Submarine acoustic techniques

Introduction

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

In water, sound can travel much farther than light or any other form of radiation because it is only minimally absorbed by the surrounding medium. For this reason, sound is the primary means of remote sensing and imaging underwater. 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 main acoustic underwater techniques are the singlebeam and multibeam echosounders, the side-scan sonar, and OAWRS (Ocean Acoustic Waveguide Remote Sensing). All consist of a projector that generates the sound waves and a signal receiver or hydrophone that receives the echo. When the transmitter can also receive, it is called a transducer.

Based on either the travel time or the energy of the reflected waves, depth or material properties can be determined, respectively. 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 image quality.

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. Most fish have a swim bladder, which can scatter more than 90% of the incident acoustic energy. 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.

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.

The echosounder is towed back and forth by a ship to survey or analyze the seabed over a larger area. The beam can be wide (“wide beams”), allowing a relatively larger surface area to be covered, but at the expense of image quality. More expensive “narrow beams” provide a clearer image but cover a smaller zone. Partly because one system provides too low an image resolution and the other covers too small an area, the singlebeam method is not efficient enough for scanning entire regions. 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. It consists of multiple single narrow beams arranged together. The transducer is positioned on the underside of the ship in such a way that it produces a fan-shaped array of sound waves. As a result, the seabed is scanned by a continuous line of measurement points perpendicular to the sailing direction of the ship. 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.

The transducer measures both the travel time and the intensity difference between transmission and reception, allowing it to determine depth and seabed characteristics, respectively. 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. A flat, hard surface reflects more acoustic energy than a soft, uneven substrate such as sand. In this way, the multibeam system can classify the seabed into different categories. To relate these acoustic classes to the actual seabed type, physical sediment samples must be collected for verification.

Side-scan sonar

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

Unlike singlebeam and multibeam echosounders, a side-scan sonar (Figure 3) is used primarily to determine the composition of the seabed rather than its depth. This is possible because different materials have specific sound-absorbing and sound-reflecting properties. Some materials, such as metal and newly formed volcanic rock, are highly reflective, whereas others, such as clay and silt, reflect much less sound. Strong reflectors produce strong echoes with high energy (high amplitude), and vice versa.

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. The result is a sonogram: an image with a two-color gradient that represents the energy spectrum of the reflected signal. 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. As with a multibeam system, a line is measured perpendicular to the ship’s direction of travel. Unlike singlebeam or multibeam systems, the side-scan sonar is usually mounted on a towfish rather than on the ship’s hull. 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 composition, 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.

Ocean Acoustic Waveguide Remote Sensing (OAWRS) is a recent development for the near-instantaneous imaging and continuous monitoring of fish populations on a very large scale. The surveyed area can cover thousands of square kilometers, which is at least ten thousand times larger than what is possible with conventional techniques. OAWRS is rarely used for seabed mapping; instead, it is mainly applied to locate distant populations of fish or other organisms, such as krill. The technique provides information on the horizontal spatial distribution of populations, their behavior, and fish abundance. 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 large spatial scale of OAWRS eliminates this unwanted effect.

This acoustic underwater technique was developed by the Massachusetts Institute of Technology (MIT) in Cambridge, United States. In 1995, submarine mountains and ridges were successfully visualized. The first successful test for fish population research took place in 2003 along the east coast of the United States. In a single second, an area larger than ten thousand square kilometers was analyzed. A second successful study was carried out in 2006 on Georges Bank, where the schooling behavior of herring during spawning was investigated.

Acoustic signals are transmitted by several vertically suspended sources. These signals are reflected by both the sea surface and the seabed, creating so-called waveguide modes, or vertical standing waves, that span the full water depth. Because the waves spread outward in a circular pattern from their source, a large three-dimensional area can be scanned. The frequencies used are close to the resonance frequency of the swim bladder, which causes most fish with swim bladders to produce strong echoes. These echoes are received by an array of receivers towed behind a ship.

In the future, it may become possible to use fixed systems instead of ship-based systems, allowing continuous long-term monitoring.

OAWRS is considered a highly promising technique because it can provide a more accurate picture of fish dynamics and abundance at regional scales. However, for high-resolution surveys, conventional fish-finding sonars (CFFS)—such as singlebeam, multibeam, and side-scan sonar systems—will remain necessary.

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

Related articles

General principles of optical and acoustical instruments

Instruments for bed level detection

Acoustic monitoring of marine mammals

Marine mammals' health as an indicator of ecosystem health - tools for monitoring

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): Submarine acoustic techniques. Available from http://www.coastalwiki.org/wiki/Submarine_acoustic_techniques [accessed on 30-04-2026] 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