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Side Scan Sonar

Side Scan Sonar (SSS) is a widely used acoustic technique in the underwater archaeologist’s toolkit. Within the AMZ cycle, SSS can be applied during the exploratory and mapping phases of a field survey to map objects on the waterbed.

What?

How does it work?

Side Scan Sonar (SSS) is an instrument used for mapping objects and structures that lie on the waterbed or partially protrude from the seabed (figure 1). A side scan sonar emits sound waves and receives the reflected signals. By analyzing the time delay and intensity of the reflected signals, this sensor creates detailed images of the underwater terrain and features.

Figure 1: Diagram of a side scan sonar (source: Wikimedia Commons).

The result is an acoustic image (echogram) of the waterbed, comparable to a black-and-white photograph (figure 2). The colours on the echogram represent the strength of the reflected signal. Solid objects and a hard seabed (such as sand) produce a strong reflection, while a soft silt surface partially absorbs the sound waves, resulting in weaker reflections. This allows for conclusions to be drawn about the composition of the waterbed (sand, silt, or clay) while also making it possible to identify potentially present objects and determine their dimensions (length, width, and height).

Figure 2: An echogram of a shipwreck located in the Markermeer (source: Periplus Archeomare).
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In-depth explanation X

The strength of a reflected sound wave is called the reflection coefficient By using a graphical representation of the reflection coefficient, the length and width of objects can be determined (figure 3). An object protruding from the waterbed will cast a “shadow” in the recordings, as sound waves are blocked behind the object. The height of the object can then be calculated based on the length of this shadow (figure 4).

Figure 3: Schematic diagram of an echogram.
Figure 4: The determination of the height of an object on an echogram.

The range and resolution of the acoustic image depend on the frequency of the transceiver (transmitter/receiver) used. The rule of thumb here is: the higher the frequencyof the emitted sound waves, the higher the resolution but the more limited the range. A standard frequency of 500 kHz and a range of 50 m (to the left and right) is typically used. In practice, a survey area is recorded twice to map all objects and structures from different angles.

What do you need?

A side scan sonar system consists of various components, the most crucial of which being the transceiver (figure 5). The transceiver itself can range from a simple system (like a ‘fishfinder’) to an advanced system with more power and a greater range. Specialized knowledge is essential for operating the system and interpreting the data. Other key components include a positioning system, and SSS data acquisition and processing software.

Figure 5: Deployment of a side scan sonar transceiver in a towed configuration (source: Rijkswaterstaat).
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In-depth explanation X

The following components make up a side scan sonar system:

  1. Transceivers: These devices convert electrical signals into high-frequencyacoustic
    signals and back. The choice of transceiver impacts both the system’s coverage and resolution—the higher the frequency, the greater the resolution. In side-scan sonar systems, the transceiver is often called a “towfish.”
  2. Positioning system: This is needed to determine the SSS’s location. It can be a Global Navigation Satellite System (GNSS) or Real-Time Kinematic (RTK) GPS. The more accurate the positioning system, the more precise the side scan results, taking into account the so-calledlayback correction (see number 6, below).
  3. Positioning software: This records the ship’s position. It also relays the position – optionally corrected for offset – to the sonar processor. The positions are then linked to the side scan data.
  4. Sonar processor: This processes the recorded acoustic signals into interpretable images. Advanced processors can enable real-time imaging.
  5. Processing Software: Various commercial programs can be used for processing, analyzing, and interpreting SSS images, such as Qinsy and SonarWiz.
  6. Optional – Ultra Short Baseline (USBL): A USBL can be used to precisely measure the distance between the SSS transceiver and the positioning system. Mounted directly below the positioning system on the vessel’s underside, the USBL communicates with the SSS transceiver via sound waves. By measuring the precise direction and distance—the so-called “layback”—between the USBL system and the side scan “towfish” and linking it to the exact GPS location, the real-time position of the fish can be determined. The USBL is only necessary when towing the SSS behind a vessel; if the side scan is attached directly to the ship, the side scan’s offset only needs to be entered into the positioning software once.

Points to consider:

  • For optimal imaging, the transceiver height above the seabed should be 10-15% of the set range.
  • Maximum range depends on the frequency used; lower frequencies allow for greater range.
  • The resolution of the echogram increases proportionally with the frequency used; for detecting objects with a minimum size of 1 m, a frequency of at least 500 kHz is recommended.

Can be used with..

The side scan sonar “fish” (transceiver) can be used in three configurations:

Fixed on a survey vessel or ROV (Remotely Operated Vehicle)

  • Advantages: Accurate positioning (the GPS antenna can be placed directly above the transceiver).
  • Disadvantages: A fixed setup is only suitable for shallower water depths of less than 25 m. Intense vessel movements are visible in the results.

Towed configuration

In this setup, the transceiver is towed behind a survey vessel on a cable.

