The stern of the decommissioned aircraft carrier USS Forrestal at Naval Station Newport, RI. The mooring lines and chains for the long-term, heavy weather mooring configuration, and the hull itself, prevent survey of the bottom underneath the footprint of the carrier by a vessel.
paired with bathymetric data via automated software applications to generate underwater topographic models. It is relatively safe to say that most hydrographic surveys, in open water, rely upon a GPS receiving system, a downward-aimed hydrographic survey echosounder (either single-beam or multi-beam), and survey software packages (such as Hypack or WinFrog or QINSy). Additional pieces of equipment, such as tide gauges, sound velocity profilers, and motion reference units may round out the survey spread in order to measure tidal water elevations relative to a project vertical datum, account for the velocity of sound to get accurate distance measurements between the echosounder and the bottom, and correct soundings for motions related to vessel heave, heading, pitch, and roll. The aim of these surveys is to develop underwater topographic models that consist of accurate horizontal positions paired with accurate vertical bottom measurements. On the occasions when a hydrographic survey must be performed in a restricted-access situation, it may be possible to utilize swath multibeam echosounders or profiling sonars from surface survey vessels to collect data under the
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structures. A portion of the footprint under a pier or relieving platform or moored vessel may be "illuminated" by the swath of the multibeam transducer or the profiling sonar when the sonars are configured in their typical downward orientation. Measuring the bottom elevation near a pier with a swath multibeam bathymetric system. It is possible to increase the size of the footprint that is covered in this survey approach by rotating the multibeam or profiling transducer off-axis. So long as the angle of rotation is accurately measured, survey software can be configured to accommodate the axial rotation and the result can be a very dense, accurate bathymetric data set in a restricted-access environment that references an accurate horizontal and vertical datum. By tilting the echosounder, it may be possible to expand the survey footprint under the pier. Unfortunately the size of the footprint available for survey may be limited by the inability to "see" the footprint with the survey transducer. There is the flexibility to rotate the transducer so that it directs more of its swath towards the area beneath a structure. However, no
Marine Technology Reporter 23
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