724 EXPLORATION GEOPHYSICS 



the second observations being designated by primes. Since 



X\ -X'n=- (Xi - Xn) 



_ (h - tn) + (t\ - t'n) 

 Tl — Tn— ——————— —^ 



M-kl 



(78) 



and the desired correction is obtained almost independently of any knowl- 

 edge of the underlying higher velocity zone, Vc. This method of measuring 

 the correction is often more accurate than that given by Equation 75. Care 

 should be taken that penetration to the underlying zone Vc is obtained, 

 and if possible, shot-points should be located at equal distances from the 

 ends of the instrument spread and the shots placed at relatively the same 

 depth, in order that the same penetration will be achieved at each shot. 



This system may be applied to split spreads by shooting shot-points at 

 each end of the split spread. For isolated shot-points this would require 

 the drilling and shooting of two extra shot holes. Though the two extra 

 shot holes need not be as deep as the one from which the reflections are 

 recorded, this extra work would consume considerable time. In the case 

 of continuous profile work on the reflecting interface, adequate informa- 

 tion on the low velocity layer is obtained more or less as a by-product of 

 the continuous profiling. It therefore becomes obvious that the thoroughness 

 of the application of the correction for the delay due to the presence of 

 the low velocity layer is often a concession on the part of accuracy to the 

 time and money available for securing data in the field. 



In general the accuracy required need not be greater than that necessary 

 to obtain pertinent and reliable information on the subsurface geological 

 structure. For example, if the attitudes of the geological markers need 

 be expressed in feet per mile rather than degrees of dip because of extremely 

 low relief of the geologic structure, then great accuracy is needed in 

 applying the correction for the low velocity zone. On the other hand, if 

 the dip of the subsurface beds exceeds 10 or 15 degrees, the accuracy in 

 the application of the correction need not be so great. 



Method of Differences. — The method of differences has been found 

 useful in estimating the thickness of the low velocity layer when a great 

 difference in velocity occurs at its lower boundary. This method was first 

 used in the solution of two-layer refraction problems measuring the thick- 

 ness of overburden lying on an irregular bedrock surface. f Because of its 

 simplicity and ease of application the method has met with considerable 

 favor. It makes use of the refracted travel times recorded from shot- 

 points placed at both ends of the line of seismometers. 



In refraction work the shock origins are in shallow holes in the over- 

 burden, while in reflection work these origins are usually at or near the 



t A. B. Edge and T. H. Laby, Principles and Practices of Geophysical Prospecting, pp. 339-341, 

 Cambridge University Press (1931). 



