section of pipe that is closed at one end and dragged a short distance across 

 the bottom to dredge up a sample. Although not representative of a single 

 point as are grab samplers, and subject to losing finer material during recov- 

 ery, dredge samplers are useful in areas where shells or gravel prevent com- 

 plete closure of grab samplers. 



113. Obtaining surficial samples, while helpful, is of limited value 

 because, as mentioned in PART III, vertical projection of surface data is 

 highly unreliable. The expense of running the tracklines for the sole purpose 

 of sampling surficial sediments is not economically justified by the value of 

 the data obtained. Samples could be taken during the geophysical survey, but 

 use of grab and dredge samplers would require the vessel to stop at each sam- 

 pling station, thus losing survey time and creating an interruption of data 

 coverage. Samplers are available that can be deployed over the side of the 

 vessel without its stopping, and these may be useful if deployed in such a way 

 as not to foul the towed seismic reflection and side-scan sonar equipment. 



Vibratory Corers 



114. Direct sampling of sub-bottom materials is essential for borrow 

 source identification and evaluation. This is usually accomplished by means 

 of a continuous coring apparatus that can obtain cores 20 to 40 ft in length 

 in unconsolidated sediments. In the types of sediment usually encountered in 

 borrow site exploration, gravity corers are not suitable for obtaining cores 

 of the requisite length, and some form of powered corers must be used. In 

 most cases, vibrator-driven coring devices have been used for this purpose 

 (Meisburger and Williams 1981) . 



115. Vibratory corers have been in use for several decades and have 

 been improved through the years as field experience has accumulated. They are 

 relatively simple devices consisting of a frame, coring tube, and drive head 

 consisting of a pneumatic, hydraulic, or electrical vibrator. While the 

 hydraulic and electrical vibrators can be used in deeper water than the pneu- 

 matic vibrators, the depth range of the pneumatic device is adequate for bor- 

 row site investigations where the maximum depth of usable borrow sources is 

 limited by practical and economic considerations. Because of their relative 

 simplicity and economy, pneumatic coring devices have been used most often for 



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