1090 Subsurface Geologic Methods 



TABLE 34 

 Velocities of Selected Rocks 



Type of rock 



Velocity 

 (ft. per sec.) 



Alluvium 



Clay, sandy clay. 



Shale 



Sandstone 



Limy sandstone 



Limestone 



Rock salt 



1,000- 2,000 

 6,000- 8,000 

 6,000-13,000 

 8,000-13,000 



12,000-14,000 

 7,000-21,000 



14,000-25,000 



values. Although several methods may be used for velocity determinations, 

 the direct technique, in which a detector is lowered to known depths in a 

 well, is favored. From such data the average velocity to any horizon, the 

 interval velocity, and the time-depth relationship may be determined. By 

 computing the mathematical formula governing the velocity from the 

 time-depth relationship, seismologists are able accurately to predict the 

 depth and disposition of subsurface strata. 



Other parameters of importance in discussing rock properties are the 

 transmission characteristics of the rocks. Significant in this connection are 

 (1) acoustic impedance, (2) spreading and dispersion, and (3) absorp- 

 tion and dissipation of energy. The average rate of flow of seismic energy 

 through the ground depends upon the amplitude and frequency of the 

 wave and the acoustic impedance of the medium. Materials for which the 

 acoustic impedance is high transmit more energy per unit area than those 

 in which the value is low, if we assume the same values of amplitude and 

 frequency for the wave. This factor also controls the reflection and re- 

 fraction of seismic waves. If, for example, the acoustic impedances of two 

 adjacent media are in the ratio of two to one (about the maximum differ- 

 ence encountered in stratified beds) , the amplitude of the refracted or 

 transmitted wave is two-thirds of the normal-incident wave, and the 

 amplitude of the reflected wave is one-third of the normal-incident wave. 

 Also of importance is the phase of the reflection: that is, whether at a 

 given reflecting horizon a compression is reflected as a compression or as 

 a rarefaction. For a compression arriving at the surface, the first move- 

 ment of the ground is upward; for a rarefaction, the first movement of 

 the ground is downward. If the acoustic impedance of the upper medium 

 at a reflecting interface is larger than that of the lower medium, a phase 

 shift of TT occurs, and the compression is reflected as a rarefaction. If the 

 acoustic impedance of the upper medium is smaller than that of the lower, 

 no phase shift occurs, and a compression is reflected as a compression. 

 Numerically, the specific acoustic impedance is the product of the longi- 

 tudinal velocity times the density referred to unit dimensions. 



The intensity of seismic waves decreases with the distance from the 

 shot point, varying inversely with the surface areas of the advancing 



