546 



SEISMIC METHODS 



[Chap. 9 



57-A. This gives, for each distance, a constant value of T, from which, 

 in turn, follows the depth by applying the graph of Fig. 9-736. If, in 

 the example, a uniform velocity (of 1881 m-sec~^) were taken to the 

 3000-m intercept, the depth would be 1097 m instead of 1375 m. 



11. Variants of refraction method using different field technique. The 

 methods discussed in the following paragraphs employ such modifications 

 of refraction technique as afford short cuts in the more elaborate methods 

 of depth calculation. These methods are (1) the arc method of structure 

 mapping, and (2) the method of differences {ABC system). They involve 

 a more direct determination of time differences which are evaluated in 

 terms of overburden thickness. 



" 02 0.4 0.6 0.8 to 12 14 16 Id ZO // 24 16 2.8 3.0 3.2 3.4 



Depth in km (h) 

 Fig. 9-736. Graph of T{K) for depth interpretation (basement rocks). 



(a) Arc method of structure mapping. In this method time differences 

 of fans are correlated with refraction profiles connecting the fan shot 

 points. Thus, a time-contour map of subsurface high-speed beds is ob- 

 tained and converted to a depth-contour map. The method is applicable 

 to low-dip structures only.^^ The shot points are generally laid out along 

 the strike in a longitudinal traverse (Fig. 9-74a), and reception points are 

 arranged in overlapping fans on the circumference of circles about these 

 shot points (tangential profiles. Fig. 9-746). Time differences are calcu- 

 lated, as shown below, and plotted for both profiles. With a suitable 

 scale the time curve will indicate the profile of high-speed formations. 



Depth calculations are based on simplifications of formulas previously 

 derived. If in eq. (9-50a) the substitution 1/vi = I/V2 sin i is introduced, 



" Jones, op, cit., 169-173 (1933). 



