1407 
with a standard deviation of 6.4%. The time constant for the ” 
game charge in free water has been observed to be 65.6 microsec, ) 
Typical pressure-time records are shown in Fig. 16. 
C. Constant Charge and Gauge Depth and Varying w/3 7p. 
Additional Investigation into e nature of surface reflection 
phenomena from very shallow charges consisted of a series of shots 
made with both charge and gauge 4 in. beneath the surface but 
with charge-to-gauge distances changing fron 60 in. to an effective 
15 in., corresponding to a variation in w1/3/R at the gauge from 
0.164 to 0.657. The 0.657 value was actually obtained with 500 gm 
at a 5 in. depth and 19 in. charge-to~gauge distance, these 
dimensions being obtained by appropriate use of scaling laws to 
obtain the desired angle of incidence and peak pressure at the 
incident point. Typical records for this shooting are shown in 
Flg@... U7). 
Results, as shown in pate V and Figs. 7 and 8, indicate 
that for high values or W1/3/R the measured duration appreximates 
the theoretical value rather well, but for decreasing W / /R 
(and this involves a decreasing angle of incidence as well as 
a decreasing peak pressure) the measured duration becomes higher 
and higher compared with the acoustic value, Pressure deviates 
in much the same way from the similarity curve for free water, 
except that the deviation does not become as noticeable until 
wl/3/R drops to about 0.3. This is what might be expected from 
the results of Sec. A above, where, as the incidence becomes 
oblique the duration begins to deviate from acoustic theory, and 
only when the incidence becomeseven more oblique does the pressure 
begin to deviate from free water similitude values, 
VI. CONCLUSIONS 
Several conclusions may be drawn from the above experimental 
measurements of shock-wave peak pressure and duration, as measured 
by a piezoelectric gauge at some point beneath the surface in the 
path of propagation of both the direct shock wave and the reflected 
negative wave. 
(i) Whenthe peak pressure at the point of surface inci-= 
dence is low, 5000 psi or less, the shock-wave peak pressure 
conforms reasonably well to free water similarity law predic- 
tions, even for the smallest angles of incidence observed. The 
duration satisfactorily approximates the computed values ob= 
tained from the geometrical structure, assuming the shock waves 
to travel with the speed of sound, 
(11) For higher pressures, at the same angles of incidence, 
peak pressures are appreciably less than the similarity values 
and the difference increases with decreasing angles, Durations 
are larger than acoustic predictions, and the difference again 
increases for decreasing angle. 
15S 
