However, it is still not inexpensive; detailed studies 

 can cost more than $500 per mile, and even 

 regional studies may cost more than SI 00 per 

 mile. In addition, much more geophysical work 

 must be done at sea than on land since this is the 

 only way, short of drilling, that either the broad 

 geological framework or specific details of the 

 geology can be determined in the oceans. 



Techniques are continually being developed to 

 increase the speed of a survey and the reliabiUty of 

 the data collected, and to decrease the costs. The 

 earUer surveys were carried out using dynamite as 

 the seismic energy source, but such data are now 

 largely collected using a great variety of non-ex- 

 plosive energy sources (electrical sparks, air guns, 

 contained gas explosions, vibrating plates, and 

 others). Seismic data also are now routinely 

 recorded on magnetic tape and treated by digital 

 computers to enhance the quality and reliabihty of 

 the data. 



Marine magnetometers are very convenient and 

 allow accurate measurement of the earth's mag- 

 netic field while underway. Marine gravimeters are 

 now being used which can determine very small 

 variations in the earth's gravity field while under- 



way at sea, even with very large movements of the 

 ship. 



Such geophysical work can be carried out in 

 any depth of water and at any distance from land. 



These geophysical techniques are indirect 

 methods for examining the rocks which comprise 

 our continental margins, and it is obviously much 

 more difficult and expensive to obtain samples of 

 these rocks at sea than on land. The most 

 satisfactory method to date has been with the 

 small coring ship which drills a hole a few hundred 

 feet into the sea floor and recovers samples of the 

 rock for further study (Figure 5). Approximately 

 100 such holes have already been cored in the U.S. 

 continental margin beyond the shelves, penetrating 

 up to 1,000 feet beneath the sea floor in waters 

 from 600 feet to nearly 5,000 feet deep. As long 

 ago as 1961, the petroleum industry drilled several 

 experimental core holes in 12,000 feet of water as 

 part of the early phase of Project Mohole. 



A group of oceanographic institutions (Joint 

 Oceanographic Institutions Deep Earth Sampling) 

 has obtained a Federal grant to contract the 

 drilling of a series of core holes to sample up to 

 2,500 feet below the sea floor in water depths as 



Outboard Profile of Caldrill 1 



^TWO POSITIONING MOTORS 



DRILLING WELL TWO POSITIONING MOTORS t^ 



176 It. 



Figure 5. Marine coring vessel 



VII-204 



