does density. Weatherby and Faust (1935) demonstrated the effect of geologic 

 age upon velocity in a study of data from some 50 well velocity surveys. It has 

 been noted (Athy and McCollum, 1937) that higher velocities are sometimes 

 encountered over structural highs. 



Lithology is the most influential factor, and its effect may be attributed 

 largely to mineral composition, porosity, degree of cementation, and the nature 



TABLE 26-1 



Longitudinal Wave Velocities 

 After Birch, "Handbook of Physical Constants' 



of interstitial fluid. Clearly, the kind of rock has a dominant effect upon velocity. 

 Igneous rocks, in general, have higher velocities than sedimentary rocks although 

 there are exceptions. Among the sediments, carbonates and evaporites exhibit 

 higher velocities than do sandstones and shales. 



The presence of interstitial fluids has a diverse effect upon velocity, which 

 has been studied by many investigators in recent years. Seismic field crews 

 long have observed the increase in velocity produced by water saturation of 

 unconsolidated surface material. Recent continuous velocity logging data (Hicks 



560 



