1392 
ABSTRACT 
This report presents the results of piezoelectric measure- 
ments of underwater explosion pressure waves generated and 
observed near the free water surface. The charge-surface~gauge 
positions were such that the rarefaction wave resulting from re= 
flection of the compression wave at the surface was propagating 
in the moving and compressed medium not far behind the shock 
front. Under these conditions, the interaction with the sur—- 
face deviates from the predictions of simple acoustic theory. 
If the angle of incidence is defined asX = tan"! (D, + Dg)/Ry 
where De and Dg are the charge and gauge depths and R is €heir 
horizontal separation, one finds that the deviations from acous= 
tic theory become more pronounced as decreases at constant 
pressure level ( at the point of reflection on the surface) or 
as the pressure level is increased at constant Q. 
The deviations readily observed by the methods employed in 
this investigation are 1) a falling-off of the peak pressure 
from the value obtained in free water at the same charge=to-= 
gauge distance, and 2) a change in the character of the rare- 
faction cut-off to zero pressure. The rarefaction becomes less 
steep-fronted and, in extreme cases, would appear to have pene- 
trated to the shock front. The decay of pressure behind the 
shock front becomes non-exponential and the pressure-time records 
become convex upward. The total duration of the positive press- 
ure phase, however, becomes longer than predicted by acoustic 
theory for the cut-off to zero pressure on the arrival of a neg= 
ative reflected front. 
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