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EXPLORATION GEOPHYSICS 



convenient during interpretation, for determining the exact location of 

 the shot with respect to the geophone spread. 



Water Shooting. — Firing of shots is usually much easier and more 

 rapid than in land operations, since shot holes obviously are unnecessary. 

 The explosives are usually detonated at shallow depths in the water. 

 Regular seismograph dynamites may be used, but for reasons of economy 

 ammonium nitrate types of explosives are commonly employed. The explo- 

 sive is packed in reasonably water-tight wrapping, often as a 50-pound 

 cartridge. Figure 566 shows some of these charges on the deck of the 



Fig. 566. — Powder technician preparing to drop a 50-pound explosive charge into water. 

 (Courtesy of United Geophysical Company.) 



shooting boat. Since several tons of explosives may be used during a day's 

 run, it is important to have the explosives packaged in single large con- 

 tainers for easy handling. 



When the explosive is detonated at an appreciable depth below the 

 surface, there is oscillation of the gas bubble resulting from the explosion.f 

 The passage of the initial shock wave imparts a high outward velocity to 

 the water surrounding the explosive. Because of this acquired momentum 

 the water continues to expand rapidly outwards for a considerable time, 

 even after the pressure in the gas sphere falls to values below the normal 

 hydrostatic pressure in the water. Eventually the outward expansion is 

 brought to a stop and the water begins to move rapidly inward, compress- 

 ing the low pressure gas sphere. Because of convergence the water picks 

 up velocity rapidly and compresses the gas to a small volume and high 

 pressure. At equilibrium, the maximum pressure is reached, the gas bubble 

 again begins to expand with explosive violence, and a second shock wave 



t Robert H. Cole, Underwater Explosions, Princeton University Press, 1948. 



