borrow source exploration programs, and will be the primary focus of the fol- 

 lowing discussion. 

 Components 



116. Pneumatic vibratory corers consist of three main components: 

 frame, core barrel, and vibrator (Figures 8 and 9). The frame is designed to 

 allow the corer to be freestanding on the sea floor. It usually consists of a 

 quadrapod arrangement with four legs connected to a vertical beam that in turn 

 supports and guides the core barrel and vibrator. Depending on the size of 

 the corer, the vertical beam is 10, 20, or 40 ft high. The core barrel 

 assembly and vibrator can freely slide up and down the beam for coring and 

 retraction. The core barrel assembly consists of a 4-in.-diam steel pipe 

 fitted with a cutter head and core catcher, and a plastic tube inner liner 

 that contains the cored material. The vibrator is usually a piston- type, 

 industrial bin vibrator that uses compressed air to drive the piston in the 

 rapid up and down movements that drive the core barrel into the bottom. The 

 compressed air supply is produced by a compressor on the support vessel, and 

 is transmitted to the vibrator and exhausted to the surface by means of air 

 hoses. Deploying and retrieving the corer requires the use of a crane, 

 A-frame, or similar hoisting equipment with a lifting capacity of about 

 10-15 tons. 



Capabilities 



117. Available pneumatic vibratory corers are capable of penetrating up 

 to 40 ft (12.2 m) of unconsolidated sediment. However, actual performance 

 depends on the nature of the sub -bottom material. Under unfavorable condi- 

 tions as little as 1 m or less may be recovered. Limited recovery occurs for 

 several reasons. Chief among these is lack of penetration. In general, stiff 

 clays, gravel, and hard-packed fine to very fine sand are usually most diffi- 

 cult to penetrate. Another reason is the "freezing" of material in the core 

 liner due to high skin friction before full penetration is reached. When this 

 happens , new material cannot enter the sampler although the core barrel con- 

 tinues to penetrate. Compaction and loss of material during recovery can also 

 cause a discrepancy between penetration and recovery, but occurs less 

 frequently. 



118. The coring operation, including setup, deploying the corer, coring 

 and recovery, is quite rapid in comparison with standard soil boring opera- 

 tions. Usually, a core 20 ft (6.1 m) long or less can be obtained in a manner 



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