SEISMIC METHODS 



901 



is generated. This subsequent shock is much lower in intensity than the 

 original shock but is of considerably longer duration and carries appre- 

 ciable energy, especially in the lower frequency components of the energy 

 spectrum. The entire process is then repeated as the gas bubble expands 

 and collapses a second time. The process may repeat two or three times, 

 sending out a weaker shock wave each time. For typical seismograph shots 

 the period of repetition is .3 or .4 of a second, and a confusing duplication 

 of reflections may result because of the multiple generation of pressure 

 pulses. 



In practice the gas bubble oscillation is avoided by detonating the shot 

 so near the surface of the water that the gas bubble breaks the surface on 

 the first excursion, and the inrushing water forces the gas and spray of 

 water upwards in a spectacular vertical jet or plume. f Figure 567 shows 

 a water and gas plume several hundred feet high. 



r" 



Fig. 567. — Water Shot. The inrushing water forces the gas and a spray of water 

 several hundred feet high in a spectacular vertical jet or plume. (Courtesy of United 

 Geophysical Company.) 



A typical shooting depth is 5 feet below the surface for a 50-pound 

 explosive charge. This depth is sufficiently shallow to prevent generation 

 of multiple pulses. The charge and one end of the firing cable are ordi- 

 narily tied to an inflated rubber balloon and thrown overboard. The shooting 

 boat pulls away to a safe distance and the shooter detonates the charge. 

 The shot instant usually is transmitted by radio to the recording boat. 



Most offshore seismograph work is done by the reflection method, 

 using continuous, dip, or correlation shooting. In some cases the shot is 

 detonated at a point offset from the center of the spread. In other cases, 



t R. L. Lay, "Repeated P- Waves in Seismic Exploration of Water-Covered Areas," Geophysics, 

 Vol. X, No. 4, Oct., 1945, pp. 467-471. 



