Multibeam echosounder

A multibeam echosounder is a type of sonar that is used to map the seabed. Like other sonar systems, multibeam systems emit acoustic waves in a fan shape beneath the transceiver of the multibeam echosounder. The length of 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, 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, as well as on small boats (as technology improved, multibeam echosounders became more compact and lighter, 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 which provided higher resolution mapping in shallow water were developed, and today 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. More recently, multibeam echsounders are also used in the renewable energy sector such as offshore windfarms.

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 multibeam acoustic frequencies have increased and the cost of components has decreased, the worldwide number of multibeam swath systems in operation has increased significantly. The required physical size of an acoustic transducer used to develop multiple high-resolution beams, decreases as the multibeam acoustic frequency increases. Consequently, increases in the operating frequencies of multibeam sonars have resulted in significant decreases in their weight, size and volume characteristics. The older and larger, lower-frequency multibeam sonar systems, that required considerable time and effort mounting them onto a ship's hull, employed conventional tonpliz-type transducer elements, which provided a usable bandwidth of approximately 1/3 octave. The newer and smaller, higher-frequency multibeam sonar systems can easily be attached to a survey launch or to a tender vessel. Shallow water multibeam echosounders, like those from Teledyne Odom, R2Sonic and Norbit, which can incorporate sensors for measuring transducer motion and sound speed local to the transducer, are allowing many smaller hydrographic survey companies to move from traditional single beam echosounders to multibeam echosounders. Small low-power multibeam swath systems are also now suitable for mounting on an Autonomous Underwater Vehicle (AUV) and on an Autonomous Surface Vessel (ASV).

Multibeam echosounder data may include bathymetry, acoustic backscatter, and water column data. (Gas plumes now commonly identified in midwater multibeam data are termed flares.)

Type 1-3 piezo-composite transducer elements, [4] are being employed in a multispectral multibeam echosounder to provide a usable bandwidth that is in excess of 3 octaves. Consequently, multispectral multibeam echosounder surveys are possible with a single sonar system, which during every ping cycle, collects multispectral swathe bathymetry data, multispectral backscatter data, and multispectral water column data. [5]

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.

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
  4. Uchino, K., (Editor), (2016), Advanced Piezoelectic materials: Science and Technology, 2nd Edition, ISBN-13 9780081014851
  5. Brown, C. J., Brissette, M., and Gazzola, V., (2019), Multispectral multibeam echo sounder backscatter as a tool for improved seafloor characterization., Geosciences, 9(3).
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