerland (i.e. (1) Zürich Nordost and Südranden, (2) Nördlich Lägern, (3) Jura Ost and (4) Jura-Südfuss). The other two sites included a length of the Thur Quaternary valley and another incorporated part of the Upper Glatt Quaternary valley.

Relatively negative residual gravity anomalies correlate with the proposed locations of Permo-Carboniferous troughs. Many buried Quaternary valleys have distinct expressions in the various gravity-based maps, whereas others are barely perceptible. Some relatively narrow buried valleys are not well-delineated as a result of the sparse sampling of the gravity data and/or due to small density contrasts between valley fills and bedrock.

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