Chap. 9] SEISMIC METHODS 549 



is great, vertical ray propagation may be assumed so that the travel 

 times are: 



(1) at B, shooting from A: Iba = h — H — 



Vi V2 Vi 



(2) at B, shooting from C: Ibc = - + - +- 



Vi V2 Vi 



(3) at C, shooting from A: tcA = ^ + ^"-^^ + - 



Vi V2 Vi 



Adding the first two equations and subtracting the third, we get 



tsA + tsc - tcA . ^^ ^ ^^ (9_g8) 



The overburden velocity Vi is determined from short-range profiles or 

 (for deep reflection shots) by providing a shot-hole receiver at yl. In 

 reflection-correlation shooting, the shot at A is a regular reflection shot 

 with five receivers set up near B and one receiver at C. This receiver is 

 then removed and the second shot placed as shown. In continuous pro- 

 filing, the reverse reflection shot automatically performs the function of 

 the weathering shot from C. 



D. Reflection Methods 



1. General Problems. Reflection methods differ from refraction pro- 

 cedures in that not the first impulses but later impulses are utilized for the 

 depth calculations. Hence, a principal problem in reflection shooting is 

 to separate reflected impulses from all others of a different character, 

 that is, not only from the first high-speed refraction impulses but also 

 from low-speed surface waves and other refraction impulses arriving after 

 the first impulses. A separation is possible (a) in regard to time, and (6) in 

 regard to amplitude. In the design and arrangement of detectors, as well 

 as in field technique, various measures are taken to accomplish this. 



A universally adopted means for time separation of impulses following 

 in rapid succession is high paper speed (10 to 15 in, see." ) and near- 

 critical damping of overall channel response. A record taken with an 

 underdamped receiver at refraction-record speed would scarcely show any 

 reflections. An overlap of interfering impulses with reflections may often 

 be eliminated by changing the distance qf the entire spread from the shot 

 point. The principal means of segregating refractions from reflections is 

 the use of a multiplicity of receivers (six, eight, or twelve). Interfering 

 impulses (such as refractions or surface waves) will arrive at each receiver 



