1086 Subsurface Geologic Methods 



quency, intensity, phase, and direction, with their associated or derived 

 phenomena such as travel time, wave length, absorption, refraction, and 

 reflection, are common to both. Owing to the rapid rate of propagation of 

 light, however, only quasi-stationary phenomena are investigated in optics; 

 in seismic work certain of the derived quantities such as travel time, re- 

 fraction, and reflection are of prime importance. 



When an artificial-force field is created in the earth by the explosion 

 of dynamite, several types of waves are generated, among which are the 

 longitudinal, transverse, Rayleigh, and Love types. The only wave utilized 

 in present-day prospecting is the longitudinal or compressional wave. 



Ricker and Lynn ^^ have recently described the use of composite re- 

 flections in mapping geologic structure. A composite reflection consists 

 of longitudinal waves to the reflecting interface and transverse waves 

 from the interface to the detectors. They list the following limitations to 

 the composite reflection method: 



1. Only the first reflecting bed seemed to be workable. 



2. A great distance was required between shot and detectors which 

 required more surveying time and greater amounts of dynamite. 



3. The depth determinations were not as accurate as with the con- 

 ventional reflection technique. 



The longitudinal wave is the fastest wave created and consequently 

 is the first to reach the receiving point. These waves that arrive at the 

 detectors may travel directly through the surface layer from the shot 

 (direct rays), may be refracted along higher speed layers at depth (re- 

 fracted rays), or may be reflected from velocity interfaces at depth (re- 

 flected rays) . A single seismic record may show the arrival of all the 

 above-mentioned rays. (See fig. 575.) 



The arrival times of direct and refracted waves on the record may be 

 recognized by the first breaks, which show progressive times of arrival 

 from the detector closest to the shot to that farthest way. By plotting 

 the arrival times against the distances, the velocities of the materials along 

 which the refracted rays have traveled (true only when refracting bed is 

 flat-lying) may be determined. As the reflected waves arriving from depth 

 strike the surface of the ground at approximately the same time, they are 

 readily recognized on the seismic record. The deeper the reflecting horizon 

 the closer together is the time of arrival of these waves. The purpose of 

 having a number of detectors in the set-up is to be able to recognize the 

 reflected waves from the extraneous noise and other disturbances, which 

 are always recorded along with the reflections. 



Snell's law, which governs the refraction of light rays, applies to 

 seismic refraction. (See fig. 576.) 



sin i V^ 



V = - = -jT- 



sm r Vo 



^Ricker, Norman, and Lynn, R. C, Composite Reflections: Geophysics, voU 15, no. 1, pp. 30-49, 

 Jan. 1950. 



