PART VI: BOTTOM AND SUB -BOTTOM SAMPLING 



110. Seismic reflection records do not provide direct information on 

 the nature of the sediment and rock units penetrated by the acoustic pulses. 

 However, inferences can often be made from consideration of factors such as 

 acoustic transparency, diffraction patterns, configuration and continuity of 

 reflectors, and apparent bedding patterns. While consideration of these and 

 other factors evident on the records is valuable in working out the sub -bottom 

 stratigraphy, this type of analysis does not provide information on the nature 

 of sub-bottom materials giving the precision, detail, or reliability needed 

 for evaluation and selection of borrow sources. Adequate information can only 

 be obtained by analysis of physical samples of bottom and sub-bottom materi- 

 als. For this reason, it is essential that cores be obtained to directly 

 determine material characteristics and suitability for fill, and to substanti- 

 ate correlation of the geological data. 



Objectives 



111. Cores are collected during the general exploration of the program 

 to provide evidence of the lithologic character of sediment and rock units 

 associated with reflector patterns, and to identify units containing poten- 

 tially usable fill material. Correlation of core and seismic reflection data 

 permits construction of an informal stratigraphic model in which the approxi- 

 mate lateral extent, thickness, and accessibility of units containing poten- 

 tially usable fill material can be determined. From this information, 

 promising areas can be selected for detailed site investigation. 



Grab Samplers 



112. There are a variety of grab- type samplers of different sizes and 

 design that are used for obtaining surficial sediment samples. Most consist 

 of a set of opposing, articulated, scoop-shaped jaws that are lowered to the 

 bottom in an open position and closed by some mechanism. In this process, a 

 sample is retrieved between the closed jaws. Many grab samplers are small 

 enough to be deployed and retrieved by hand; others require some type of lift- 

 ing gear. A simple and inexpensive dredge -type sampler can be made of a 



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