Detailed Seimic Velocities in Sediments Along a 250 km Profile in the Barents Sea

Abstract

Seismic measurements are the most powerful, indirect method of observing the deeper structure of the earth, and they are extensively used in investigations of marine sediments. An important output from seismic measurements is the velocity-depth variation, and in marine seismic investigations, seismic velocity is frequently used as an indicator of type, age and consolidation of sediments. The present paper will mainly be concerned with the interpretation of travel time graphs and conversion of travel times to velocity depth values, but in addition an attempt is made at geophysical interpretation of the derived velocity profiles in order to demonstrate the importance of vertical and horizontal variations of seismic velocities. A continuous travel time graph is frequently approximated by fitting a series of line segments to the curve, the solution giving rise to a model consisting of layers of constant velocity. The method is satisfactory with respect to quantitative evaluation of sedimentary deposits, but the velocity-depth function obtained in this way is of limited value in detailed geophysical and geological interpretations. Increasing the number of line segments used to approximate the travel time curve results in a solution approaching the real velocity distribution. The approximations induce a large number of discontinuities in the solution, and it is important that the investigator makes a distinction between 'artificial' and physically significant discontinuities. The authors' impression is that, in many cases, solutions presented in connection with sonobuoy data give more information about the number of line segments necessary to approximate the travel time graph than about layering of sediments.