and to one half of the lane spacing in a parallel direction. As 

 the survey data were read into the computer, the depth data from 

 fixes falling within a given cell were weighted with respect to 

 their spatial offset from the center of the cell, an average was 

 calculated, and the resulting depth value was assigned to that 

 cell. Prior to these calculations, each individual data point was 

 corrected for sound velocity variations, transducer draft, and 

 tidal amplitude. This depth matrix was then used to generate depth 

 contour charts. The accuracy of the depth measurements assigned to 

 each cell was affected by the instrument accuracy of the fathometer 

 (0.5% of indicated depth) , the number of depth measurements in that 

 cell, the vertical motion of the research vessel, the difference in 

 depth between the center of the grid cell and the actual position 

 of the vessel, and the accuracy in the prediction of the change in 

 tidal height for the duration of the survey. Error analysis of the 

 depth measurements was performed with respect to the conditions at 

 the time of the surveys, and it was determined that the error on 

 the depths presented in the contour plots for the Brenton Reef 

 disposal site is approximately +8 cm. 



2.3 Side Scan Sonar and Sub-bottom Surveys 



A Klein Associates Model 520® side scan sonar system was 

 used in this project; this system consists of a 100 kHz towfish 

 interfaced to a 48 centimeter (19 inch) wet paper recorder through 

 a lightweight Kevlar® tow cable with simultaneous dual-channel 

 recording of the side scan sonar at variable ranges. During the 

 surveys, a position fix was printed every minute and a mark was 

 made on the recorder simultaneously. These were later used to 

 determine the position of individual side scan targets. Position 

 data were also recorded every second on magnetic disk by the INDAS. 

 Towfish layback (distance behind ship) was estimated from catenary 

 curves provided by Klein Associates for various speeds and checked 

 by comparison of bottom features observed during the bathymetric 

 survey . 



The side scan sonar surveys were run concurrently with 

 the bathymetric surveys at the Brenton-A, Brenton-B, and Prudence 

 areas. The lane spacing at each of the three areas (Figures 2-1 

 and 2-2) was chosen to provide the minimal acceptable coverage for 

 the side scan sonar system. The width of the area covered by the 

 side scan was controlled by the towfish height above bottom and the 

 settings on the instrument. While the side scan yielded the most 

 economical broad-scale physical characterization of the bottom, 

 spatial bathymetric detail was limited with the wide survey lane 

 spacing. 



The side scan sonar survey at the Brenton Reef Disposal 

 Site could not be performed due to the density of lobster pots in 

 the area. However, a sub-bottom survey, using an ORE, Inc. Model 

 1032® sub-bottom profiling system, was run concurrently with the 



