area. In the field, the two radar transponders were positioned at known 

 geographic locations with reply to the interrogator at frequencies of 

 9,310 and 9,490 megahertz. The transponder replies were received by the 

 dual -channel X-band receiver with the time interval between the interro- 

 gation and transponder response precisely measured by crystal -control led 

 digital circuits. To obtain reasonable angles of intersection, trans- 

 ponders were leapfrogged from site-to-site down the coastline as the 

 survey progressed southward. On several occasions during the 1974 survey 

 only one transponder could be read. At these times, the ship position 

 was tracked by triangulation from the one operating beacon and distance 

 from shore as determined by conventional radar. 



c. Acoustic Profiling . During the two field surveys five types of 

 acoustic profiling equipment were used: three different continuous seis- 

 mic reflection profilers, a fathometer, and a side-scan sonar profiler. 

 The latter two instruments and one seismic profiler were used in the 

 1974 survey. 



Seismic reflection profiling is a widely used technique for deliniat- 

 ing 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 be- 

 tween acoustically dissimilar materials. In general, the compositional 

 and physical properties which commonly differentiate sediments and rocks 

 also produce acoustic reflections at interfaces between different mater- 

 ials. Thus, an acoustic profile is roughly comparable to a geologic 

 cross section. During 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. Depth can be determined by assuming a constant velocity for sound 

 in water and unconsolidated shelf sediments. General seismic profiling 

 techniques are discussed in detail in Miller, Tirey, and Mecarini (1967) 

 and Moore and Palmer (1968) . 



Seismic reflection profiling for the 1970 survey was conducted by 

 Ocean Science and Engineering, Inc. (OSE), Washington, D.C., under con- 

 tract to CERC with the OSE Dual -Frequency Seismic Profiler. The system 

 consists of a 3.5-kilohertz crystal transducer and a variable power 

 sparker with a 3-meter hydrophone array for a receiver. The initial 

 operating power level of the sparker was 200 joules; however, reverber- 

 ations in shallow water caused a loss of detail in the uppermost sediments 

 so the sparker was operated at the 60- to 90-joule level. Acoustic data 

 were processed, amplified, and recorded on a Gifft recorder. The spring 

 1974 survey used a Bolt, Beranek, and Newman, Inc. (BBN) Acoustipulse 

 system (tracklines are shown in Fig. 4). The system utilized a electro- 

 magnetically driven diaphragm to produce a broadband acoustic pulse; the 

 motion of the diaphragm (or transducer) is controlled to minimize the 

 formation of a secondary or "bubble pulse." Up to three transducers, 

 each capable of over 1,000 joules, can be mounted on a catamaran to give 

 a depth penetration of 90 to 150 meters (300 to 500 feet) . 



