Ch. 4— Technologies for Exploring the Exclusive Economic Zone • 157 



Photo credit: Bonnie McGregor, U.S. Geological Survey 



The box core retrieves relatively undisturbed cores but 

 only of the first few feet of sediment. 



pling of marine placers. It has been widely and 

 reliably used in offshore programs around the 

 world. Other systems may work weU but the Becker 

 drill has gained the confidence of investment 

 bankers, who must know the extent and tenor of 

 a deposit with a high degree of accuracy before in- 

 vesting money in development. For developing 

 commercial deposits, it is particularly important 

 that the method used be one with a proven record. 



The Amdril, available in several different sizes, 

 is another type of percussion drilling device. Un- 

 like the Becker Hammer Drill, Amdrils are sub- 

 mersible and virtually independent of the support 

 ship's movements. As a result, this drill can oper- 

 ate in much deeper water than the Becker drill. 

 Rather than using the reverse circulation method, 

 an independent pipe supplies air to the casing to 

 raise the drill cuttings. Although the Amdril can- 



not sample boulders or bedrock, it is capable of 

 sampling gravel (unlike vibratory corers) using an 

 airlift system. One type of Amdril has successfully 

 sampled marine sands and gravels off Great Brit- 

 ain. »* 



A somewhat similar system, the Vibralift, devel- 

 oped by the Mississippi Mineral Resources Insti- 

 tute, has proved successful in sampling a variety 

 of mineral deposits, including heavy minerals in 

 dense and semi-hard material. The Vibralift is ba- 

 sically a counterflush system. It utilizes a dual wall 

 drill pipe driven into the sediment by means of a 

 pneumatic vibrator. Water under pressure is in- 

 troduced to the annular space of the dual pipe via 

 a hose from a shipboard pump and is jetted into 

 the inner pipe just above the cutting bit. In this 

 way, the core rising in the inner pipe during the 

 sample drive is broken up by the water jets and 

 transported up the pipe through a connecting hose 

 and finally to a shipboard sample processor. Ad- 

 ditional lift is obtained by routing exhaust air from 

 the vibrator into the inner pipe. Samples are col- 

 lected in a dewatering box to minimize the loss of 

 fine materi3il.^' 



Several types of vibratory corers have been de- 

 veloped over the years. Designs vary by length of 

 core obtained (6 to 12 meters), by core diameter 

 (5 to 15 centimeters), by water depth limits of oper- 

 ation (25 to 1,000 meters), by method of penetra- 

 tion (electric, hydraulic, and pneumatic), by port- 

 abOity, etc. Vibratory corers have been widely used 

 for scientific and reconnaissance sampling. This 

 method is probably the best low-cost method for 

 coring sand and gravel deposits. Relatively un- 

 disturbed and representative cores can be retrieved 

 in unconsolidated sediments such as most sands, 

 clay, and gravel. However, the effectiveness of 

 vibratory corers decreases in dense, fine, relatively 

 consolidated sands and in stiff clays. Some progress 

 has been reported in sampling dense, fine-grained, 

 heavy mineral placers with a jet bit that does not 

 disturb the core.^^ Vibratory corers will not pene- 

 trate boulders or shale. This type of sampling de- 

 vice is less expensive and more portable than the 

 Becker Hammer Drill and is, therefore, probably 



"Ibid., p. 31. 



°^R. Woolsey, demonstrated at Underwater Mining Institute Con- 

 ference, Biloxi, MS, November 1986. 

 '"^Ibid. 



