21 



2.1 .4 Bathymetric Data Processing 



During data analysis, the raw bathymetric data from INDAS/PINSS were corrected 

 for changes in tidal height and sound velocity. In 1992, these data were standardized to 

 Mean Low Water. Tidal height corrections were based on the observed National Oceanic 

 and Atmospheric Administration (NOAA) data for the New London, Connecticut, tidal 

 station. The 1995 and subsequent surveys utilized six-minute observed tidal data obtained 

 via the National Oceanographic and Atmospheric Administration (NOAA), Ocean and 

 Lake Levels Division's National Water Level Observation Network. 



Observed tide data are downloaded through the Internet in a station datum or 

 referenced to Mean Lower Low Water (MLLW) and based on Coordinated Universal 

 Time. For the 1995 and 1997 NLDS surveys, data from the NOAA tide station 8461490 

 in New London Harbor, New London, Connecticut were downloaded in the MLLW datum 

 and corrected to local time. Tide differences based on the entrance to West Harbor, 

 Fishers Island, New York, were applied to the observed data. 



In August 1995, tidal data were also collected on-site with a Seabird Instruments, 

 Inc. SBE 26-03 Sea Gauge wave and tide recorder. The tide gauge recorded pressure 

 values every six minutes and provided, after conversion, a constant record of tidal 

 variations in the survey area. These observed tidal data were later used to compare and 

 verify the corrected NOAA data. 



The bathymetric data were analyzed using SAIC's Hydrographic Data Analysis 

 System (HDAS), Version 1.03. Raw bathymetric data were imported into HDAS, 

 corrected for sound velocity, and standardized to MLLW using the NOAA observed tides. 

 The bathymetric data were then processed to produce depth models of the survey area. A 

 model is a depth matrix used to generate graphical representations of the survey area (i.e., 

 three-dimensional plots and depth contours). A detailed discussion of the bathymetric data 

 acquisition and analysis is given in the DAMOS navigation and bathymetry reference 

 report (Murray and Selvitelli 1996). 



The depth model for each bathymetric survey performed over NLDS was then 

 subjected to depth difference routines in HDAS to detect and quantify changes in seafloor 

 topography over time. The end result of the depth difference comparison is a graphical 

 representation of a disposal mound or mounds. However, due to a variety of factors (tidal 

 corrections, changes in sound velocity through the water column, the slope of the bottom, 

 and vertical motion of the survey vessel) comparisons of sequential bathymetric surveys 

 can only reliably detect changes in depth of 20 cm or greater. These factors often 



Monitoring Cruise at the New London Disposal Site, 1992 - 1998 



