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EXPLORATION GEOPHYSICS 



OFFSHORE SEISMOGRAPH OPERATIONS 



Seismograph operations offshore involve some novel and spectacular 

 features, but they are basically very similar to operations on land. Because 

 of the greater variety of activities involved, water operations are more 

 elaborate and costly. However, the changes* necessary to adapt land equip- 

 ment and methods to water work are relatively simple. Some of the aspects 

 will be discussed under the headings of instrumentation, shot generation, 

 surveying, interpretation, and operations. 



Instrumentation. — As regards seismic wave transmission, the behavior 

 of water in many respects is much the same as that of rock. Hence the 

 geophone may be submerged anywhere in the water or placed on the solid 

 bottom. The instruments commonly used on land may also be used in 

 water work, due provision being made to prevent water reaching vital 

 parts. The electrical output of most land type seismometers is propor- 

 tional to the velocity of the ground or water motion, at frequencies above 

 the natural frequency of the geophone. Suitable pressure-sensitive units 

 may also be used in place of velocity-type geophones. However, their use 

 to date (1949) is not widespread. 



Fig. 565. — Seismometer spread in offshore seismograph operations as viewed from stern 

 of boat. (Courtesy of United Geophysical Company.) 



In some earlier water operations, geophones were individually placed 

 on bottom from a surface boat. A heavy iron base or grill was provided to 

 insure the geophone's coming to rest in a vertical position. In some cases 

 geophones were "poled" into soft mud bottoms. 



In most current operations, the geophones are fastened integrally into 

 the cable system, and the entire assembly of cables and geophones is towed 

 or dragged as a unit by the recording boat. Figure 565 shows such an 



