Echo sounding

Not to be confused withechography.

Illustration of echo sounding using amultibeam echosounder.

The MTVZA sounder received from the Meteor M2-2 satellite by an amateur station

Echo soundingordepth soundingis the use ofsonarforranging,normally to determine thedepthof water (bathymetry). It involves transmitting acoustic waves into water and recording the time interval between emission and return of a pulse; the resultingtime of flight,along with knowledge of thespeed of soundin water, allows determining the distance between sonar and target. This information is then typically used for navigation purposes or in order to obtain depths forchartingpurposes.

Echo sounding can also be used for ranging to other targets, such asfish schools.Hydroacoustic assessments have traditionally employed mobile surveys from boats to evaluate fish biomass and spatial distributions. Conversely, fixed-location techniques use stationary transducers to monitor passing fish.

The wordsoundingis used for all types of depth measurements, including those that don't usesound,and is unrelated in origin to the wordsoundin the sense of noise or tones. Echo sounding is a more rapid method of measuring depth than the previous technique of lowering asounding lineuntil it touched bottom.

History

German inventorAlexander Behmwas granted GermanpatentNo. 282009 for the invention of echo sounding(device for measuring depths of the sea and distances and headings of ships or obstacles by means of reflected sound waves)on 22 July 1913. Meanwhile, in France, physicistPaul Langevin(connected withMarie Curie) and better known for his research work innuclear physicswas recruited byfrench navylaboratories at the beginning of WW 2 and conducted (then secret) research on active sonars for anti-submarine warfare (using apiezoelectric transmitter). His work was developed and implemented by other scientists and technnicians such as Chilowski, Florisson and Pierre Marti. Though a fully operational échosondeur (sonar) was not ready for use in wartime, successful trials both off Toulon and in the Manche (Channel)took place as early as 1920 and french patents taken for civilian uses. Oceanographic ships and french High-sea fishing assistance vessels were equipped with Langevin-Florisson and Langevin Marti recording sonars as early as the mid/late 20s

One of the first commercial echo sounding units was the Fessenden Fathometer, which used theFessenden oscillatorto generate sound waves. This was first installed by the Submarine Signal Company in 1924 on the M&M liner S.S. Berkshire.

Technique

Further information:Sound speed profile

Diagram showing the basic principle of echo sounding

Distance is measured by multiplying half the time from the signal's outgoing pulse to its return by thespeed of soundin thewater,which is approximately 1.5 kilometres per second [T÷2×(4700 feet per second or 1.5 km per second )] For precise applications of echosounding, such ashydrography,the speed of sound must also be measured typically by deploying asound velocity probeinto the water. Echo sounding is effectively a special purpose application ofsonarused to locate the bottom. Since a traditional pre-SIunit of water depth was thefathom,an instrument used for determining water depth is sometimes called afathometer.

Most chartedoceandepths use an average or standard sound speed. Where greater accuracy is required average and even seasonal standards may be applied to ocean regions. For high accuracy depths, usually restricted to special purpose or scientific surveys, a sensor may be lowered to measure the temperature, pressure and salinity. These factors are used to calculate the actualsound speedin the local water column. This latter technique is regularly used by US Office of Coast Survey for navigational surveys of US coastal waters.

Types

Single beam

beam shape of a single-beam echosounder on aUSV

A single-beam echo sounder is one of the simplest and most fundamental types of underwater sonar. They are ubiquitous in the boating world and used on a number of different marine robotic vehicles. It operates by using a transducer to emit a pulse through the water and listen for echos to return. Using that data, it's able to determine the distance from the strongest echo, which can be the seafloor, a concrete structure, or other larger obstacle.Afishfinderis an echo sounding device used by both recreational and commercial fishers.

Multibeam

This section is an excerpt fromMultibeam echosounder.[edit]

Multibeam sonar is used to map the ocean floor

Amultibeam echosounder(MBES) is a type ofsonarthat is used tomap the seabed.It emitsacoustic wavesin a fan shape beneath itstransceiver.Thetimeit 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 andecho sounders,MBES usesbeamformingto extract directional information from the returning soundwaves, producing aswatheofdepth soundingsfrom a single ping.

Common use

As well as an aid to navigation (most larger vessels will have at least a simple depth sounder), echo sounding is commonly used forfishing.Variations in elevation often represent places where fish congregate. Schools of fish will also register.

Hydrography

In areas where detailedbathymetryis 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 acousticfrequencyof thetransducer.

An example of a precision dual frequency echosounder, theTeledyneOdom MkIII

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

A screen grab of the difference between single and dual frequency echograms

Most hydrographic operations use a 200 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 kHz and 24 kHz.

The beamwidth of the transducer is also a consideration for the hydrographer, as to obtain the bestresolutionof 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 calledmultibeam 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 theInternational Hydrographic Organization(IHO) for surveys that are to be undertaken to IHO standards.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 anuncertainty budgetto 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,and the NOAA 'Field Procedures Manual'.

See also

• Acoustical oceanography
• Alexander Behm– inventor
• AUV
• Bathymeter
• Depth gauge
• Fessenden oscillator
• Fisheries acoustics
• Hydroacoustics
• Hydrographic survey
• Sonar
• Depth sounding
• Underwater acoustics

External links

Media related toEcho soundingat Wikimedia Commons

• "How Echoes Tell Depth of Water Under Ship"Popular Mechanics Monthly,July 1930– drawing of details of early depth finders using echoes
• ELAC (1982)An Introduction to Echosounding.Honeywell-ELAC-Nautik GmbH, Kiel, 88 pp,(pdf 27.5 MB)