1J_ 



it was run through the HYPACK® Sort routine to reduce its size. Because of the rapid rate at 

 which a survey echosounder can generate data (approximately ten depths per second), the 

 along-track data density for a single-beam survey tends to be very high (multiple soundings 

 per meter). In most cases, these data sets contain many redundant data points that can be 

 eliminated without any effect on overall data quality. The Sort routine examines the data 

 along each survey line and then extracts only the representative soundings based on a user- 

 specified distance interval or search radius. The output from the Sort routine is a merged 

 ASCII-XYZ (horizontal position and corrected depth) file that may contain anywhere fi^om 

 2-10% of the original data set. These greatly reduced, but still representative, data sets are 

 far more efficient to use in the subsequent modeling and analysis routines. For the NLDS 

 survey, the data were sorted at distance intervals of 5 and 10 m for later analysis. 



233 Bathymetric Data Analysis 



The goal of the data analysis was to create seafloor surface models fi-om the fully 

 processed bathymetric data, and then to evaluate these models in an attempt to identify any 

 unique topographic features and account for any observed differences between consecutive 

 surveys. For the NLDS survey, the analysis technique used to evaluate the 2000 survey and 

 compare it with the most recent 1 997 survey has been used routinely during past D AMOS 

 Program monitoring surveys. This technique entails calculating and then mapping the 

 difference in depth between similarly gridded data sets for the two surveys. With this 

 technique, the sorted ASCII-XYZ files were imported into ESRI's Arc View® software, and a 

 grid system was defined over the NLDS survey areas. Because the survey track-lines were 

 spaced at 25 m intervals, a cell-size of 12.5 m (along- track) by 25 m (cross-track) was 

 specified to ensure sufficient data coverage to fill each cell. An Arc View® gridding routine 

 was then run to average all of the single-beam data points that fell within each cell and 

 generate a single depth value that was assigned to the center of each cell. The end result of 

 this process was a matrix of depth values that defined a three dimensional surface model of 

 the survey area. A similar grid-filling process was performed over both the NL-91 and D/S 

 Mound Complex and the Seawolf Moimd survey areas using both the 2000 and 1997 data 

 sets. The two grids for both areas were then depth differenced in an attempt to highlight 

 areas of significant change between the two surveys. 



2.4 REMOTS® Sediment-Profile Photography 



Remote Ecological Monitoring of the Seafloor (REMOTS®) is a benthic sampling 

 technique used to detect and map the distribution of thin (<20 cm) dredged material layers, 

 map benthic disturbance gradients, and monitor the process of benthic recolonization over 

 the disposal mound. This is a reconnaissance survey technique used for rapid collection, 

 interpretation and mapping of data on physical and biological seafloor characteristics. The 

 DAMOS Program has used this technique for routine disposal site monitoring for over 20 



Monitoring Cruise at the New London Disposal Site, August 2000 



