2.0 METHODS 



After completion of a survey at Central Long Island Sound Disposal Site (Morris 

 1994), the survey team devoted a day to collect additional bathymetric and subbottom 

 profile data at CSDS to confirm the presence of the fine-grained dredged material deposit, 

 measure its thickness, and determine its stability over two years. The combination of 

 bathymetric and subbottom profiling techniques has been particularly effective when used 

 to map distinctive layers of dredged material deposits in the New York Mud Dump and in 

 Long Island Sound. 



The July 8, 1994 survey at CSDS was set up over a 1000 m x 750 m grid (29 lanes 

 at 25 m lane spacing) centered on 41°12.775' N, 72°21.857' W and oriented east and 

 west. Throughout the survey, both the bathymetric and subbottom profilers were operated 

 simultaneously along each survey lane. The 208 kHz transducer for the bathymetric 

 survey was attached to the side of the vessel. The acoustic transducers of the subbottom 

 system were mounted on a towfish and towed 5 m below the water surface and 

 approximately 15 m behind the vessel. After completing 20 lanes, the survey was 

 interrupted by an electrical storm, and survey operations, which were limited to one day, 

 were suspended. 



2.1 Bathymetry 



The SAIC Integrated Navigation and Data Acquisition System (INDAS) provided 

 the precision navigation required for all field operations. This system uses a Hewlett- 

 Packard 9920® series computer to collect position, depth, and time data for later analysis, 

 and to provide real-time navigation. A Del Norte Trisponder® System provided 

 positioning to an accuracy of +3 m. Shore stations were established in Connecticut at 

 known benchmarks at Cornfield Point (41° 15.790' N, 72°23.040' W) and Lynde Point 

 Light (41° 16.290' N, 72°20.590' W) in Old Saybrook, Connecticut. DAMOS 

 Contribution No. 60 (SAIC 1989) contains a detailed description of INDAS and its 

 operation. 



An ODOM DF3200 Echotrac® Survey Recorder with a narrow-beam 208 kHz 

 transducer recorded depth to a resolution of 3.0 cm (0.1 ft) as described in DAMOS 

 Contribution No. 48 (SAIC 1985). Depth values transmitted to the computer were 

 adjusted for speed of sound and transducer depth. Before starting the bathymetric survey, 

 a SeaBird Instruments, Inc. SEACAT SB A 19-01 conductivity-temperature-depth profiler 

 (CTD) was used to calculate a sound velocity profile. During analysis, all depth values 

 were converted to Mean Low Water (MLW) after compensating for vessel draft and tidal 

 fluctuations. Position and depth were also checked to identify and eliminate any outlying 



Bathymetric and Subbottom Survey at the Cornfield Shoals Disposal Site, July 8, 1994 



