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FREe SURFACE 
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i aaa Orr Theory 
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47. For a given charge at a given depth, the fractional decreases in 
impulse and energy will become greater as (r' - r) decreases; this will 
occur as the depth of P from E N increases. The resulting influence of 
surface cut off on the variation of damage with distance and orientation of 
explosion relative to the target is discussed later. Therefore it follows 
that damage to a surface vessel by the pressure pulse should tend to be less 
near the water line than near the bottom of the vessel, subject of course to 
the effect of any marked difference in strength of the ship's structure at 
different levels. 
Reflection of the pulse at the sea-bed. 
4.8. Pressure-time records of underwater explosions often show a pressure 
pulse, following, and partly superimposed on, the main pulse. The obvious 
interpretation of the second pulse is that it represents the reflection of the 
main pulse on the bottom. The time delay between the direct pulse and the 
reflected pulse agrees with expectations, but the magnitude of the reflected 
pulse is extremely variable, elthough it correlates in a general way with the 
hardness of the bottom. Tis the peak pressure in the reflected wave from 
a rock bottan might be as large as 30-50% of the direct pulse, while for a 
muddy bottom, the reflected pulse might be altogether absent. Should the 
bottom consist of a layer of mud or sand over rock, the reflection appears to 
occur on the rock surface. 
49-6 The sea bed is so variable that it is difficult to make quantitative 
predictions of the reflected pulse. Indeed, no satisfactory theory has so 
far been developed except in two elementary limiting cases. The first 
limiting case is that where the sea bed is so very soft that it behaves 
exactly the same as water itself, and therefore gives no reflection. The 
second limiting case is that where the sea bed is completely rigid, and the 
reflected wave is therefore (in the theory of sound approximation at any rate) 
identical with the incident pulse. No actual sea bed is rigid, but a rock 
bottom might behave as an almost perfect reflection for very weak pulses 
(iee. of order a few hundred p.sei. peak pressure). 
Charge well away from sea=-bed 
50. First, the explosion will be assumed sufficiently far from the sea-bed 
for the initial events in the inner region found the charge to be unaffected 
by the sea=bed. The pressure pulse sent out by the explosion will then be 
the same as for a charge in mid-water independent of the sea-bed and on 
reaching the sea=-bed the pressure in the pulse will be assumed small enough 
for the simple linear theory of sound pulses to apply. 
