770 EXPLORATION GEOPHYSICS 



recording, sensitivity of recording equipment, and terrain of area under 

 investigation. This so-called "optimum distance" is established by experi- 

 ment and then used as a guide for the study of a given area. 



Correlation refraction shooting differs from early refraction methods 

 in that the earlier methods were concerned only with the first arrivals on 

 the seismogram. The instrument designs used then were for precise timing 

 of the first arrivals, and as a consequence it was seldom that any but the 

 first arrivals were recorded. 



This early work demanded long and continuous recording distances in 

 order to follow certain high-speed subsurface boundaries. Correlation 

 refraction work does not make use of continuous refractions from a 

 boundary, but depends for its effectiveness on correlation of refractions 

 obtained from independent arrangements of shot-point and receiver sta- 

 tions. These correlations are made on the basis of arrival time, sequence 

 of arrival, wave character of the recorded arrival, and horizontal velocity 

 of the recorded impulse. In comparison with reflection correlations we 

 have one additional criterion in refraction correlation, i.e., the recorded 

 surface velocity of the arrivals. In practice this velocity is referred to as 

 "move-out" or "step-out" time and is often diagnostic of a certain sub- 

 surf ace geologic formation. 



In refraction correlation work, elevations are theoretically established 

 on certain velocity discontinuities, and dips of these surfaces are estimated. 

 With a sufficient number of these observations in an area, it is possible to 

 draw a structural contour map. 



The correlation of refraction methods has been applied in Russia, in 

 the Edwards Limestone area of Texas, and experimentally in California.f 



Special seismic instruments are not necessary for use in correlation 

 refraction work. Only a slight modification of the modern reflection type 

 of equipment, embodying an adequate amplitude control, is required to 

 obtain sufficient separation of the various arrivals of refracted energy on 

 the seismogram. By recording several refracted wave arrivals on a single 

 seismogram it is possible to study the geological structure of a number of 

 refracting layers, using only one distance between shot-point and center 

 of instrument spread. In practice this distance usually varies from three 

 to six miles to obtain structural information to depths from 3000 to 5000 

 feet. However, recording distances as long as twenty miles have been used 

 in some cases. 



Figure 466 is an illustration of a multiple-event refraction record from 

 Crockett County, Texas, recording distance 30,250 - 33,000 feet. This is 



t G. A. Gamburtsev, Izvestiya Akademii Nauk SSSR, "Seriya Geograficheskaya i Geofiziches- 

 kaya," Bull. Acad. Set., U.S.S.R. Serie Geogr. et Geoph., 1942, pp. 26-47. 



L. W. Gardner, "Correlation of Refraction Shooting," Geophysics, Vol. XI, No. 1, Jan., 1946, 

 pp. 59-65. 



J. A. Gillin & E. D. Alcock, "The Correlation Refraction Method of Seismic Surveying," 

 Geophysics, Vol. XI, No. 1, Jan., 1946, pp. 43-51. 



A. J. Barthelmes, "Application of Continuous Profiling to Refraction Shooting," Geophysics, 

 Vol. XI, No. 1, Jan., 1946, pp. 24-42. 



