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 from bottom and subbottom interfaces are received by one or more hydrophones. 

 Returning signals are amphfied and fed to a recorder which graphically plots the two-way 

 signal travel time. 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. 



A more detailed discussion of general seismic profiling techniques can be found in several 

 technical pubhcations (Ewing 1963; Hersey, 1963; Miller, Tirey, and Mecarini, 1967; Moore 

 and Palmer, 1968). 



Seismic reflection profiles for this study were made with an Alpine Engineering Sparker. 

 Two sound sources, one operated at 50 to 100 joules and the other operated at 100 to 200 

 joules, were fired alternately 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 for obtaining cores from the survey area. The 

 apparatus consists of a standard 20-foot core-barrel (nominal 3-inch diameter) liner, 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 influenced 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 tlie core is driven and returned, the Uner containing 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 

 Uner walls and give some distortion. 



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

 Precision Radar System which accurately locates 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 1:24,000 and 1:80,000. 



e. Processing. Seismic records are analyzed to estabhsh the principal bedding, erosional, 

 constructional, and structural features in upper subbottom strata. After preliminary 

 analysis, typical records are reduced to detailed cross-sectional profiles showing aU 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 are mapped. If possible, the uppermost mapped reflector is correlated witli 

 core data to provide a measure of continuity between cores. 



