Underwater acoustic observation

Several acoustic techniques are widely used to observe the underwater environment. This article introduces four examples: singlebeam and multibeam echo sounding, side-scan sonar, and Ocean Acoustic Waveguide Remote Sensing (OAWRS). These techniques illustrate different applications, ranging from water-depth and seabed mapping to observation of fish aggregations over large areas.

Introduction

Figure 1: Singlebeam. The transducer emits sound (blue cone), which is reflected (white rings) and partly received again.

Sound generally propagates much farther through seawater than electromagnetic radiation such as visible light or radar, especially at low frequencies. Sound generally propagates much farther through seawater than electromagnetic radiation such as visible light or radar, especially at low frequencies, making acoustics the principal means of underwater remote sensing beyond the optical range. Instruments that use sound to identify objects in the water column and to determine depth are called SONARs (SOund NAvigation and Ranging).

There are two types of SONAR: passive and active. Passive SONAR systems listen to sounds produced by animals (e.g. whales) or objects (e.g. ships and submarines). Active SONAR systems generate specific sound waves themselves and then analyze their reflection (echo). These “echo sounders” have more diverse and complex applications and are therefore discussed further here.

The four acoustic underwater observation techniques discussed here are the singlebeam and multibeam echosounders, the side-scan sonar, and OAWRS (Ocean Acoustic Waveguide Remote Sensing). Active sonar systems use acoustic transducers to transmit and receive sound. A transmitting transducer is often called a projector and a receiving transducer a hydrophone; the same transducer may perform both functions, although many systems use separate transmit and receive arrays.

Travel time is used to determine range or depth, whereas the strength and character of the returned signal provide information about the acoustic properties and structure of the target. The characteristics of SONAR systems are partly determined by the transmitted frequency. Lower sound frequencies undergo less absorption and therefore travel farther than higher frequencies. Although lower frequencies allow a larger area to be monitored, this is usually accompanied by a loss of spatial resolution. Standards and best practices for underwater acoustic observation are described in the Standard Ocean Mapping Protocol of the US National Ocean Mapping, Exploration, and Characterization Council.

The simplest technique is the singlebeam echosounder, developed at the beginning of the 20th century. The development of this and subsequent SONAR systems is largely due to military research, as they are ideal tools for detecting submarines and mines. In the scientific field, they are used, among other things, for producing bathymetric maps of seabed topography and for studying fish populations and dynamics.

Fish can be detected because of their reflective properties. For many fish species with a gas-filled swim bladder, the swim bladder dominates the acoustic target strength and can contribute about 90% or more of the total backscattered sound. Some species, such as Atlantic mackerel, do not have a swim bladder but still reflect sound through their bones and muscle mass. Fish without a swim bladder therefore produce weaker echoes. Signal strength also depends on fish size, orientation, depth and acoustic frequency, so acoustic intensity cannot simply be converted into biomass without calibration and biological sampling.

Singlebeam echosounder

Until the beginning of the 1960s, singlebeam echosounders (Figure 1) were mainly used for depth measurements. This system uses a single vertically directed acoustic pulse (“ping”), comparable to a searchlight. The transducer receives part of the echo, from which the depth is calculated based on the travel time of the pulse. For a vertically directed singlebeam echo sounder, the depth [math]h[/math] (distance to the bottom) is [math]h \approx \frac{1}{2} c \Delta t[/math], where [math]c[/math] is sound speed and [math]\Delta t[/math] is the two-way travel time.

Singlebeam transducers are commonly hull-mounted, pole-mounted, centreboard-mounted or otherwise attached to the survey platform. The survey vessel follows a series of survey lines, while the downward-looking transducer records depths beneath the vessel. Because singlebeam measurements sample only a narrow corridor along each line, seabed features between survey lines can remain undetected.

The beam can be wide (“wide beams”), allowing a relatively larger surface area to be covered, but at the expense of spatial resolution. More expensive “narrow beams” have a smaller seabed footprint and therefore provide higher spatial resolution. Singlebeam surveys provide measurements only along the vessel tracks and therefore require closely spaced survey lines to achieve detailed areal coverage. The method has limitations in both time and space, meaning that only a partial picture can be obtained of, for example, fish dynamics and abundance.

Multibeam echosounder

Figure 2: Multibeam. The transducer emits sound in several adjacent individual beams.

Around 1960, the multibeam echosounder (Figure 2) was developed. A multibeam echosounder uses transducer arrays and electronic beamforming to obtain many simultaneous depth measurements across a fan-shaped swath perpendicular to the vessel track. Each ping produces many depth soundings across a swath perpendicular to the vessel track; successive pings provide nearly continuous seabed coverage The width of this line on the seabed is called the swath width, which can be expressed either in meters or as the angular width in degrees.

