2. Field and Laboratory Procedures. 



The exploration phase of the ICONS program uses seismic reflection profiling 

 supplemented by cores of the marine bottom. Additional supporting data for the studies are 

 obtained from the National Ocean Survey (formerly U.S. Coast and Geodetic Survey) 

 hydrographic boat sheets and related published literature. Planning, and seismic-reflection 

 profiling, coring, positioning, and analysis of sediment obtained in the cores are detailed in 

 Geomorphology and Sediment Characteristics of the Nearshore Continental Shelf, Miami to 

 Palm Beach, Florida. (Duane and Meisburger, 1969.) However, a brief description of 

 techniques is germane to this paper and follows. 



a. Planning. Survey trackUnes were laid out by the CERC Geology Branch staff in either 

 of two line patterns: grid and reconnaissance lines. A grid pattern (line spacing about 1 

 statute mUe) was used to cover areas where a more detailed development of bottom and 

 subbottom conditions was desired. Reconnaissance lines are one of several continuous zigzag 

 lines followed to explore areas between grids, and to provide a means of correlating sonic 

 reflection horizons between grids. Reconnaissance lines provide sufficient information to 

 show the general morphologic and geologic aspect of the area covered, and to identify the 

 best places for additional data collection. 



Selection of core sites was based on a continuing review of the seismic profiles as they 

 became available during the survey. This procedure allowed core-site selection based on the 

 best information available; it also permitted the contractor to complete coring in one area 

 before moving his base to the next area. 



b. Seismic Reflection Profiling. Seismic reflection profiling is a technique in wide use for 

 delineating subbottom structures and bedding planes in sea floor sediments and rocks. 

 Continuous reflections are obtained by generating repetitive high -energy, sound pulses near 

 the water surface and recording "echoes" reflected from the bottom-water interface, and 

 subbottom interfaces between acoustically dissimilar materials. In general, the 

 compositional and physical properties which commonly differentiate sediments and rocks 

 also produce acoustic contrasts. Thus, an acoustic profile is roughly comparable to a 

 geologic cross section. 



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

 and receiving instruments behind a survey vessel which follows 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 amplified and fed to a recorder which graphically plots the two-way 

 signal travel time. Assuming a constant velocity for sound in water and shelf sediments, a 

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

 by frequent navigational fixes keyed to the chart record by an event marker, and by 

 interpolation between fixes. 



A more detailed discussion of seismic profiling techniques can be found in a number of 

 technical pubUcations. (Miller, et al., 1967), (Moore and Palmer, 1968.) 



c. Coring Techniques. A pneumatic vibrating hammer-driven coring assembly was used 

 for obtaining cores from the survey area. The apparatus consists of a standard core barrel, 

 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 



