GEOPHYSICAL METHODS OF INVESTIGATING 243 



and should if possible be deduced by (18c) above, because 

 the average vertical velocity encountered will in some cases 

 vary from 5,000 ft. per second for shallow depths up to 

 12,000 ft. per second for greater depths. An error of 

 1/1,000 sec. in the seismic time here will show an error of 

 5 to 12 ft. in the length of the seismic path. Hence the time 

 record should read direct to 1/1,000 sec. and at very least 

 1/100 sec. 



The charge of explosives should be planted at about 

 10 ft. down to prevent it blowing out, and greater accuracy 

 will be had if we take h, U . . . etc., the seismic paths 

 from shots Si, S2 . . . etc., and their times ti to . . . etc., 

 for getting the true depth h and the true average vertical 

 velocity V thus 



l,-^ = h^ -^ Si'- . . . etc. 

 hence 



a general equation for all points. Plotting this in the lineal 

 form ax -\- y -\- C = with values of x as functions of y 

 {t^ as functions of S^) we get the straight-line graph with 

 the slope a = V^ and^ the ordinate intercept C — h^ in the 

 usual way and so by normal coordinate geometry for 

 gradients and interpolated values. The charges in the hole 

 seldom exceed a few pounds, though in major geophysical 

 work, as in the deep lying anticlines of the Persian oil 

 areas, over a hundred pounds have been employed in one 

 shot, the depth feature being in the region of 4,000 ft. in 

 places. 2 



The method adds decided advantages in an extensive 

 field in that it provides data as to subsurface conditions 

 simultaneously with the above. One of these advantages 

 is the structural image we get of the underground from a 

 study of the curves when plotted as above. This will be 

 best appreciated by an example or two. 



1 Ibid., p. 70. 



2 Professor Rankine, lecture before Loughborough Scientific Society, 1929. 



