702 



EXPLORATION GEOPHYSICS 



the reflections because the difference in reflection times between successive 

 traces usually is small. 



The theory of computation for dip shooting has been given in a preced- 

 ing section. The computation of data in correlation shooting is essentially 

 a special case in which the reflections, individually, persist over considerable 

 area, usually with relatively small dip. This problem is treated in the 

 following section. 



Fig. 430. — Typical dip record. A7" is the difference in reflection times to the two end seis- 

 mometers. Note 0.005 sec. lag between bottom two traces due to change of conditions in low 

 velocity layer. (Courtesy of Western Geophysical Company.) 



Reflection Correlation Shooting 



Although the character of a reflection furnishes no definite information 

 regarding the composition of the constituent layers of the subsurface, it 

 is often observed that certain strata yield a similar and sometimes distinc- 

 tive reflection throughout certain areas. In these cases, therefore, it is 

 possible to trace and map specific geologic strata without dip information. 

 Such mapping is done by character correlation, i.e., selecting corresponding 

 phases of those reflections which have substantially similar wave forms on 

 records obtained at two or more separate seismometer spreads. 



Under the general heading of correlation shooting may be placed four 

 type classifications of seismic work. These may be enumerated as pulse 

 correlation, spot reflection correlation, correlation of a reflection with a 

 definite geological marker bed, and refraction correlations. 



Pulse Correlation. — Pulse correlation is an extension of the process 

 used to recognize a reflection on a single seismogram to include all seismo- 

 grams obtained in a given area. A single-pulse continuous correlation is 



