25 
Motion of bubble near sea surface 
71. The sea surface behaves as a free surface and repels the bubble 
downwards. For an explosion near the sea surface there are, therefore, 
two opposing effects; the repulsion of the bubble by the sea surface, and 
the tendency for the bubble to rise under the net hydrostatic force. Consider 
similar charges exploded at different depths, excluding extremely shallow 
depths for which the bubble vents on its first expansion. As the charge 
depth decreases the repulsion of the free surface increases; the gravity 
rise will also increase, but more siowly because the operative "depth" 
affecting this rise is measured from a point 33 ft. above sea level (since 
the atmospheric pressure is equivalent to a head of 33 ft. of sea water.) 
For a shallow charge the repulsion can be the stronger effect and the 
bubble will sink, whilst for a deep charge the surface effect will be smal] 
and the bubble will rise. At some intermediate depth the two effects can 
exactly cancel and the bubble neither rises nor falls. Therefore, the 
major effect of the proximity of the sea surface on the motion of the 
bubble is a downwards repulsion tending to lessen and even reverse the 
upwards motion due to gravity. 
Motion of bubble near ueesbed's 
72. Experiments suggest that sea-beds in fact behave qualitatively like 
a rigid surface in the sense that they tend to attract the bubble. With 
very soft sea-beds and/or charges some distance from the sea-bed, this 
downwards attraction is weak and manifests itself only as a small decrease in 
the rise due to the net hydrostatic force. With charges close to. or in 
contact, with hard sea-beds, the attraction is strong and the bubble 
tends to stay on or near the sea-bed until it breaks up into small 
bubbles. 
73. An important additional effect observed experimentally is that the 
proximity of the sea-bed tends to suppress the oscillations and the 
bubble pulses. Thus with 1 oz. charges and a gravel sea-bed three 
pubble pulses were detected for an explosion at 6 ft. from the sea-bed 
whereas explosions at 1 ft. 6 in. or less from the sea-bed gave only a 
single bubble pulse. Experiments with 1 oz. charges and an artificial 
(steel plate) rigid sea-bed showed a similar qualitative effect but the 
second pulse was evident at a charge distance of 1 ft. though not at a 
charge distance of 6 in. Charges of 1 lb. to 5 1b. exploded on or near 
a mud sea~bed also indicated this suppression as the position of the 
charge approached the sea-bed, and with the charge actually on the sea-bed 
there was rarely more than one bubble pulse which was of a broken nature. 
Therefore, the major effects of the proximity of the sea-bed on the 
bubble motion are first, a downwards attraction tending to lessen or even 
reverse the upwards motion due to net hydrostatic force, and secondly, a 
suppression of the later oscillations and bubble pulses. 
Visible surface phenomena 
74. The visible surface effects, often very spectacular, consist 
essentially of a spray dome produced by the pressure pulse and a 
subsequent plume produced by venting of the gas bubble. The dome and 
plume are not of direct significance in connection with damage. However, 
a knowledge of surface phenomena may enable the expert to estimate the 
size and depth of an explosion if photographs of the dome and plume are 
available. This can be of operational value. 
