the reduction to datum are the uphole method and the refraction method. The 

 uphole method is used when the shothole-seismometer distance is short enough 

 to warrant the assumption that shothole conditions are duplicated under the 

 seismometer, and the shot is below the LVL. Otherwise, a refraction technique 

 is indicated. Occasionally the immediate subsurface is so complex that the 

 usual methods of reduction to datum yield unacceptable results. In such areas, 

 the interpreter may resort to an isochron (often colloquially called an isopach) 

 technique wherein reflection times to deep horizons are compared with those 

 of a selected shallow reflector. This shallow horizon is essentially a datum. 



Once the irregular immediate subsurface variations have been removed 

 by reducing reflection times to datum, computation of depth, dip, and offset can 

 proceed. Reference to Figure 26-7 will demonstrate the interdependence of 

 these three quantities, and it becomes apparent that each must be known before 

 the position of the reflector segment can be established. Actually depth and 

 offset are obtained from the coincident wave path {VT /2) and the dip 

 (sin' 1 VAT<f>/Ax) and all will be well if the seismometer profile lies along 

 dip azimuth. If this is not the case, then sin"* VAT<f>/Ax represents only an 

 apparent dip, and it is necessary to shoot in two directions (usually an x-spread), 

 each yielding apparent dips from which both value and direction of true dip 

 are computed by a method similar to that used by the surface geologist for the 

 same purpose. 



The conversion of reflection times to depths and stepout times to dips 

 demands a knowledge of velocity distribution both vertically and horizontally, 

 and we must recognize that time presentations so often used in preliminary 

 phases of a seismic survey are in truth depth representations involving an 

 unspecified velocity. 



VELOCITY In the discussion of the geometry of wave 



DISTRIBUTION paths, we have tacitly assumed that the vel- 



ocity has remained constant to a particular 

 reflecting horizon. This we know is not true. Each uniform stratum in a layered 

 medium makes its unique contribution to the average velocity through the 

 medium. A many layered section would have such a complex velocity distribu- 

 tion that even though it were known it would be useless as such to the inter- 

 preter. He would be obligated to simplify the distribution in order to apply 

 it economically to his needs. 



The earliest simplification was the concept that an average velocity pre- 

 vails to a given depth or to a particular reflector. In essence, this is the same 

 as the assumption of constant velocity. It implies straight ray paths with the 

 resulting simplified geometry. The integration of the contributions of a multi- 

 tude of layers inherent in the average velocity concept makes its use practical in 



573 



