1094 Subsurface Geologic Methods 



frequently made and leads to rather simple mathematical relationships. 



2. The velocity zones are all parallel to the reflecting horizons. 



3. The velocity zones at the surface are horizontal and dip at a 

 rate that varies as a function of the depth. This assumption undoubtedly 

 fits the actual subsurface conditions most accurately, but it unfortunately 

 leads to complex mathematical relationships that are difficult to apply. 



4. An average velocity is assumed from the surface to the reflecting 

 horizon. The ray paths thus become straight lines, and this greatly sim- 

 plifies calculations. 



SURFACE 



A GEOLOGICAL INTERFACE 

 SEDIMENTARY CHANGE 



B GEOLOGICAL INTERFACE 

 LITHOLOGICAL CHANGE 



S SEISMIC REFLECTION 



C GEOLOGICAL INTERFACE 

 PALEONTOLOGICAL CHANGE 



D SCHLUMBERGER KICK 

 RIGHT HAND, LEFT HAND 

 OR BOTH. 



E SEDIMENTARY BASEMENT 

 COMPLEX INTERFACE 



Figure 577. A seismic reflection (S) may correlate with any of the horizons (A, B, 

 C, D, or E), or it may not precisely coincide with any of them. The important 

 thing is that S is conformable with A, B, C, D, or E, if weathering corrections 

 and velocity changes are properly accounted for. A seismic event such as S' 

 seldom, if ever, will occur. (After Handley. Reproduced permission Oil and 

 Gas Jour.) 



The assumption to be made concerning the attitude of the iso-vel- 

 ocity surfaces depends upon the geology of the area concerned. For ex- 

 ample, on the Gulf Coast where an extremely thick section of geologically 

 young, incompetent sediments are arched upward or pierced by flowage of 

 a deep salt layer, the assumption may be made that the iso-velocity sur- 

 faces are parallel with the reflecting horizons. This is reasonable since 

 the uplift in many cases occurred contemporaneously with or after deposi- 

 tion of the sediments and is reflected through the sedimentary section to 

 the surface. The reflecting horizons are approximately parallel not only 

 to each other, but to the iso-velocity surfaces. This assumption leads to 

 the use of extremely simple computing devices.^^ 



^^ See U. S. Patent no. 2,535,220 — G. M. McGuckin, Apparatus for Solving Seismic Problems. 



