Translations:Appendix K: Exercises/6/en

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4. Explain how well-log measurements of velocity and density provide a link between seismic data and the geology of the substrata. A sonic log is a plot of interval velocity as a function of depth. A strong low-frequency component with a distinct, blocky character represents gross velocity variations. This low-frequency component normally is estimated by velocity analysis of CMP gathers. In many sonic logs, the low-frequency component is an expression of the general compaction-derived increase of velocity with depth. The sonic log also has a high-frequency component superimposed on the low-frequency component. These rapid fluctuations can be attributed to changes in rock properties that are local in nature. For example, the limestone layer can have interbeddings of shale and sand. Porosity changes also can affect interval velocities within a rock layer. Seismic impedance is defined as the product of density and velocity. Because the vertical density gradient is in most cases much smaller than the vertical velocity gradient, the impedance contrast in those cases between rock layers is caused mostly by the velocity contrast only.