Multibeam echosounder

A multibeam echosounder is a type of sonar that is used to map the seabed. Like other sonar systems, multibeam systems emit sound waves in a fan shape beneath a ship's hull. The amount of time it takes for the sound waves to bounce off the seabed and return to a receiver is used to determine water depth. Unlike other sonars, multibeam systems use beamforming to extract directional information from the returning soundwaves, producing a swath of depth readings from a single ping.

Multibeam sonar is used to map the ocean floor

History and progression
A multibeam image of the USS Susan B. Anthony (AP-72) shipwreck off the coast of France.

Multibeam sonar sounding systems, also known as swathe (British English) or swath (American English), originated for military applications. The Sonar Array Sounding System (SASS) was developed in the early 1960s by the US Navy, in conjunction with General Instrument to map large swaths of the ocean floor to assist the underwater navigation of its submarine force.[1][2] SASS was tested aboard the USS Compass Island (AG-153). The final array system, composed of sixty-one one degree beams with a swath width of approximately 1.15 times water depth, was then installed on the USNS Bowditch (T-AGS-21), USNS Dutton (T-AGS-22) and USNS Michelson (T-AGS-23).[1]

Starting in the 1970s, companies such as General Instrument (now SeaBeam Instruments, part of L3 Klein) in the United States, Krupp Atlas (now Atlas Hydrographic) and Elac Nautik (now part of the Wärtsilä Corporation) in Germany, Simrad (now Kongsberg Maritime) in Norway and RESON now Teledyne RESON A/S in Denmark developed systems that could be mounted to the hull of large ships, and then small boats (as technologies improved and operating frequencies increased).

The first commercial multibeam is now known as the SeaBeam Classic and was put in service in May 1977[3] on the Australian survey vessel HMAS Cook. This system produced up to 16 beams across a 45-degree arc. The (retronym) term "SeaBeam Classic" was coined after the manufacturer developed newer systems such as the SeaBeam 2000 and the SeaBeam 2112 in the late 1980s.

The second SeaBeam Classic installation was on the French Research Vessel Jean Charcot. The SB Classic arrays on the Charcot were damaged in a grounding and the SeaBeam was replaced with an EM120 in 1991. Although it seems that the original SeaBeam Classic installation was not used much, the others were widely used, and subsequent installations were made on many vessels.

SeaBeam Classic systems were subsequently installed on the US academic research vessels USNS Thomas Washington (T-AGOR-10) (Scripps Institution of Oceanography, University of California), the USNS Robert D. Conrad (Lamont-Doherty Earth Observatory of Columbia University) and the RV Atlantis II (Woods Hole Oceanographic Institution).

As technology improved in the 1980s and 1990s, higher-frequency systems suitable for high-resolution mapping in shallow water were developed, and such systems are widely used for shallow-water hydrographic surveying in support of navigational charting. Multibeam echosounders are also commonly used for geological and oceanographic research, and since the 1990s for offshore oil and gas exploration and seafloor cable routing.

In 1989, Atlas Electronics (Bremen, Germany) installed a second-generation deep-sea multibeam called Hydrosweep DS on the German research vessel Meteor. The Hydrosweep DS (HS-DS) produced up to 59 beams across a 90-degree swath, which was a vast improvement and was inherently ice-strengthened. Early HS-DS systems were installed on the RV Meteor (1986) (Germany), the RV Polarstern (Germany), the RV Maurice Ewing (US) and the ORV Sagar Kanya (India) in 1989 and 1990 and subsequently on a number of other vessels including the RV Thomas G. Thompson (US) and RV Hakurei Maru (Japan).

As the cost of components has decreased, the number of multibeam systems sold and in operation worldwide has increased significantly. Smaller, portable systems can be operated on a small launch or tender vessel unlike the older systems that required considerable time and effort to attach to a ship's hull. Some multibeam echosounders such as the Teledyne Odom MB2 also incorporate a motion sensor at the face of the acoustic transducer, allowing even faster installation on small vessels. Multibeam echosounders like this are allowing many smaller hydrographic survey companies to move from traditional single beam echosounders to swath systems.

Multibeam data includes bathymetry, acoustic backscatter, and water column data. Gas plumes now commonly identified in midwater multibeam data are termed flares.

A multibeam echosounder showing the transmit array (larger black rectangle) and receive array (narrower rectangle) - Odom MB1

Theory of operation

A multibeam echosounder is a device typically used by hydrographic surveyors to determine the depth of water and the nature of the seabed. Most modern systems work by transmitting a broad acoustic fan shaped pulse from a specially designed transducer across the full swath acrosstrack with a narrow alongtrack then forming multiple receive beams (beamforming) that are much narrower in the acrosstrack (around 1 degree depending on the system). From this narrow beam, a two way travel time of the acoustic pulse is then established utilizing a bottom detection algorithm. If the speed of sound in water is known for the full water column profile, the depth and position of the return signal can be determined from the receive angle and the two-way travel time.

In order to determine the transmit and receive angle of each beam, a multibeam echosounder requires accurate measurement of the motion of the sonar relative to a cartesian coordinate system. The measured values are typically heave, pitch, roll, yaw, and heading.

To compensate for signal loss due to spreading and absorption a time-varied gain circuit is designed into the receiver.

For deep water systems, a steerable transmit beam is required to compensate for pitch. This can also be accomplished with beamforming.

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References
  1. Albert E. Theberge Jr. and Norman Z. Cherkis (22 May 2013). "A Note on Fifty Years of Multi-beam". Hydro International. Archived from the original on 14 July 2014. Retrieved 30 June 2014.
  2. U.S. Naval Research Laboratory/Marine Physics Branch (Code 7420). "GOMaP GLOBAL OCEAN MAPPING PROJECT". U.S. Naval Research Laboratory. Archived from the original on 2 July 2014. Retrieved 30 June 2014.
  3. Harold Farr, Marine Geodesy, Volume 4, Issue 2 1980, pages 77 – 93
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