772 EXPLORATION GEOPHYSICS 



believed to be an example of an event from a high velocity refractor pre- 

 ceded by a lower amplitude arrival from a lower velocity refractor. 



Figure 467 is an illustration of a multiple-event refraction record from 

 West Texas, recording distance 44,000 - 46,250 feet. At least three out- 

 standing refracted arrivals are present on this seismogram. 



Figure 468 is an illustration of a multiple-event refraction seismogram 

 from West Texas (same general area in which Figure 467 was obtained). 

 This record was made with lower frequency instruments than the above. 

 It is of interest to note that this seismogram was made with the use of 

 only 40 pounds of dynamite buried at a depth of 90 feet and recorded at 

 the extreme recording distance of 88,000 -90,250 feet, or about 17 miles. 



Figure 468A illustrates a type of multiple-event refraction seismogram 

 obtained in California where no high-velocity marker beds (such as lime- 

 stone formations) are known to exist in the geological column. This record 

 was obtained from the explosion of 300 pounds of 60 per cent Vibrogel 

 placed at a depth of 180 feet in the outcrop of the basement complex, 

 located 21,000 feet east of the center of the instrument spread. From well 

 logs it was known that the basement is at least 5,000 feet below the location 

 of the instrument set-up. By placing the shot-point directly in the high- 

 velocity material and the instrument spread above several thousand feet 

 of sedimentary formations overlying the high-velocity basement rocks, 

 the serious problem of securing wave penetration from the surface through 

 the sediments to the refracting horizons was eliminated. 



The first arrivals of the refracted energy of this seismogram (2.228 + 

 .021 on trace one) are correlated with the basement complex as the refract- 

 ing horizon. The later arrivals (2.884 + .021 on trace one) are correlated 

 with corresponding refracting horizons within the sedimentary formations 

 above the basement rocks. It is of interest to note the difference in the 

 step-out times of the two outstanding energy arrivals : 0.078 sec. for the 

 basement arrivals and 0.113 sec. for a layer above the basement. This 

 step-out difference, the sequence of refracted energy arrivals and the 

 character of the recordings are useful criteria for refraction correlation 

 work. Also, it should be noted that the refraction from the deeper-lying 

 basement arrived ahead of the refraction from the shallower-lying sedi- 

 mentary formations, due to the higher velocity in the basement. 



A test of the refraction correlations was obtained in this case by 

 continuous recording of refraction profiles along lines radiating from the 

 shot-points placed in the outcrop of the basement complex. It is obvious 

 that equal arrival travel-times should be observed at identical distances 

 and elevations for the case of horizontal stratification of the relatively 

 constant velocity layers. When different travel times are observed at equal 

 horizontal distances and elevations relative to the shot-point, such data 

 may be interpreted as indicating that the refraction horizons are non- 

 parallel with the horizontal. In this manner possible ridges and valleys in 

 the basement complex surface may be mapped and the general geologic 



