Hydrographic survey
The IHO publishes Standards and Specifications[2] followed by its Member States as well as Memoranda of Understanding and Co-operative Agreements[3] with hydrographic survey interests.
Such surveys commonly are conducted by national organizations or under their supervision or the standards they have approved, particularly when the use is for the purposes of chart making and distribution or the dredging of state-controlled waters.
[7] Commercial entities also conduct large-scale hydrographic and geophysical surveying, particularly in the dredging, marine construction, oil exploration, and drilling industries.
[12] For many centuries, a hydrographic survey required the use of lead lines – ropes or lines with depth markings attached to lead weights to make one end sink to the bottom when lowered over the side of a ship or boat – and sounding poles, which were poles with depth markings which could be thrust over the side until they touched bottom.
[12] In 1904, wire-drag surveys were introduced into hydrography, and the United States Coast and Geodetic Survey′s Nicholas H. Heck played a prominent role in developing and perfecting the technique between 1906 and 1916.
Prior to the advent of sidescan sonar, wire-drag surveying was the only method for searching large areas for obstructions and lost vessels and aircraft.
[14] Between 1906 and 1916, Heck expanded the capability of wire-drag systems from a relatively limited area to sweeps covering channels 2 to 3 nautical miles (3.7 to 5.6 km; 2.3 to 3.5 mi) in width.
Sidescan sonar could create images of underwater obstructions with the same fidelity as aerial photography, while multibeam systems could generate depth data for 100 percent of the bottom in a surveyed area.
[12][15] Vessels were freed from working together on wire-drag surveys, and in the U.S. National Oceanic and Atmospheric Administration (NOAA), for example, Rude and Heck operated independently in their later years.
Unlike other sonars and echo sounders, MBES uses beamforming to extract directional information from the returning soundwaves, producing a swath of depth soundings from a single ping.
A multispectral multibeam echosounder is the culmination of many progressive advances in hydrography from the early days of acoustic soundings when the primary concern about the strength of returning echoes from the bottom was whether, or not, they would be sufficiently large to be noted (detected).
The operating frequencies of the early acoustic sounders were primarily based on the ability of magneostrictive and piezoelectric materials whose physical dimensions could be modified by means of electrical current or voltage.
Eventually it became apparent, that while the operating frequency of the early single vertical beam acoustic sounders had little, or no, bearing on the measured depths when the bottom was hard (composed primarily of sand, pebbles, cobbles, boulders, or rock), there was a noticeable frequency dependency of the measured depths when the bottom was soft (composed primarily of silt, mud or flocculent suspensions).
During that same time period, early side scan sonar was introduced into the operational practices of shallow water hydrographic surveying.
In fact, important information was deduced about the shape of the bottom and manmade items on the bottom, based on the regions where there were absences of detectable echo amplitudes (shadows)[23] In 1979, in hopes of a technological solution to the problems of surveying in "floating mud", the Director of the National Ocean Survey (NOS) established a NOS study team to conduct investigations to determine the functional specifications for a replacement shallow water depth sounder.
It simultaneously pinged at two acoustic frequencies, separated by more than 2 octaves, making depth and echo-amplitude measurements that were concurrent, both spatially and temporally, albeit at a single vertical grazing angle.
Admittedly, the along-track insonification and receiving beam patterns were different, and due to the absence of bathymetric data, the precise backscatter grazing angles were unknown.
Given that side scan sonar, with its across-track fan-shaped swath of insonification, had successfully exploited the cross-track variation in echo amplitudes, to achieve high quality images of the seabed, it seemed a natural progression that the fan-shaped across-track pattern of insonification associated with the new monotone higher frequency shallow water MBES, might also be exploited for seabed imagery.
The inherent precision of the bathymetric data from a multispectral multibeam echosounder is also a benefit to those users that may be attempting to employ the acoustic backscatter angular response function to discriminate between different sediment types.
Apart from obvious cost savings, this also gives a continuous survey of an area, but the drawbacks are time in recruiting observers and getting a high enough density and quality of data.
[citation needed] Although the accuracy of crowd-sourced surveying can rarely reach the standards of traditional methods, the algorithms used rely on a high data density to produce final results that are more accurate than single measurements.
