Abstract
AbstractThe article is based on data from 74 seismic well velocity surveys in N.W. Germany.The effect of overburden is illustrated by means of diagrams representing interval velocities against depth. To eliminate observational errors and lithological variations from well to well, interval velocities from about the same depth are averaged. Thus, relatively uniform curves are obtained for lower Tertiary, Upper Cretaceous, and Lower Cretaceous (Albian), making the effect of overburden better evident than from individual observations. The vertical gradient of velocity for these geological sections are given. Furthermore, they are compared with curves for Lower Tertiary and Cretaceous obtained in N. America. Due to its high lime content, the Upper Cretaceous of N.W. Germany has a much higher velocity than that observed in the Cretaceous of N. America. The curves for shales of the Lower Tertiary and Lower Cretaceous (Albian), however, are similar in both countries. In older formations, on the other hand, e.g. Jurassic and Triassic, lithologic character has the dominant influence and the effect of overburden is rather small and often scarcely noticeable.The data considered cover a depth range down to 2,700 meters. The interval velocities are generally based on the use of various equal intervals.
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