Editing Hydrographic survey (section)

===Multibeam Echosounders===
{{main|Multibeam echosounder}}
{{Duplication|dupe=Multibeam echosounder|section=yes|date=January 2023}}

A multibeam echosounder (MBES) is a type of [[sonar]] that is used to [[seafloor mapping|map the seabed]]. It emits [[acoustic wave]]s in a fan shape beneath its [[transceiver]]. The [[time of flight|time]] it takes for the sound waves to reflect off the seabed and return to the receiver is used to calculate the water depth. Unlike other sonars and [[echo sounder]]s, MBES uses [[beamforming]] to extract directional information from the returning soundwaves, producing a swath of [[depth sounding]]s from a single ping.

Explicit inclusion of phraseology like: ''"For all MBES surveys for LINZ, high resolution, geo-referenced backscatter intensity is to be logged and rendered as a survey deliverable."''<ref>New Zealand Hydrographic Authority, (2016), Ver. 1.3 of Contract Specifications for Hydrographic Surveys, Land Information New Zealand</ref> in a set of contract survey requirements, is a clear indication that the wider hydrographic community is embracing the benefits that can be accrued by employing MBES technology and, in particular, are accepting as a fact that a MBES which provides acoustic backscatter data is a valuable tool of the trade.{{citation needed|date=January 2023}}{{clarify|How is this a clear indication?|date=January 2023}}

The introduction of multispectral multibeam echosounders<ref>Costa, B., (2019), Multispectral acoustic backscatter: How useful is it for marine habitat mapping and management?, ''Journal of Coastal Research'', 35(5), pp 1062-1079</ref> continues the trajectory of technological innovations providing the hydrographic surveying community with better tools for more rapidly acquiring better data for multiple uses. 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).<ref>Owaki, N., (1963), A note on depth when the bottom is soft mud, ''International Hydrographic Review'', XL, No. 2, pp 41-43</ref> It was observed that higher frequency single vertical beam echosounders could provide detectable echo amplitudes from high porosity sediments, even if those sediments appeared to be acoustically transparent at lower frequencies.

In the late 1960s, single-beam hydrographic surveys were conducted using widely spaced track lines and the shallow (peak) soundings in the bottom data were retained in preference to deeper soundings in the sounding record.  During that same time period, early side scan sonar was introduced into the operational practices of shallow water hydrographic surveying. The frequencies of the early side scan sonars were a matter of engineering design expediency and the most important aspect of the side scanning echoes was not the value of their amplitudes, but rather that the amplitudes were spatially variable. 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)<ref>Fish, J. P., & Carr, H., A., (1990), ''Sound underwater images: A guide to the generation and interpretation of side scan sonar dat.'' Orleans, MA: Lower Cape Pub.</ref>  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.<ref>Huff, L. C. (1981), A Study of Future Depth Recorder Requirements, International Hydrographic Review, LVIII (2)</ref>  The outcome of the study was a class of vertical-beam depth sounders, which is still widely used.  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.{{clarify|How does this help? what is a vertical grazing angle?|date=January 2023}}

The first MBES generation was dedicated to [[seafloor mapping|mapping the seafloor]] in deep water. Those pioneering MBES made little, or no, explicit use of the amplitudes, as their objective was to obtain accurate measurements of the bathymetry (representing both the peaks and deeps).  Furthermore, their technical characteristics did not make it easy to observe spatial variations in the echo amplitudes.<ref>Lurton, X., (2010), An Introduction into Underwater Acoustics: Principles and Applications, 2nd ed, {{ISBN|978-3540784807}}, Springer</ref> Subsequent to the early MBES bathymetric surveys and at the time when single frequency side scan sonar had begun to produce high quality images of the seabed that were capable of providing a degree of discrimination between different types of sediments, the potential of the echo amplitudes from a MBES was recognized.<ref>deMoustier, C., (1986), Beyond bathymetry: Mapping acoustic backscattering from the deep seafloor with Sea Beam, JASA Vol 79, pp 316-331</ref>

