Page 589 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

588 CHAPTER 21 Practical Applications
farther from the line under evaluation when the search was called off. This is a case of an impossi- ble PSD search due to temperature and decompression issues. The search area was difficult to define due to conflicting witness testimonies. The bottom conditions were tedious and difficult, with uneven and rock-strewn conditions causing a high number of sonar targets as well as having any number of places to trap the victim in bottom holes and crevasses.
21.3.4.4.3 Scenario 2
The victim was on a field trip in Western Canada and decided to swim across a 1 mile glacial lake. The victim drowned somewhere during the swim across. A witness last saw the victim swimming with difficulty and took a sight line on the last seen point.
Several days of searching with side scan sonar produced thousands of sonar targets. Discriminating those for evaluation and elimination took more time than the search team had bud- geted for the search. The bottom conditions were difficult for divers but acceptable for ROVs, with 25 ft (8 m) visibility and an average depth of 125 ft (38 m) of fresh water. The search area was sim- ply too large due to the soft “last seen” point. The victim was eventually recovered when the lake was drained for reservoir maintenance.
21.3.4.4.4 Scenario 3
An example of a successful drowning victim recovery is the search for the bodies from an over- turned public water taxi in the eastern United States. The accident happened on a cold, windy day during frontal system passage when the vessel capsized due to the turning moment as the vessel attempted to turn back into port. Rising wind conditions overturned the dual-hulled vessel, trapping the victims within the cabin. Six passengers drowned during the ensuing rescue attempt. Data gath- ered during the planning phase of the body recovery included:
1. Extensive witness interviews as well as interviews of the survivors and vessel operators 2. Navigational charts
3. Survey and work permit application charts for subsurface harbor construction
The initial search focused on finding a debris field before the tedious task of body search was initiated. Initial search efforts focused on the north side (upwind side on the day of the accident) of the harbor near where the vessel capsized. After several days of searching without results, the data was reanalyzed. A trajectory of the vessel track due to wind drift after the capsizing was plotted, and then the search area was redrawn based upon witness interviews of the recovery efforts. What seemed insignificant during the original search effort took on more importance—an LST (“landing ship tank” or military surplus landing vessel for loading/offloading equipment in the con- struction industry) had been dispatched to render assistance. Once a reestimation of the LST’s posi- tion at the time of intercept of the water taxi was made, the search area was refocused on that position. Once the search was resumed in the new area, the severed canopy from the water taxi was found with side scan sonar on the first search line. The location of the debris field formed the new central datum from which the new search area could be drawn. The victims could not be (and were not) far from that point.
Within 3 days of finding the debris field, all of the remaining victims were located and recov- ered. They were located with side scan sonar and then reacquired with an acoustic lens sonar mounted to an ROV. An example of the detailed search capability of an acoustic lens sonar is the