SEISMIC METHODS 723 



velocity layer from that which would be expected from purely geometric 

 considerations of reflected rays. The "line-up" in fact is largely deter- 

 mined by the low velocity zone in those cases where split spreads are used 

 for mapping of deep beds. 



Furthermore, the reflection pattern must be examined not only with 

 a view to identifying reflections but also with a view to reading reflec- 

 tion time ditferences accurately on the records. This is the case because 

 the evaluation of the reflection time difiference is usually based on the 

 trend of several traces adjacent the end traces, as well as the end 

 traces themselves. 



Time Corrections for End Shots, Low Velocity Layer, — The fore- 

 going corrections can be applied equally well to any set-up in which the 

 seismometers are placed in symmetrical pairs and the shot-point lies in the 

 plane of symmetry. However, it is often desired to record with end-shots, 

 or instrument set-ups in which the shot-point is placed outside the spread 

 of the seismometers, usually not more than two or three hundred feet from 

 the nearest seismometer. In this case the reflection time is taken at the 

 nearest instrument to the shot-point; the shot-point distance is generally 

 small enough so that this time may be considered approximately equal to 

 the reflection time which would have been recorded at the shot-point. 

 Correction of the reflection time for the delay in the low velocity layer is 

 made exactly as in the case of symmetrical spreads about the shot-point. 

 However, the step-out time correction of t is a little more difficult. Apply- 

 ing Equation 75 and assuming the number of seismometers to be n, with 

 the nearest seismometer at a distance Xi and the most distant seismometer 

 at the distance X„ from the shot-point, we may write Equation 76 as an 

 expression of the difiference in vertical travel time (tx — t„) through the 

 low velocity layer at the respective positions of seismometers number 1 

 and n. 



n-r,- y- 



^'^^m' 



(76) 



This expression involves the additional term, {Xi—Xn)/Vc introduced by 

 the set-up asymmetry. Obviously small uncertainties in Vc will introduce 

 a large percentage of error in the determination of {ti — Tn). This difficulty 

 is eliminated by shooting at the other end of the set-up, at a depth and 

 distance sufficient to insure that the refracted waves will penetrate below 

 the low velocity layer. In this case 



n-r„ = Ei 



v"^^^ 



(77) 



