Seismic reflection surveys of marine areas are made by towing sound- 

 generating sources and receiving instruments behind a survey vessel which 

 follow predetermined survey tracklines. For continuous profiling, the sound 

 source is fired at a rapid rate, and returning signals are amplified and fed 

 to a recorder which graphically plots the two-way signal traveltime. Assuming 

 a constant speed for sound in water and shelf sediments, a vertical depth 

 scale can be constructed to the chart paper. Horizontal location is obtained 

 by frequent (2-minute intervals) navigational fixes keyed to the chart record 

 by an event marker. 



General seismic profiling techniques are discussed in Moore and Palmer 

 (1968) and American Association of Petroleum Geologists (1977). 



Seismic reflection profiles for this study were made with an engineering 

 "sparker" (Alpine Geophysical Associates, Inc., 1965). Two sound sources, one 

 operated at 100 joules and the other at 200 joules, were fired alternately at 

 a pulse rate of 4 per second during the survey. Returns from the sound 

 sources were recorded by a dual-channel recorder which displayed the results 

 on a single strip chart. 



c. Coring Techniques. An Alpine Geophysical Associates, Inc. pneumatic, 



vibrating hammer-driven coring assembly was used to obtain cores from the 

 survey area. The apparatus consists of a standard 5-meter-long core barrel, 

 acrylic liner, and shoe and core catcher, with the driver element fastened to 

 the upper end of the barrel. These are enclosed in a self-supporting frame 

 which allows the assembly to rest on the bottom during coring, thus not influ- 

 enced by limited motion of the support vessel in response to waves. Power is 

 supplied to the vibrator from a deck-mounted air compressor by means of a 

 flexible hoseline. After the core is driven and returned, the liner contain- 

 ing the cored material is removed and capped. Denser materials (compact 

 sands; partially lithified rocks) are more difficult to penetrate than loose 

 materials. Cohesive materials cause drag on the liner walls and give some 

 distortion. 



d. Navigation . Position location was determined during the survey by use 

 of the Decca Navigator Mark 12 Receiver and Track Plotter which accurately 

 located the vessel with respect to two onshore reference points. Navigation 

 fixes were made at 2-minute intervals during all seismic reflection survey 

 work and at each core position. Final plots of trackline and core location 

 compiled from survey data were prepared at scales of 1:80,000. 



e. Processing . Seismic records were analyzed to establish the principal 

 bedding, erosional, constructional, and structural features in upper subbottom 

 strata. After preliminary analysis, typical records were reduced to detailed 

 cross-sectional profiles showing all reflective interfaces within several 

 hundred feet of the bottom. Selected reflectors, considered significant 

 because of their extent and relationship to the general structure and geology 

 of the study area, were then mapped. If possible, the uppermost mapped 

 reflector was correlated with core data to provide a measure of continuity 

 between cores. 



Cores were visually inspected and logged aboard ship. After delivery to 

 CERC, the cores were sampled every foot by drilling through the liners and 

 removing samples of representative material. After preliminary analysis, a 

 number of representative cores were split to determine details of the bedding. 



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