Studies on elastic wave velocities in rocks have proved increasingly useful in the geologic investigation of sedimentary basins. Velocities of sediments are

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closely interrelated with their lithology, structure, and fabric, and strongly reflect their alteration through diagenetic processes. Perhaps the most important of these processes, the compaction of clastics, causes progressive and unilateral reduction of pore space and expulsion of fluids. Compaction status--especially of shales--can be measured by interval velocities which increase irreversibly up to the maximum burial depth. In any specific basin, therefore, shale velocities indicate later uplift and thus aid in recalculating the thickness of eroded sediments. Shale velocities, when observed vertically and parallel with the bedding plane, exhibit an anisotropy which probably decreases with growing lateral tectonic stress. Rapidly deposited shales and shales embedded in evaporites, how retarded compaction and consequently delay in velocity increase. Psammites and carbonates have more complicated velocity patterns due to the complexity of their diagenesis. Evaporites show little compaction because of their primary lack of porosity.

All these phenomena are reflected in velocities and have been studied in basins where numerous well-velocity data were available (e.g., NW German basin). Seismic velocity data, collected through advanced modern techniques, serve the same purpose. Although their accuracy is still comparatively limited, an almost quantitative interpretation may be reached with the aid of mathematical and statistical treatment of sedimentation models.

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