= pound demolition blocks were prepared by taping or tying 
the blocks together. Sometime s 3z pound demolition blocks 
were used alsoe The 50 pound charges of demolition blocks 
were prepared by replacing the top and bottom of the 50 pound 
boxes by slats and removing all the wax paper. The Mk 4eT 
charges were fired without the associated tail assembly, and 
the Mk 50 charges were fired with the tail assembly. The 
Mk 54 charges were fired in their carrying casese 
IV. Reduction to Surface of Reference 
It is usually desirable to reduce the travel time 
data to a surface of reference from which to measure seismic 
depthse in the case of deep sea refraction profiles sea 
level is chosen as the most convenient surface of reference. 
The refraction, reflection, and direct wave travel times are 
corrected to bring them to this surface of reference. The 
correction to the first reflection is the time if would t ake 
sound in water to travel from A to B plus the time from C to D 
(see Figure 13). This is obtained by multiplying the depth 
of the shot plus the depth of the hydrophone, expressed in 
seconds, by cos 86, where © is the angle of incidence of the 
reflected wave on the bottom. The correction for the second 
and third reflections is obtained in a similar manner. The 
correction for the refracted waves is equal to the product of 
the sum of these depths by the cosine of the angle () whose 
sine is the ratio of the waimam water velocity rea to the 
velocity in the refracted layer (cy). The correction for the 
