Editing Echo sounding (section)

== Hydrography==
In areas where detailed [[bathymetry]] is required, a precise echo sounder may be used for the work of hydrography.  There are many considerations when evaluating such a system, not limited to the vertical accuracy, resolution, acoustic beamwidth of the transmit/receive beam and the acoustic [[frequency]] of the [[transducer]]. 
 
[[File:Odom Mk3 Echosounder.jpg|thumb|An example of a precision dual frequency echosounder, the [[Teledyne Technologies|Teledyne]] Odom MkIII]]

The majority of hydrographic echosounders are dual frequency, meaning that a low frequency pulse (typically around 24&nbsp;kHz) can be transmitted at the same time as a high frequency pulse (typically around 200&nbsp;kHz). As the two frequencies are discrete,{{clarify|date=September 2024}} the two return signals do not typically interfere with each other. Dual frequency echosounding has many advantages, including the ability to identify a vegetation layer or a layer of soft mud on top of a layer of rock.

[[File:DF SBES Wiki.jpg|thumb|A screen grab of the difference between single and dual frequency echograms]]

Most hydrographic operations use a 200&nbsp;kHz transducer, which is suitable for inshore work up to 100 metres in depth.  Deeper water requires a lower frequency transducer as the acoustic signal of lower frequencies is less susceptible to attenuation in the water column.  Commonly used frequencies for deep water sounding are 33&nbsp;kHz and 24&nbsp;kHz.

The beamwidth of the transducer is also a consideration for the hydrographer, as to obtain the best [[Sensor resolution|resolution]] of the data gathered a narrow beamwidth is preferable.  The higher the operating frequency, the narrower the beamwidth.  Therefore, it is especially important when sounding in deep water, as the resulting footprint of the acoustic pulse can be very large once it reaches a distant sea floor.

A multispectral multibeam echosounder is an extension of a dual frequency vertical beam echosounder in that, as well as measuring two soundings directly below the sonar at two different frequencies; it measures multiple soundings at multiple frequencies, at multiple different grazing angles, and multiple different locations on the seabed. These systems are detailed further in the section called [[multibeam echosounder]].

Echo sounders are used in laboratory applications to monitor sediment transport, scour and erosion processes in scale models (hydraulic models, flumes etc.). These can also be used to create plots of 3D contours.

===Standards for hydrographic echo sounding===

The required precision and accuracy of the hydrographic echo sounder is defined by the requirements of the [[International Hydrographic Organization]] (IHO) for surveys that are to be undertaken to IHO standards.<ref>{{cite journal|last=International Hydrographic Bureau|title=IHO Standards for Hydrographic Surveys|date=February 2008|issue=5th Edition|url=http://www.iho-ohi.net/iho_pubs/standard/S-44_5E.pdf|url-status=dead|archive-url=https://web.archive.org/web/20111008191705/http://www.iho-ohi.net/iho_pubs/standard/S-44_5E.pdf|archive-date=8 October 2011|df=dmy-all}}</ref>  These values are contained within IHO publication S44.

In order to meet these standards, the surveyor must consider not only the vertical and horizontal accuracy of the echo sounder and transducer, but the survey system as a whole.  A motion sensor may be used, specifically the heave component (in single beam echosounding) to reduce soundings for the motion of the vessel experienced on the water's surface.  Once all of the uncertainties of each sensor are established, the hydrographer will create an [[uncertainty budget]] to determine whether the survey system meets the requirements laid down by IHO.

Different hydrographic organisations will have their own set of field procedures and manuals to guide their surveyors to meet the required standards.  Two examples are the US Army Corps of Engineers publication EM110-2-1003,<ref name="USACE Hydrographic Manual">{{cite web |url=http://140.194.76.129/publications/eng-manuals/em1110-2-1003/toc.htm |title=EM 1110-2-1003 (01 Jan 02) |access-date=2011-06-09 |url-status=dead |archive-url=https://web.archive.org/web/20110720182750/http://140.194.76.129/publications/eng-manuals/em1110-2-1003/toc.htm |archive-date=20 July 2011 |df=dmy-all }}, USACE publication EM 1110-2-1003.</ref> and the NOAA 'Field Procedures Manual'.<ref name="NOAA Field Procedures Manual">[http://www.nauticalcharts.noaa.gov/hsd/docs/Field_Procedures_Manual_May_2011.pdf] {{Webarchive|url=https://web.archive.org/web/20110516143244/http://www.nauticalcharts.noaa.gov/hsd/docs/Field_Procedures_Manual_May_2011.pdf|date=16 May 2011}}, NOAA Field Procedures Manual.</ref>