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Pseudo-diagraphies de vitesse en offshore profondNormal access

Author: M. Lavergne
Journal name: Geophysical Prospecting
Issue: Vol 23, No 4, December 1975 pp. 695 - 711
DOI: 10.1111/j.1365-2478.1975.tb01554.x
Organisations: Wiley
Language: English
Info: Article, PDF ( 1021.44Kb )

Summary:
Pseudo-velocity-logs are tentative determinations of subsurface velocity variations with depth, using both information of seismic amplitude and reflection curvature.

A rigorous theoretical method would consist in

While this method works with synthetic examples, it is not generally applicable to field cases, one of the reasons being the poor reliability of desynthesization in the presence of noise.

The present method is a compromise between a rigorous and a practical process: the complete desynthesization is not performed; deconvolution and demultiplication are done by more classical techniques using real amplitudes; absolute velocities are determined to fit both the reflection coefficients and the rms velocities. It leads to pseudovelocity-logs, accurate enough to show lithologic variations, smoothed enough to preserve the signal/noise ratio.

Examples are shown of Flexichoc profiles recorded in 2500 m (8000–9000 feet) deep areas of the Mediterranean Sea. Pseudo-velocity-logs show 1000 m (3000 feet) of a velocity-increasing-with-depth Plio-pleistocene marl formation, overlying Miocene evaporites. Intercalations of high and low-velocity layers in the evaporites seem to indicate vertical facies variations.

The Pseudo-velocity-log, associated with other lithologic determination processes, should become a geological tool for deep offshore exploration.


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