722 



EXPLORATION GEOPHYSICS 



An example of a low velocity zone refraction record is shown in 

 Figure 441. This record illustrates a type which is especially designed 



for an accurate determination of the 

 AT correction for extreme points of 

 the spread. At each end of the spread 

 two seismometers separated by only 

 a few feet are used to obtain the 

 first arrival-times. This form is 

 sometimes used when more than one 

 seismometer feeds the end traces of 

 a dip record* and the condition of 

 the low velocity layer at each seis- 

 mometer is to be determined. The 

 record shows that the magnitude 

 of the velocity Vc is approximately 

 8200ft./sec. (Compare Figure 442.) 

 It will be recalled that relations 

 between dip time difference AT and 

 the magnitude of dip were obtained 

 on the basis of seismometer stations 

 at the same elevation. Consequently, 

 when the base of the low velocity 

 layer is at dififerent elevations at the 

 seismometer stations, one must 

 apply an elevation correction, AT^;, 

 to the dip time difference AT. This 

 correction, AT^j, is equal to the difference of elevation divided by the 

 velocity Vc in the consolidated zone. The problem again arises as to 

 variation of thickness H of the low velocity layer over the length of the 

 spread and may be handled by scattered up-hole shooting, as already 

 mentioned. 



The treatment of the low velocity layer has presumed a constant 

 value for the velocity Vc in the consolidated zone, the assumption being 

 based precisely upon the consolidated nature of the lower zone. Although 

 generally uniform, Vc does vary over the length of a spread in some 

 regions. A test of the constancy of this velocity may be attained by 

 employing so-called checking shot-points placed on a line normal to and 

 passing through the center of the spread, as will be evident from the 

 following considerations : The difference Af„ of first arrival-times to 

 symmetrically located seismometer stations depends on the low velocity 

 layer conditions alone only if the velocity Vc in the consolidated zone is 

 constant; hence, a test of the constancy of Vc consists in observing how 

 constant the difference A?„ remains for successive shot-points placed in 

 the prescribed manner, preferably on opposite sides of the first shot-point. 

 The pattern or "line-up" of a reflection will be modified by the low 



* See section on Multiple Detectors, p. 853. 



Fig. 442. — Refraction diagram corresponding 

 to Figure 441. Open circles correspond to ar- 

 rival-times at seismometers located on one side 

 of the shot-point and solid circles correspond 

 to arrival-times at seismometers located on the 

 other side of the shot-point. The irregularity 

 of the low velocity layer is indicated by the scat- 

 tered character of the points. 



