DEPTHS FROM SEISMIC TIME-TRAVEL CURVES 129 
-687, A = 43.4" 
.750, A +0 = 48.6° 
4,400/6, 400 
sin A 
sin (A + 6) 
4,400/5, 860 
6 = dip angle = 5.2° down from shot point A. 
The dip down from shot point B was determined in a similar manner 
and was found to be 1.4°. It is seen from the time-travel curve that a 
dip downward on the shot point B profile occurs between stations d 
and e (Fig. 4). This dip was calculated to be 7.4° by the same method 
as the other dips were determined. The thickness of the V3 bed is 
unusually difficult to compute in this case as the surface of the V, bed 
is not on a steady dip between the two shot points. The method used 
here is essentially a cut and try method, but it must be remembered 
that all geophysical interpretations are largely accomplished by 
setting up conditions which will give the observed results. By using 
5,860 as the value of V, in relations (8)—(10) the angles of refraction 
of the Mintrop paths were determined and the paths laid off as shown 
in dotted lines in Figure 4. From other profiles of more regular charac- 
ter, the depth of the V4 surface at each end of the profile in question 
was found to be nearly the same. The time required to traverse the V1 
and V2 beds along a path through the V4 bed from shot point A to shot 
point B was determined by measuring the sections of this path through 
V, and Ve, dividing the results by the respective velocities, and the 
result was found to be 0.254 seconds. The correction for X, which 
was the sum of the distances from the actual shot points to the re- 
spective secondary shot points on the surface of the V3 bed was found 
to be 350 meters. The value of T was determined by subtracting 0.254 
sec. from the time codrdinate of the point at the end of each profile. 
This time codrdinate was, of course, the same for both end points. 
The thickness of the V3 bed under each shot point was then determined 
by substituting the corrected values of X and T in relation (1) with 
the values of V4, Ts, and X, being used in place of the values of Vs, Ts, 
and X». The thickness of the third bed under each shot point came out 
to be 606 meters. The depth of the V, surface in the immediate vi- 
cinity of each shot point was thus determined and plotted on the sub- 
surface chart as shown in Figure 4. By drawing lines through the 
depth points at angles previously determined, the surface of the V, 
bed was obtained at each end of the profile. The slope of the V, sur- 
face which connects the dips at each end of the line was previously 
determined to be 7.4°. From the seismic times of the stations marked 
d and ¢ on the surface, it is apparent that the sharp dip occurs be- 
tween d and e. Hence, by drawing in the 7.4° dip from the surface of 
789 
