476 



SEISMIC METHODS 



[Chap. 9 



Pennn/l^anlan 



than at right angles thereto. 

 In metamorphics the speed of 

 propagation of elastic waves is 

 therefore greater in the direc- 

 tion of strike than at right angles 

 thereto/^ This elasiic aniso- 

 tropy also plays a part in sedi- 

 mentary rocks and accounts for 

 some irregularities encountered 

 occasionally in seismic prospect- 

 ing. McCoUum and Snell^^ 

 found that in shales the veloc- 

 ity parallel to the stratification 

 was as much as 50 per cent 

 higher than at right angles thereto. 



With an increase in depth of burial the porosities of sedimentary rocks 

 are reduced. This decrease depends on the amount of porosity originally 

 present and the type of sediment concerned. It is caused by the fact that 

 collodial matter in sediments undergoes dehydration with increasing pres- 

 sure and that their soft mineral grains become granulated. 



tiOOO 



ZOPO 4OO0 6000 



Mean depth of section In feet 



Fig. 9-20. Differential velocities (to 2000 

 feet, from 2000 to 3000 feet, and 3000 to 4000 

 feet) plotted against mean depths for sec- 

 tions of different ages (after Weatherby and 

 Faust). 



5 



\ 



/soo 



r 



depth In feet 



Fig. 9-21. Variation of wave velocity with depth (California). 



Rieber.) 



(Adapted from 



Fig. 9-21 shows the variation of velocity with depth in the San Joaquin 

 Valley in California for various unconsoUdated members of the Tertiary 

 formation (from refraction observations). Not only in sedimentary but 



" Iso-time curves are elliptical and have the same shape as equipotential curves 

 in anisotropic media (see pp. 700 and 706). 

 "Physics, 2(3), 174 (March, 1932). 