  • Advantages: Vessel movements have less effect on the results, and the sonar fish can be towed at a fixed height above the bottom by adjusting the cable length.
  • Disadvantage: Positioning may be less accurate, depending on the cable length. When there are large variations in water depth, the cable length must be continuously adjusted.

Sectorscan configuration

In this configuration, the transceiver is mounted vertically on a tripod on the seabed, allowing the sensor to scan 360-degrees around.

  • Advantages: The transceiver can perform a full 360-degree scan, providing a complete view of the surroundings without repositioning the transceiver.
  • Disadvantages: Because the transceiver is fixed to a tripod, the scan is limited to one location, making it difficult to survey larger areas.

Archaeological Applications

Place in the Dutch archaeological heritage management process

Within the archaeological heritage management process (IVO opwater), the SSS is used to map objects on the seabed from the water’s surface during the exploratory and mapping phases of a survey (KNA protocol 4103, p. 5-7). By combining all the collected line recordings into a mosaic (similar to an orthophoto), large-scale structures—such as continuous dragmarks from fishing nets or variations in surface sediments—can be visualized (Figure 6).

Afbeelding 6: Side scan sonar mosaic of the Enkhuizerzand research area in the Markermeer (source: Periplus Archeomare).

What types of archaeological materials/landscapes

The SSS can be used to map objects such as shipwrecks and determine their dimensions (length, width, and height). Based on the strength of the reflections, it is possible to gauge whether the wrecks are wooden (weak reflection) or metal (strong reflection). SSS can be used in all types of water: fresh, salt, and brackish water.

Larger structures, such as submerged dikes and villages, can also be mapped using SSS by creating a composite mosaic.

 

Limitations/uncertainties

To successfully deploy the side scan sonar, it is important to take into account the limitations and uncertainties of this method.

Limitations:

  • Special software is required for both data acquisition and interpretation.
  • It is only suitable for mapping objects and structures at the underwater surface, not for covered objects within the seabed.
  • Higher frequencies yield better resolution but reduce maximum range.
  • Accurate identification of objects often requires additional investigations, such as through diving surveys or visual inspections with an ROV. For example, the difference between a polyester and a wooden wreck is not discernible on sonar images, necessitating visual confirmation for validation.
  • The correct interpretation of side scan sonar images takes significant experience.

Uncertainties:

  • The accuracy of positioning decreases in water deeper than 10 m.
  • Waves, currents, and turbidity can affect its performance.

References/further reading

Mazel, C. (1985). Side scan sonar training manual. Klein Associates.

Van den Brenk, S., & Van Lil, R. (2023). Archeologisch bureauonderzoek en inventariserend veldonderzoek Enkhuizerzand, Markermeer. Periplus Archeomare rapport 23A003-01.

Van den Brenk, S., & Van Lil, R. (2021). Inventariserend veldonderzoek (opwaterfase), de Banjaard, Noordzee. Periplus Archeomare rapport 20A015-03.

Van den Brenk, S., & Van Lil, R. (2021). Inventariserend veldonderzoek (opwaterfase) Haringvliet. Periplus Archeomare rapport 20A015-07.

Van den Brenk, S., & Van Lil, R. (2021). Inventariserend veldonderzoek (opwaterfase) Grevelingenmeer. Periplus Archeomare rapport 20A015-08.

Van den Brenk, S. (2021). Archeologisch bureauonderzoek en inventariserend veldonderzoek Friese Kust, IJsselmeer. Periplus Archeomare rapport 21A012-04.

Van den Brenk, S. (2023). Archeologisch bureauonderzoek en inventariserend veldonderzoek Kreupel en Vrouwenzand, IJsselmeer. Periplus Archeomare 22A002-08.

Van den Brenk, S., Jongejan, E., & Onstwedder, C. (2020). Inventariserend veldonderzoek (opwaterfase), gebied ten westen van Noorderhaaks Noordzee en Gemeente Texel. Periplus Archeomare rapport 19A005-06.

Van den Brenk, S., Eelman, H., & Bartels, M. H. (2018). Inventariserend veldonderzoek (opwaterfase), Texelstroom Noord en Scheer, westelijke Waddenzee. Periplus Archeomare rapport 18A010-01.

Van den Brenk, S., Cassée, R., Van Lil, R., Aay, C., Postema, F., Van Meurs, T., De Weerd, H., & Van Oosterwijk, R. (2020). Inventariserend veldonderzoek (opwaterfase) Hoornse Hop. Periplus Archeomare rapport 20A015-06.

ScienceDirect. (n.d.). Side-scan sonar. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/sidescan-sonar

Wikimedia Commons. (n.d.). Side-scan sonar.svg. https://commons.wikimedia.org/wiki/File:Side-scan_sonar.svg?lang=nl