WELL VELOCITY DETERMINATIONS IN THE ENGLISH TRIAS, PERMIAN AND CARBONIFEROUS

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Abstract
To obtain inferences useful in predicting the overburden velocities, a statistical study of velocity data was undertaken on 46 well velocity surveys confined mainly to Eastern England. Five formations were particularly studied: Keuper, Bunter, Permian, Coal Measures and Millstone Grit. Their interval velocities plotted against the mean depth of the interval below the surface supplied five equations of the form V=k. H1/n, which cover the range of depth explored down to 7000 ft. Comparison of these results shows that the interval velocities increase with depth and in the Keuper and Permian attain a maximum value nearly twice that in the remaining three formations. The interval velocities of these formations are confined within a relatively narrow band of 9000–12000 ft/sec. and the formations cannot be distinguished satisfactorily by their interval velocities alone. More practical results were obtained considering the relation between the vertical time and depth to the top of the Permian, Coal Measures, Millstone Grit and Carboniferous Limestone. For each of these a linear relation T=k. H+ To, was obtained from which the vertical time T can be predicted from the known depth with a probable error of ± 4 millisecs.

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