498 SEISMIC METHODS [Chap. 9 



terms of depth. However, they enter indirectly into the interpretation 

 of a seismogram, since impulses due to any new phase are characterized 

 by a change in both frequency and intensity. The only wave parameter 

 employed for depth calculations in present practice is the time interval 

 elapsed between the instant of the explosion and the arrival of the first or 

 later impulses. According to the type of wave used and the manner in 

 which travel times are observed and analyzed, the following seismic meth- 

 ods are distinguished: (1) fan shooting, (2) refraction, and (3) reflection. 



The objective of the first method is to determine whether there is an 

 intervening medium of different velocity between shot point and receiving 

 points. Fan shooting is a reconnaissance method, capable of covering a 

 large area in a comparatively short time. Indications obtained by it may 

 be detailed by the refraction or reflection methods. In many ways the 

 fan shooting method is comparable to resistivity mapping of electrical 

 prospecting. When distances between receivers and shot are kept fairly 

 constant, the depth penetration also remains about the same. In con- 

 trast to the fan shooting method, refraction a»d reflection methods in- 

 volve absolute determinations of depths to geologic formations. In the 

 refraction method, this determination involves an observation of the varia- 

 tion of travel time with interval between shot point and receiver. Hence, 

 it is comparable with the resistivity-sounding method of electrical 

 prospecting. 



While in fan and refraction shooting primarily the first impulses from 

 high-speed beds within the range of the shot distance are evaluated, the 

 reflection method is based on the determination of travel times of impulses 

 arriving subsequently in the seismogram. The interval between shot and 

 receiving points is no longer a factor controlling depth penetration. Were 

 it not for ray curvature and absorption it would be possible to penetrate 

 to any depth with any given spread. This method can be used for both 

 reconnaissance and detail. However, because of difficulties in correlating 

 records through large distances, its main application is to detailed survey- 

 ing. In refraction shooting, the distance between shot and receiving points 

 is roughly a multiple of the depth penetration (generally from 3 to 5); 

 in reflection shooting, it is a fraction thereof (from tV to ^). 



To obtain depth in refraction shooting, the travel time must be deter- 

 mined as a function of distance. Hence, observations in a number of 

 distances are required. In reflection shooting, one distance would the- 

 oretically be sufficient if the velocity is known. In practice, however, 

 more distances are necessary, since it is not possible to differentiate be- 

 tween a refracted and a reflected impulse in a single record. 



In field application the distinction between the three seismic methods is 

 not so sharp as it may appear from the above description. To calibrate 



