1322 EUGENE McDERMOTT 
SHOT FECOROER 

Vi, 
Fic. 6.—Curvature of path due to increasing velocity with depth. 
matter of fact, as the accurate determination of relative depths is of 
greatest importance, it is generally possible to assume an approximate 
velecity which meets the general requirements of accuracy for absolute 
depths. Where a well is available, it is possible.to check this velocity 
without lowering a recording instrument into it provided the reflecting 
strata are not too close together, and the reflections may therefore be 
definitely identified. In general, in the course of a shooting program, 
wells are thus checked as they are encountered. An approximate velocity 
may be determined by arranging several instruments in profile. The 
foregoing equation of the path of the disturbance is obviously a linear 
equation between X? and 7°. Thus, if X? is plotted against 7°, the straight 
line obtained has a slope equal to V7. The velocity may thus be deter- 
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