Page 584 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

• Streams: These normally possess easy access with clear visibility and shallow depths (but possibly higher currents). Streams are normally searched visually or with shallow water, snorkeling divers.
• Lakes: The lake environment is probably the easiest underwater search environment due to its usual lack of currents, generally good visibility, and relative lack of concentrated surface traffic.
• Estuaries and littoral waters: In general, the closer the body of water is to mountainous terrain, the higher the water flow and the higher the visibility of the water (with some exceptions). In areas of generally flat terrain with field run-off, expect low visibility and difficult search conditions. The challenge of estuaries and littoral areas is the sheer size of the search area. Covering a large search area without high-speed, high-coverage search sensors (such as a towed side scan sonar or magnetometer) will, in all likelihood, be time and/or cost prohibitive.
Bottom conditions affecting searches
• Sea grass: Sea grass and other bottom growth near the surface is fed by photosynthesis and suspended in the water column via air-filled pockets within the structure of the leaves.
As explained in Chapter 15, air is highly reflective to sonar and will cause both a sonar echo (possibly displaying a false target) and a sonar shadow on the backside of the echo. If a significant presence of sea growth is encountered in the search area, prepare for a long and difficult search experience with plenty of false targets and fouled search equipment.
• Rocky bottom: This type of bottom condition generally reflects sonar signals well, causing a noisy environment for searching and numerous false targets. In order to maintain a consistent sonar display, the gain on the system must be adjusted to a lower setting. However, this sometimes masks the sonar target along with the clutter. The only true straight lines in the environment are man-made, which does help in target discrimination. Rocky bottoms are also difficult for tether management of the ROV since tethers tend to snag around rocky outcroppings and get trapped in overhangs.
• Muddy bottom: Muddy bottoms permit easy sonar searches due to the absorption of the sonar signals, allowing a higher gain setting. Anything with a higher consistency than mud appears quickly on the sonar display. ROV systems attempting to navigate near muddy bottoms are often plagued with ineffective cameras due to low visibility. Also, when items fall into a muddy bottom condition, they are often enveloped within the mud, causing impossible visual clues and rendering them invisible to high-frequency sonar systems.
• Sandy bottom: Of the bottom conditions for searching, the sandy bottom is probably the easiest environment for locating and discriminating targets, both visually and with sonar. The surface tension of the sandy bottom is such that items will not normally bury in a short period of time, nor will the sonar reflectivity produce the false target data present in other less favorable conditions.
21.3.4.2 Wide area search
A wide area search is conducted with equipment and sensors mounted on or towed from a rela- tively high-speed platform that can cover a wide search area in a short period of time. Examples of wide area search systems include light detection and ranging (LIDAR) or MAD equipment mounted aboard an airborne platform; towed or boat-mounted side scan sonar; towed video camera and bathymetry equipment.
21.3 Public safety diving 583