Page 585 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition
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584 CHAPTER 21 Practical Applications
21.3.4.3 Narrow area search
The narrow area search is conducted with relatively immobile sensor equipment mounted aboard a fixed or slow-moving platform. Examples of narrow area search equipment include:
• Diver performing a rope-guided search
• Tripod-mounted, mechanically or electrically scanning sonar moved from location to location
while covering a fixed area
• ROV performing a grid search
• Low-tech cable drags
An earlier project involved a classic narrow area search for evidence (in conjunction with the US Bureau of Alcohol, Tobacco, and Firearms) on a suspected arson case involving a boat belong- ing to a high-profile politician. The boat burned in the slip of a floating dock; therefore, investiga- tors were certain that the clues rested on the muddy bottom, 62 ft (20 m) below the surface. The visibility on the bottom of the muddy lake in the southern United States was less than 2 ft (70 cm). The search area was less than 100 ft (30 m) in diameter. A tripod-mounted, mechanically scanning sonar was lowered to the bottom to image the entire search area. Two separate ROVs were used to recover all of the items of interest from the search area (over 40 items were recovered in 6 hours) with the sonar used to direct the ROVs to each target.
21.3.4.4 Some search examples
Two good examples of the effect of the environment involved separate unsuccessful drowning victim searches in 2000, both of which were in rocky bottom lake conditions. The rocky bottom caused such a large false target population that the entire search budget was expended identifying rock outcrop- pings. The victims were both eventually recovered, which allowed the refinement of the search tech- niques used.
For illustration of the search process, the efforts made on the two unsuccessful body searches are discussed below, as well as those on a successful search. These will serve as a primer to the issues involved with such an endeavor, as well as highlight the difficulties inherent in body searches.
21.3.4.4.1 Drowning considerations
Prior to conducting a mission that may involve a drowning victim, the following rules of thumb should be considered:
• For drowning victims, the body has up to a 1:1 glide ratio to its landing spot on the bottom. That glide ratio must be adjusted based upon currents for an actual track over the ground. A depiction of this phenomenon is shown in Figure 21.19.
• The average adult weighs 812 pounds (46 kg) in water (for an expanded table of weights as well as an in-depth analysis of the drowning issue, see Teather’s work on the subject). If the ROV is equipped with a manipulator, it should be sufficient to bring the victim to the surface if a strong hold can be maintained to the body.
• The average temperature crossover point for a body to either remain negatively buoyant or become positively buoyant is approximately 52F (12C). Check the water temperature to determine the likelihood that the victim is still near the last seen point.
