158 • Marine Minerals: Exploring Our New Ocean Frontier 



Photo source: P. Johnson, Office of Technology Assessment 



Vibracore ready for deployment from side of ship. 



VIbracores can retrieve relatively undisturbed samples 



in many types of unconsolidated sediment. 



more suitable for reconnaissance work than the 

 Becker drill; however, given limitations in the type 

 of deposit that can be sampled, vibratory corers 

 would be less appropriate for proving certain mine 

 sites. 



Vibracore systems, properly designed and oper- 

 ated, have successfully evaluated thin (i.e., less than 

 12 meters), surficial, unconsolidated deposits of 

 fine-to-coarse-grain material, such as sand and 

 gravel, shell, heavy minerals, and phosphorite. 

 Vibratory corers are inadequate for the more dis- 

 seminated precious mineral placers such as gold, 

 platinum, and diamonds, due to system limitations 

 in sampling host gravels typically containing cob- 

 bles and boulders. Vibratory corers are also use- 

 less for any deposit where the thickness of overbur- 

 den and/or zones of interest exceed the penetration 

 limits of the system. 



The costs of offshore sampling vary widely, de- 

 pending on such factors as water depth, mobiliza- 

 tion costs, weather, navigation requirements, and 

 vessel size and availability. One of the most im- 

 portant factors in terms of unit costs per core is the 

 scope of the program. Costs per hole for a small- 

 scale program wUl be higher than costs per hole for 

 a large-scale program. Table 4-9 shows typical costs 

 of offshore vibracore programs in shallow and deep 

 water. Costs per core are seen to vary between 

 about $2,500 and $7,000. 



An alternative or supplementary strategy to tak- 

 ing the large numbers of samples that would be 

 needed to prove a mine site is to employ a small, 

 easily transportable dredge in a pilot mining 

 project. Each situation is unique, but for some cases 

 the dredge may be less expensive and may be bet- 

 ter at reducing uncertainty than coring or drilling. 

 Such a program was recently completed with a pi- 

 lot airlift dredge off the coast of west Africa. Four 

 tons of phosphorite concentrate were recovered for 

 an economic evaluation.^' Dredging would cause 

 significantly more environmental disruption and 

 may, unlike other sampling methods, require an 

 environmental impact statement. 



Crusts 



Cobalt-rich ferromanganese crusts were discov- 

 ered during the 1872-76 expedition of the HMS 

 Challenger, but detailed studies have only recently 

 begun. In general, existing coring and other de- 

 vices developed to sample shallow-water placers are 

 not appropriate for sampling crusts in deep water; 

 therefore, new sampling technologies must be de- 

 veloped. An important consideration in develop- 

 ing new technology is that crusts and underlying 

 substrate are usually consolidated and hard and 

 therefore not as easily penetrated by either dredges 

 or coring devices. Moreover, crusts are found at 

 much greater depths than most unconsolidated de- 

 posits. The most desirable crusts are believed to oc- 

 cur between 800 and 2,500 meters water depth; 

 thus, sampling equipment must at least be able to 

 operate as deep as 2,500 meters. Crusts known to 

 date rarely exceed 1 2 centimeters (5 inches) in thick- 

 ness; therefore, there is no requirement for long 

 samples. 



''A. Woolsey and D. Bargeron, "Exploration for Phosphorite in 

 the Offshore Area of the Congo," Marine Mining, vol. 5, No. 3, 1986, 

 pp. 217-237. 



