14 



water depth. First, the depths are estimated using 1500 m/s as the speed of sound; the 

 speed of sound will be higher in sand and the well-consolidated, homogeneous BBC 

 (approximately 1700 m/s; Hamilton 1971). In addition, one lane was digitized to compare 

 the subbottom depths with bathymetry, and the scale printed on the subbottom cross section 

 was found to be inaccurate. 



2.3.3 Sidescan Data Collection and Analysis 



A Marine Sonics PC Scan side-scan sonar system was used for side-scan sonar data 

 collection. This system is a single frequency (300 kHz) system that collects digital data 

 directly to a PC-compatible computer. Bottom coverage was 100% for each pass. Several 

 N-S passes were made with the cell on one channel, and the best image was processed 

 using Adobe Photoshop® to enhance the visual resolution and add annotations. The image 

 was not geo-registered, so that the aspect ratio of the cell (width vs. length) has been 

 compressed in the vertical (N - S) direction because of survey vessel speed. 



2.4 Videosled Survey 



On 3 December 1997, CR Environmental, Inc. (CR) performed a towed video sled 

 survey at the CAD in Boston Harbor to demonstrate the effectiveness of using high 

 resolution underwater video in detecting the coverage of the sand cap. The CR 

 Underwater Video Sled is equipped with a high resolution Sony Hi8 video camera, and two 

 250 watt Deep Sea Power lights with variable light output. The system has a 100 meter 

 cable, portable monitor and VCR, and isolated power for shock protection. 



The operations were performed from CR Environmental' s 32-foot survey vessel 

 Cyprinodon equipped with a hydraulic winch, A-frame and a large enclosed pilot house for 

 survey equipment. A Northstar DGPS and the Coastal Oceanographies' HYPACK 

 navigation software package were utilized to provide the boundaries of the confined 

 disposal cell and a pre-programmed set of tracklines. The layback of the video sled to the 

 navigation anteimae was estimated to be approximately 50 ft. By examining the real-time 

 video display, the vessel captain adjusted the towing speed and the pay out of the tow cable 

 to achieve the best towing angle and bottom coverage. Two north-south transects were 

 made through the disposal cell. Operations were performed at slack low tide and water 

 visibility was estimated as 5 to 10 ft. 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



