Selection o£ individual core sites is based on a continuous inspec- 

 tion of the seismic records as they become available from the contractor 

 during the survey. This procedure of picking core locations based on 

 geologic conditions revealed on the seismic records allows core-site 

 selection to be based on the best available information and thus maxi- 

 mizes usefulness of both sources of data. It also permits the contrac- 

 tor to complete the required work of obtaining geophysics and cores in 

 one area before moving his base of operations to the next area. 



b. Seismic Reflection Profiling . Seismic reflection profiling is a 

 technique widely used for delineating subsea floor geologic structures 

 and bedding surfaces in sea floor sediments and rocks. Continuous reflec- 

 tions are obtained by generating repetitive, high-energy, sound pulses 

 near the water surface and recording "echoes" reflected from the sea 

 floor-water interface and subbottom interfaces between acoustically dis- 

 similar materials. In general, the compositional and physical properties 

 Ce-g«> porosity, water content, relative density) which commonly differen- 

 tiate sediments and rocks also serve to produce acoustic contrasts which 

 show as dark lines on the geophysical paper records. Thus, a seismic 

 profile is roughly comparable to a geologic cross section. 



Seismic reflection surveys of marine areas are made by towing variable 

 energy and frequency sound-generating sources and receiving instruments 

 behind a survey vessel which follows the predetermined survey track lines. 

 A dual energy source (100- to 200- joule engineering sparker) was used for 

 this survey. In continuous profiling, the acoustic source is fired at a 

 rapid rate (usually 4 pulses per second) and returning echo signals from 

 sea floor and subbottom interfaces are received by an array of towed hydro- 

 phones. Returning signals are amplified and fed to a recorder which graph- 

 ically plots the two-way signal travel time. Assuming a constant velocity 

 for sound in water at 4,800 feet per second and for typical shelf sediments 

 at 5,400 feet per second, a vertical depth scale is constructed to fit the 

 seismic record. Detailed seismic profiling techniques are discussed in 

 several technical publications (Ewing, 1963; Hersey, 1963; van Reenan, 

 1963; Miller, Tirey, and Mecarini, 1967; Moore and Palmer, 1968; Barnes, 

 et al., 1972; Ling, 1972). 



Geographic position of the survey vessel is obtained by frequent navi- 

 gational fixes keyed to the record by an event marker. Navigation for 

 this particular survey was achieved by lose of the Alpine Precision Range 

 System, Model 4350. This system uses a Decca Transar for radar rangings, 

 the Alpine Model 4270 X-Band Transponder as a point reflection source, the 

 Precision Range System for measuring the distance of strobe alinement to 

 the reflectors, and a Remote Autotrack Plotter. The plotter was coupled 

 electronically to the range system to provide a real-time true-motion plot 

 of the survey vessel course. Accuracy for this survey was about ±100 feet 

 in 80 nautical miles. 



c. Coring Techniques . The sea floor coring device used in this study 

 was a pneumatic, vibrating piston coring assembly designed to obtain core 

 samples (20-foot or 6-meter maximum length; 4- inch or 10.2-centimeter 



