and Berry, 1956) indicate that the presence of liquids increases the seismic 

 velocity; in fact, for a given rock framework, a separation among gas, oil, and 

 water-filled zones may be made on the basis of comparative velocities. Hicks and 

 Berry (1956) illustrate these relations in the idealized logs shown in Figure 26-2. 

 Contrarily, laboratory data of Hughes and Kelly (1952) and Wyllie, Gregory 

 and Gardner (1956) show that air-saturated cores under high pressures exhibit 

 velocities that are greater than the same cores water-saturated under the same 

 high pressures. These apparent contradictions appear to be dependent upon 

 the pressure range and the relative pressures of the rock framework and the 

 filling fluid. From field experience, it would seem then that introduction of 

 interstitial liquids tends to increase formation velocity within the pressure 

 ranges encountered in prospecting. 



Figure 26-2. Idealized logs from Miocene sands and shales. The high-porosity sand exhibits 

 an increased velocity as the filling fluid changes from gas to oil and from oil to water. 

 Also a comparison of the water sands reveals an inverse relationship between wave 

 speed and porosity (Hicks and Berry, 1956). 



The seismic energy resulting from a concentrated charge in a spherical 

 cavity in an isotropic homogeneous material radiates as a compressional or 

 longitudinal pulse whether or not the material is absorptive. If, as in earth 

 materials, the medium is absorptive, the higher frequency components con- 

 tained in the original pulse are attenuated; the pulse breadth increases and its 



561 



