OCEANOGRAPHY IN THE UNITED STATES 291 



underlying mantle was first defined by Prof. A. Mohorovicic in 1912 as the 

 depth at which earthquake waves showed a marlied change in velocity. This 

 boundary is commonly called the Moho. There are intermediate layers in the 

 crust which also are identified by their seismic velocities. 



4. The feasibility of drilling to the mantle depends on two things: (1) the 

 depth to the Moho ; (2) the maximum reach of di'illing tools. 



Thus our study looked carefully at both. We find that there are places 

 both in the Atlantic and the Pacific basins where the total distance from the 

 water surface to the mantle is less than 9.5 kilometers (about 31,000 feet). 

 We also find that leading members of the oil industry believe that "a 50,000 

 foot hole would be possible if there were any reason to drill it and if the best 

 of deep-drilling equipment and technology were assembled." (The deepest 

 hole to date is 25,340 feet. ) This is on dry land. 



Drilling from floating platforms anchored in shallow water is becoming 

 routine ; four large vessels and a dozen small ones have built up a backlog 

 of experiences both in the Gulf of Mexico and off the California coast. Two 

 of these vessels, Cuss I and Nola I (made from Navy freight-barge hulls), 

 are completely self-sustaining and have successfully ridden out storms. They 

 are desi,gned to drill holes 12,000 feet deep in water as much as 500 feet deep. 



5. The average depth of the ocean (and the depth at which it will be necessary 

 for us to drill) is about 4 kilometers or 13,000 feet. 



The ocean floor is paved with soft sedimentary material whose usual thick- 

 ness is about 1,()00 feet. Beneath that is a "second layer" 3,000 to 6,(XK) feet 

 thick which may be hardened sediments, limestone, basalt, or more soft sedi- 

 ments screened by a hard rock layer above. We do not know. Beneath the 

 second layer is a third layer of even more uncertain composition, and beneath 

 that is the Moho and the mantle. Table 1 shows that a drill capable of reaching 

 18,000 feet below the sea surface will sample all of the intermediate layers. 



6. It is thus evident that, if one of the existing rigs can be modified so that it 

 (1) will reach 18,(X)0 feet and (2) maintain its position above a deep-water hole 

 site, we will have made a great step forward. It is the opinion of the opera- 

 tors of both the Cvss and the Nola drilling ships that this is feasible and both 

 groups are now making studies of exactly how their ships could be modified to 

 do this job. 



Phase I of the Moho project involves modifying a drilling vessel and testing 

 it out in deep water to see how it performs mechanically. This will give us 

 much valuable engineering information which can be used in the design of the 

 ultimate vessel. 



7. Although phase I is not intended to reach to the Moho, it is man's first 

 attempt to drill or to do any heavy work on the bottom in deep water. The 

 implications of this on oceanic work are enormous. But besides obtaining de- 

 sign data and practical experience it will return scientific data of great 

 importance. 



As seen in table 1, the experimental holes of phase I will reach completely 

 through the sedimentary layers and sample the second and third layers. By 

 so doing, scientists expect to uncover a great deal of otherwise unobtainable in- 

 formation about the history of the ocean and the earth (both biological and 

 physical). 



8. When the test drilling has been completed we will go back to the drawing 

 boards and design a phase II floating drilling rig capable of reaching to at least 

 31,000 feet. It will be built, tested, and taken to a site where it appears that 

 the most valuable information about the deep crust and the mantle can be ob- 

 tained. There it will drill to the Moho. 



If phase I is largely completed by July 1 of 1960 (which is possible if we can 

 get the money to proceed at once) the phase II Mohole rig could be ready in 

 another year (July 1961) and the mantle reached by the end of 1962. 



We have tentatively estimated the cost of this project at about $15 million, 

 broken down as follows: Million 



Preliminary and phase I $3. 



Phase II drilling 9.5 



Scientific work 2.5 



Total 15. 



