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2.3 Bathymetric Data Acquisition and Analysis 



2.3.1 Bathymetric Data Acquisition 



During the bathymetric survey, HYPACK® was interfaced with an Odom Hydrotrac® 

 survey echosounder, as well as the Trimble DGPS system. The Hydrotrac® uses a narrow- 

 beam (3°), 208-kHz transducer to make discrete depth measurements and produce a 

 continuous digital data output and an analog record of the seafloor. The Hydrotrac® 

 transmits approximately 10 digital depth values per second (depending on water depth) to the 

 data acquisition system. Within HYPACK®, the time-tagged position and depth data were 

 merged to create continuous depth records along the actual survey track. These records 

 could be viewed in near real-time to ensure adequate coverage of the survey area and verify 

 data quality. 



2.3.2 Bathymetric Data Processing 



The bathymetric data were fully edited and processed using the HYPACK® data 

 processing modules. Raw position and sounding data were edited as necessary to remove or 

 correct questionable data. Sound velocity and draft corrections also were applied. In 

 addition, the sounding data set was reduced to the vertical datum of Mean Lower Low Water 

 (MLLW) using observed tides obtained ft-om the National Oceanic and Atmospheric 

 Administration (NOAA). 



During bathymetric survey data acquisition, an assumed and constant water column 

 sound velocity was entered into the Odom echosounder. In order to account for the variable 

 speed of sound through the water column, a Seabird Instruments, Inc. SEACAT SBE 19-01 

 Conductivity, Temperature, and Depth (CTD) probe was used to obtain sound velocity 

 profiles at the start, midpoint, and end of each field survey day. An average sound velocity 

 was calculated for each day from the water column profile data, and then entered into a 

 HYPACK® sound velocity correction table. Using the assumed sound velocity entered into 

 the echosounder and the computed sound velocity from the CTD casts, HYPACK® then 

 computed and applied the required sound velocity corrections to all of the sounding records. 



Observed tide data were obtained through NOAA's National Water Level 

 Observation Network. The NOAA six-minute tide data were downloaded in the MLLW 

 datum and corrected for tidal offsets. SAIC used the water level data available from the 

 operating NOAA tide station in New London, CT (Station 8461490). 



After the bathymetric data were fully edited and referenced to MLLW, cross-check 

 comparisons on overlapping data were performed to verify the proper application of the 

 correctors and to evaluate the consistency of the data set. After the full data set was verified. 



Monitoring Cruise at the New London Disposal Site. August 2000 



