E-9 

 A second pinger was mounted on the trawl wire at 50 

 meters above the lead depressor. Travel time from this 

 second pinger to the bottom was used to continuously mon- 

 itor the height of the depressor above the sea bed. 

 Echos from the depressor can be detected down to water 

 depths as great as 1000 meters, allowing the depressor to 

 be towed at a well determined elevation. In general, at 

 a tow speed of 1 knot the camera settles to the bottom 

 when the depressor is within 50 meters of the bottom. 

 Settling rate of the sled is 35 meters/minute which equates 

 to a descent angle of 50 degrees. The descent rate is a 

 function of the amount of negative buoyancy. As adjusted 

 for the field program, the sled would not track effectively 

 down slopes steeper than about 45 . The ability to track 

 downslope is also a function of the depressor elevation. 

 Attempts to fly the depressor only a few meters above the 

 bottom literally pull the sled downhill. Such low flying 

 is risky, however, because it reduces the ability of the 

 tow cable to pull the sled free of obstacles that might en- 

 tangle it (ledges, large boulders, abandoned traps, etc.) 



The percent of time in contact with the bottom changes 

 from run to run depending mostly on bottom roughness, 

 operator experience, and water depth. In benign terrains, 

 such as thalweg floors or flat shelves, the bottom contact 

 might exceed 80% of the time. During transects along 

 gullied canyon walls it may be reduced to less than 40% of 

 the time . 