Accurate multibeam bathymetry requires simultaneous measurement of vessel position and motion and correction for acoustic refraction using the water-column sound-speed profile.

The system measures the travel time and strength of the returned signal, providing bathymetric information and, after appropriate corrections, seabed backscatter information. Using computer processing, the seabed topography can then be visualized. Depth is usually represented using color codes, where blue areas are deeper than red areas. The intensity of the reflected signal provides information about the hardness, texture, and morphology of the seabed. Backscatter strength depends on acoustic impedance, grain size, roughness, seabed slope, incidence angle and acoustic frequency. Hard or coarse substrates often yield stronger backscatter than fine soft sediment, but the relationship is not unique; interpretation therefore requires consideration of seabed morphology and preferably ground-truth observations.

Side-scan sonar

Figure 3: Side-scan sonar. Two transducers emit sound waves from a towfish.

Side-scan sonar is primarily an imaging technique. It reveals spatial variations in seabed backscatter, morphology and objects; seabed type can often be inferred when the acoustic image is combined with bathymetry and ground-truth observations.

Different seabed materials, roughnesses and morphological features produce different backscatter patterns. Hard or coarse surfaces often return stronger echoes than fine soft sediment, while objects and relief also produce characteristic acoustic shadows. The returned signal depends on seabed acoustic impedance, roughness, slope, incidence angle and frequency, so different substrate types often—but not uniquely—produce different backscatter signatures. The amount of reflected energy can be measured, and with knowledge of the acoustic properties of common materials, an image of the seabed texture can be formed. Successive pings form a sonogram in which image brightness represents relative backscatter strength and acoustic shadows reveal the relief of seabed objects and bedforms. The best results are obtained in calm seas and when the survey vessel follows a straight course.

The system consists of two transducers mounted on either side of the device, allowing measurements to be made both to the left and right of the ship. Each ping insonifies narrow fan-shaped strips on both sides of the track; successive pings form a continuous swath image of the seabed.

Unlike singlebeam or multibeam systems, the side-scan sonar is usually mounted on a towfish rather than on the ship’s hull (Figure 3). A towfish is a torpedo-shaped carrier that can be towed close to the seabed. It is also possible to mount the sonar on submersibles or on autonomous underwater vehicles (AUVs).

Because side-scan sonar usually provides little information about depth, while multibeam systems provide less detailed information about backscatter texture/seabed character, the two techniques are often used together as complementary methods.

Ocean Acoustic Waveguide Remote Sensing (OAWRS)

Figure 4: Schematic representation of the OAWRS system used in the Gulf of Maine in 2006.

OAWRS exploits low-frequency propagation in the ocean acoustic waveguide, where sound interacts repeatedly with the sea surface and seabed and can remain detectable over tens of kilometers. The echoes are received by an array of receivers towed behind a ship. The world’s navies depend upon ocean waveguide acoustics as the primary underwater remote sensing tool for both surface and sub-surface vessels as well as in fixed installations.

The technique also provides information on the horizontal spatial distribution of fish populations, their behavior, and fish abundance. Fish aggregations can almost instantaneously be imaged over areas of order 10³ km². Frequencies can be selected to exploit strong swim-bladder scattering, including resonance for particular species, sizes and depths.. With more traditional survey methods, the research vessel often comes so close to the organisms that their normal behavior is disturbed, leading to a less realistic representation of natural conditions. The long sensing range of OAWRS reduces the need for the survey vessel to pass directly through each aggregation and can therefore reduce vessel-induced disturbance.

OAWRS is considered a highly promising technique because it can provide a much more spatially synoptic picture of fish distributions and dynamics at regional scales. For local high-resolution observations and quantitative fisheries surveys, conventional fisheries echosounders and sonars remain necessary.

Studies have shown that the sound waves produced by OAWRS may affect whale behavior, indicating that potential ecological impacts must be carefully considered. See also Underwater noise.

Related articles

General principles of optical and acoustical instruments

Instruments for bed level detection

Passive acoustic monitoring (PAM) of marine mammals

Health biomarkers in marine mammals

External links

http://en.wikipedia.org/wiki/Multibeam_echosounder

http://nl.wikipedia.org/wiki/Side_scan_sonar

http://nl.wikipedia.org/wiki/Singlebeam

References

The main author of this article is Van Beveren, Elisabeth Please note that others may also have edited the contents of this article. Citation: Van Beveren, Elisabeth (2026): Underwater acoustic observation. Available from http://www.coastalwiki.org/wiki/Underwater_acoustic_observation [accessed on 2-09-2026] For other articles by this author see Category:Articles by Van Beveren, Elisabeth For an overview of contributions by this author see Special:Contributions/Elisabeth Van Beveren