INTERPRETING RESULTS ON DEPTH CALCULATION 45 
both considered and are plotted on the profile. The sharp break with 
large amplitude is the first impulse on records 2B and 2B’, while on 
the longest three records the sharp break is preceded by an absorbed 
impulse which is characteristic of the igneous basement. Several other 
profiles in the same general area as profile B all had a small wave of 
short period on the second horizon which required large dynamite 
charges to bring out. A curve drawn through the points represented 
by the first small impulse or ‘forerunner’ does not intersect the curve 
drawn through the points representing the sharp breaks of the third 
horizon until after the basement velocity curve intersects both of 
them. Should the small wave or “‘forerunner’’ b2 interpreted as a thin 
lens-like formation overlying the horizon which gives the sharp break, 
or is it a separate acoustic horizon? If the former, it might be dis- 
regarded as far as depth calculation is concerned, but if it is the latter 
its thickness must be considered. 
THEORETICAL CONSIDERATION 
If the conditions shown in Figure 1 be assumed, it is possible to 
derive equations for the time-distance curves which these velocities 

330 
322! 
981 
te 2.285 
Va 
Fic. 1.—Assumed conditions of depth and velocity. 
375 