With Marty Klein's introduction of dual frequency (nominally 100&nbsp;kHz and 500&nbsp;kHz) side scan sonar, it was apparent that spatially and temporally coincident backscatter from any given seabed at those two widely separated acoustic frequencies, would likely provide two separate and unique images of that seascape. 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.  However, the overlapping sets of side scanning across-track grazing angles at the two frequencies were always the same.{{clarify|explain or link jargon. what is a backscatter grazing angle and why does it matter?|date=January 2023}}

Following the grounding of the {{ship||Queen Elizabeth 2||2}} off [[Cape Cod]], [[Massachusetts]], in 1992,<ref>{{Cite web |last1=Lusk |first1=Barry |date=May 12, 2009 |url=https://www.hydro-international.com/content/article/grounding-of-the-queen-elizabeth |title=Grounding of the Queen Elizabeth 2 |publisher=Hydro International}}</ref> the emphasis for shallow water surveying migrated toward full bottom coverage surveys by employing MBES with increasing operating frequencies to further improve the spatial resolution of the soundings.  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.  Images acquired under the initial attempts at MBES bottom imaging were less than stellar, but fortunately improvements were forthcoming.

Side scan sonar parses the continual echo returns from a receive beam that is perfectly aligned with the insonification beam using time-after-transmit, a technique that is independent of water depth and the cross-track beam opening angle of the sonar receive transducer. The initial attempt at multibeam imagery employed multiple receive beams, which only partially overlapped the MBES fan-shaped insonification beam, to segment the continual echo returns into intervals that were dependent on water depth and receiver cross-track beam opening angle. Consequently, the segmented intervals were non-uniform in both their length of time and time-after-transmit. The backscatter from each ping in each of the beam-parsed segments was reduced to a single value and assigned to the same geographical coordinates as those assigned to that beam's measured sounding. In subsequent modifications to MBES bottom imaging, the echo sequence in each of the beam-parsed intervals was designated as a snippet.<ref>Lockhart, D., Saade, E., and Wilson, J., (2001) New Developments in Multibeam Backscatter Data Collection and Processing, Marine Technology Society Journal Vol. 35, pp 46-50.</ref> On each ping, each snippet from each beam was additionally parsed according to time-after-transmit.  Each of the echo amplitude measurements made within a snippet from a particular beam was assigned a geographical position based on linear interpolation between positions assigned to the soundings measured, on that ping, in the two adjacent cross-track beams. The snippet modification to MBES imagery significantly improved the quality of the imagery by increasing the number of echo amplitude measurements available to be rendered as a pixel in the image and also by having a more uniform spatial distribution of the pixels in the image which represented an actual measured echo amplitude.

The introduction of multispectral multibeam echosounders{{clarify|what makes a MBES multispectral?|date=January 2023}}<ref>Brown, C. et al., (2018), Multispectral Multibeam Echo Sounder Backscatter as a Tool for Improved Seafloor Characterization, ''Geosciences'' 8, 455</ref> continued the progressive advances in hydrography.  In particular, multispectral multibeam echosounders not only provide "multiple look" depth measurements of a seabed, they also provide multispectral backscatter data that are spatially and temporally coincident with those depth measurements. A multispectral multibeam echosounder directly computes a position of origin for each of the backscatter amplitudes in the output data set.  Those positions are based on the backscatter measurements themselves and not by interpolation from some other derived data set.  Consequently, multispectral multibeam imagery is more acute compared to previous multibeam 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. Multispectral multibeam echosounders reinforces the fact that spatially and temporally coincident backscatter, from any given seabed, at widely separated acoustic frequencies provides separate and unique images of the seascape.<ref>Gaida, T, C., et al., (2019) Mapping the Seabed and Shallow Subsurface with Multi-Frequency Multibeam Echosounders, ''Remote Sens.'' 12, 52</ref>