Subsurface Methods as Applied in Geophysics 1089 



Rock Properties 



Both refraction and reflection of seismic waves are controlled by the 

 physical properties of the rocks involved. The velocity of a longitudinal 

 wave is determined by the following relations between the elastic constants 

 and the density: 



V- 



k + 4/3re 

 Vl = ■ 



where k is the bulk modulus 

 n is the shear modulus 

 S is the density 



It has been found that a number of factors influence the seismic velocity 

 of rocks. Both the mineralogic composition and the crystalline structure 

 affect the wave speed in igneous rocks. The velocity increases with a de- 

 crease in silica content and with an increase in the size of the mineral 

 grains. 



In sedimentary rocks the velocity varies with the depth, geographic 

 location, lithology, local structural position, and geologic age. In general, 

 velocity increases with depth owing to the increase of the value of the 

 elastic moduli with pressure of the overlying strata. The geographic loca- 

 tion influences the velocity because of the effects of metamorphism, severe 

 folding, and regional variations in lithology. In the San Joaquin Valley 

 of California it is noted that the velocities for corresponding depths or 

 for corresponding reflection times are very much higher near the margins 

 of the valley than in the central portion. The mineralogic composition, 

 size, porosity, interstitial fluids, and degree of cementation of sediments 

 all tend to influence the velocity, increases in velocity being observed 

 when (1) the lime content of sands or shales increases, (2) the pohosity 

 decreases, (3) the interstitial fluid content of unconsolidated rocks in- 

 creases, (4) the interstitial fluid content of consolidated rocks decreases, 

 and (5) the degree of cementation increases. Velocities are generally 

 higher over a structure owing to the induration of sediments lying above 

 uplifts and the increase in the elastic constants. In general, velocity in- 

 creases with geologic age. Two factors are probably involved: (1) the 

 older sediments are more deeply buried, and (2) the younger sediments 

 show lesser degrees of consolidation and cementation. It has been ob- 

 served that most sedimentary and metamorphic rocks are anisotropic; that 

 is, the velocity of the material is higher parallel to the bedding planes or 

 schistosity than at right angles thereto. Although in certain shales the 

 velocity may be as much as 50 percent higher parallel to the stratifica- 

 tion than at right angles, an average figure would probably be about 

 10 percent. 



Because the success of the seismic method depends on the accurate 

 determination of the velocity, extreme care is exercised in obtaining 



