neath a fault complex. In this particular case, even though a balanced split 

 spread were used, the stepout between seismometers Nos. 1 and 11 would not 

 be zero, and hence a false dip and an incorrect coincident point location would 

 be plotted. Quarles (1950) shows results of seismic surveys in areas of faulting 

 wherein this effect is well demonstrated. 



Thus wave-path geometry serves as a foundation for development of com- 

 puting equations, as an aid in spread selection, and as a basis for interpreting 

 perplexing problems which might arise. It is estimated that over 50 million 

 sketches of ray-path diagrams have been made by geophysicists all over the 

 world. 



COMPUTATION No computation procedure can be expected to 



PRINCIPLES yield results which will exactly answer the 



highly complex problems presented by the 

 earth. What the geophysicist must do, then, is to develop and use computation 

 techniques which, though approximations to the unattainable ultimate, still 

 serve to evolve a subsurface picture of reasonable accuracy and with economy 

 of computation time. 



For computation purposes, the subsurface is roughly divided into two 

 zones — the immediate subsurface and the deeper subsurface. Directly beneath 

 the ground surface is a highly irregular zone of low velocity material called the 

 low velocity layer (LVL in fig. 26-1). Rapid variations in the immediate sub- 

 surface not related to deeper subsurface structures could either mask the deep 

 structures or build false structures. For example, if a line of shotpoints is shot 

 across an area wherein the LVL decreases in thickness toward the center, then 

 the uncorrected seismograms would indicate a seismic high in the central 

 region. Thus since the principal objective of seismic prospecting is to find 

 traps favorable for the accumulation of oil, the effect of the irregular immediate 

 subsurface must be removed by computation. This procedure usually is termed 

 reduction to datum. It is generally assumed that beneath the datum an overall 

 regularity of velocity distribution exists. 



We generally recognize 3 classes of datum surfaces: regional, floating, and 

 fixed. The regional datum is based on a regional conformity to the isovelocity 

 surfaces. It is perhaps the most accurate of the datum surfaces listed, but it 

 cannot be described until much velocity information has been gained in an 

 area either through surface shooting for velocity or from velocity surveys in 

 wells. A floating datum is one whose elevation parallels the ground elevations 

 and it is presumed that the isovelocity surfaces parallel the ground surfaces. A 

 fixed datum is a reference surface whose elevation is constant. 



In datum reduction, the shotpoint and the seismometers are mathematically 

 transferred or "reduced" to datum. Two of the basic procedures employed in 



572 



