Seismic Investigations in the North Sea, 3 to 7

Abstract

Summary Parts 1 and 2 of this paper explained why a seismic crustal investigation programme was started in the North Sea, and gave a description of the sedimentary structure off Flamborough Head as resulting from five reverse refraction lines. These lines cover the 200-kilometres long crustal refraction line which was shot using a variant of the velocity-depth or common reflection point method. The present paper gives in part 3 the results of this 200-km line. The line appeared to be situated over a zone in which the crust shows a considerable thinning. This violates the presumptions of the velocity-depth method. Structurally, the problem could only be solved by also using the records of the U.K. Atomic Energy Authority seismic station at Eskdalemuir. Part 4 presents the revision of a crustal refraction line at the Doggerbank, also using the velocity-depth method, and shot in 1964. In part 5 the anomalous station term of Eskdalemuir is discussed. It is suggested that a high velocity/low density body is situated under Eskdalemuir, possibly consisting of anorthosites. Part 6 deals shortly with a crustal reflection experiment in 1966 at the site of the 1965 refraction line off Flamborough Head. The negative outcome of this experiment is ascribed to insufficient instrumentation and strong water reverberations occurring in our region. Part 7 concludes the paper with a general discussion on the structure of the Earth's crust under the North Sea, on structural effects in both the seismic refraction and the reflection method, and on the use of crustal seismic experiments in general.