Page 583 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

582 CHAPTER 21 Practical Applications
Once all of the data had populated the GIS map, the data points should clump onto the site of the actual wreck. As an example, the R.W. Gallagher was a US Merchant Marine tanker sunk in July 1942 by the German submarine U-67. The DMA, USCG, NOS, and NOAA databases had that wreck in three separate locations—dives were conducted, and it was not at any of those loca- tions. Regardless of the success of such searches, the bottom line is that without proper planning, the search team is probably destined for failure.
21.3.4.1.2 Murder victim search
A second example involved the search of a water-filled abandoned rock quarry for a murder victim who was purportedly cut in half, stuffed into two separate acid-filled sealed barrels, and rolled off a cliff into a 100 ft water depth isolated location. Although the victim was not found, the data gath- ered to ensure an efficient search included:
1. Satellite and aerial photos from Terraserver
2. County survey data from county records
3. Company records of excavation from landowner
4. Extensive witness interviews
5. Sheriff’s investigators rolled water-filled test barrels off the cliff to test the trajectory and then
marked the search area off with buoys.
21.3.4.1.3 Environmental considerations
One would prefer to swim in the warm waters of the equatorial regions on sunny cloudless days. Unfortunately, the PSD does not have the luxury of knowing the time and choosing the place of the team’s deployment. Most PSDs have within their jurisdiction aquatic areas comprised of lakes, riv- ers, canals, and coastal waterways where most human activities prevail. These areas are normally of low visibility and/or of a high current/tidal flow that create difficult search conditions. Visual instruments are of limited use for such environments, making the intelligent use of other sensors practically a requirement.
In theory, with low visibility conditions, items of interest should be first trapped by a nonvisual sensor and then held in a steady-state condition (e.g., for a sonar target, it should be held “in sight” on the sonar screen) so the diver or ROV can positively navigate to the target. If conducting area searches, it is most convenient to use search lines of equal depth to limit the maneuvering to x, y coordinates without throwing in the additional z factor of constantly changing depth.
Smooth, flat bottom conditions are the optimum type of underwater topography for searching with sonar. Since the target can be insonified on sonar much more easily on a flat bottom, the sensors will pick up minimal “false targets.” For rocky bottom conditions, more time must be allocated to clear the area, since the target may have fallen against or into some of the rocks. This would make identification visually or acoustically difficult. Additional details on environmental and bottom condi- tions that should be kept in mind when preparing for a search are provided in the following sections.
Types of environments
• Rivers: Of the search environments confronting the public safety dive team, the most difficult class is the river environment. Rivers normally drain from higher elevations and have both suspended particulate matter and high currents. Rivers also often have any number of bottom obstructions, highly concentrated surface vessel traffic, and soft, loosely consolidated or unconsolidated mud bottoms.