902 



EXPLORATION GEOPHYSICS 



two shooting boats are used in line with, but off some distance from, each 

 end of the geophone spread. 



Along the California coast, state regulations require that explosives 

 be jetted into the bottom where the water depth is less than 100 feet. This 

 regulation is based on the assumption that less damage is done in killing 

 fish by buried shots. Special jetting spoons have been developed to rapidly 

 jet and bury charges in the bottom sediments from a surface jetting boat. 



Surveying. — One of the most troublesome problems in offshore seis- 

 mograph work is the accurate surveying of the geophone spread and shot 

 locations. Within sight of the shore, sextant shots may be used for loca- 



FiG. 568. — A marine seismic fleet consisting of two survey boats, a shooting boat and 

 a recording boat. These boats are (SF-1) fadar-equipped; note radar antennae and 

 reflectors. (Courtesy of American Exploration Company.) 



tions if adequate known targets are present on shore ; or locations may be 

 determined by triangulation from shore station's. As the distance offshore 

 becomes greater, or when fog or haze is prevalent, other survey methods 

 are necessary. A combination of running on course by gyro-compass 

 bearings or by a system of back-sighting, together with distance measure- 

 ments with a wire line machine, can be used. However, as distances from 

 shore become of the order of 20 or 30 miles or more, it is nearly essential 

 to use some electronic means of surveying.* Certain of these methods are 

 limited to line-of-sight operating conditions ; hence the antennas or reflectors 

 must be mounted as high above the surface as possible in order to overcome 

 limitations caused by the earth's curvature. (See Figure 568.) The target 

 stations are often boats anchored at known positions rather than land 

 stations. 



* See page 905 for description of radio location systems. 



