6. Photography of Underwater 

 Explosions 



The optically visible effects of underwater explosions constitute 

 valuable experimental tools in measurement. The advantage peculiar 

 to techniques based on optical phenomena is the fact that for many- 

 purposes no distortion of the phenomenon results from the measure- 

 ment, a state of affairs which it is often difficult and sometimes impos- 

 sible to achieve by use of mechanical or electrical devices. A second 

 advantage lies in the fact that photographic records can give infor- 

 mation over a field of view, rather than being point measurements. 

 Largely because of these characteristic features, photography of under- 

 water explosions has supplied a great deal of both qualitative and quan- 

 titative information and is potentially capable of supplying much more. 

 It is the purpose of this chapter to review some of the techniques which 

 have been developed, suggest their advantages and limitations, and give 

 illustrations of their appHcations. 



6.1. Surface Phenomena 



The simplest and most obvious photographic data obtainable from 

 underwater explosions are records of the phenomena visible above the 

 surface. Apart from their pictorial interests, such records can be made 

 to yield quantitative information as to the propagation of shock waves 

 and the motion of water around the gas bubble formed by the explosion 

 products. The surface phenomena arising from these causes are dis- 

 cussed in detail in sections 10.1 and 10.2, but the types of measurement 

 involved are mentioned here to indicate some of the technical require- 

 ments. 



The initial arrival of the shock wave at the surface is made evident 

 even at great depths by a region of darkened water, often described as 

 the slick. The front of this region spreads out from a point above the 

 explosion with a speed which is at first infinite (neglecting relativity 

 effects !) and decreases asymptotically to the velocity of sound in water, 

 or about 5,000 ft. /sec. The higher speed at points closer to the charge 

 is the result both of the geometry of the source and surface and of super- 

 sonic velocity of finite amplitude waves. Measurements of the dis- 

 turbance are thus capable in principle of application to measurements 

 of depths of explosions and to determination of shock front velocities. 

 In either case, the large velocities involved require motion pictures 

 taken at high speed if detailed records are to be obtained, as motion pic- 



210 



