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Abstract

This study provides results on buried tunnel valleys from the greatest number of released three-dimensional (3D) seismic surveys in the Danish North Sea sector to date. The valley infill sediments and the fluid content from buried Pleistocene tunnel valleys beneath the North Sea floor cause significant seismic velocity effects. Analysis of conventional 3D seismic data, high-resolution two-dimensional (2D) seismic data and wire-line log data from exploration wells reveal a multistage cut-and-fill history of tunnel valleys in the Danish North Sea. The seismic infill facies of the valleys is analysed and the associated velocity effects quantified. The valley infill generates both velocity pull up and push down of underlying seismic horizons. In contrast to expectations, our analysis shows no distinct correlation between particular seismic facies and a particular travel-time effect. Infill units characterized by the same seismic facies within the same valley can thus provide different travel-time effects. The anomalous velocities can be used as indicators of shallow gas and till content within the valleys leading to low and high velocities, respectively. Detailed investigations suggest that tunnel valley formation and sediment infill occurred over several episodes, with partial reuse of the kilometre-scale valleys and strong indications of ice-free periods between some of the formation episodes.

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