904 



EXPLORATION GEOPHYSICS 



close to that of sound in water. These waves appear to be reflected from 

 irregularities on or near the bottom, and return to the geophone spread to 

 be recorded as apparent reflections with very great moveout times. 



The phenomena described above are mentioned as features peculiar to 

 water shooting. They are not typical of usual water seismic operations. 

 Generally speaking, the quality of records in water shooting is comparable 

 to that of land records. Records varying from excellent to very poor or 

 useless are obtained in different regions. The noise level due to relative 

 motion of the water and geophone spreads may be somewhat higher than 

 typical noise levels on land. In some cases heavier dynamite charges are 

 necessary to compensate for this difference, to give a better signal noise- 

 level ratio. 



Fig. 569. — ^Type of boat suitable for offshore operations. (Courtesy of United 

 Geophysical Company.) 



Operations on Water. — Seismograph operations on water generally 

 lend themselves to high speed and large production. Usually the cable 

 system is dragged along the line of profile, and shots are fired at the desired 

 intervals. The geophones are at rest at the time of shooting, but the record- 

 ing boat may keep advancing slowly in order to maintain steerageway, 

 playing out slack cable which is pulled aboard again after the shot. Survey- 

 ing is a rapid and relatively simple matter, especially with suitable electronic 

 equipment. The shooting boat progresses with the recording boat and can 

 fire shots as rapidly as required. In some operations the shooting boat is 

 dispensed with, and the shot is released from the recording boat along a 

 guiding line to a paravane near the desired shot location. The shot is deto- 

 nated through a trailing firing line from the recording boat. 



Production depends to a considerable extent on weather conditions. 



