By Dennis Wilson (MMA)
Introduction
The methodology surrounding the field of maritime archaeology is under going a continual evolution, and has been since its establishment in the late 1950s as a sub discipline of archaeology. From the invention of SCUBA onwards, maritime archaeologists have been using technology to overcome the constraints of the underwater environment and efficiently gather information about underwater cultural heritage sites. Today, maritime archaeologists have placed an emphasis on the protection of underwater cultural heritage (UCH) through the adoption of principles like in situ preservation and a movement toward stronger legislation (See UNESCO 2001). This movement toward in situ preservation and UCH identification is facilitated in part by the growing availability of capable remote sensing survey equipment to maritime archaeologists everywhere. These technologies have made it vastly easier to locate, and therefore protect UCH sites in an increasingly economic fashion. Side scan sonar is a pivotal tool in site surveying and continuing technological advancements have made it more readily available to the archaeological community. While we know the technology is out there, it is important for young archaeologists to know the capabilities and limitations of side scan sonar, and its potential for future research.
What is Side Scan Sonar?
Fig. 1 The image created will show shadows based on vertical anomalies, and a thick black line which characterizes the side scan sonar "blind spot".
Side scan sonar traditionally operates via a torpedo shaped “fish” that is towed behind a survey vessel, allowing the side scan transducers to radiate sonar beams laterally from both sides of the fish. The result is a two dimensional image created based on the time it takes for an acoustic signal to travel from the transponders to an object on the seabed and back. Objects of varying density producing visible changes in the final image (Atallah, 2005). Side scan images are characterised by a thick black line through the centre of the image caused by a gap between the transponders on either side of the fish (see figure 1), however the image will show the operator three things; the seabed, the water surface, and shadowed images of anomalies (Green, 2001). This black and white image is similar to what a photograph of the seabed would look like, but the use of sound instead of light means its accuracy is unaffected by murky or black water.
Fig. 2 Side scan sonar image of a scuttled Tub Boat. Note the distinct shadow and gouge in the sand created by the vessels impact with the seabed.
The interpretation of side scan sonar images is a complex ordeal and hardly produces conclusive results, but a keen eye can often separate real targets from interference. The presence of shadows in images is key to sound data interpretation. Shadows are an indication of an object standing as a vertical protrusion from the seabed and help to convey the shape of a target in detail. However, sonar data is affected by the conditions in which it is recorded, “noise” can create images that resemble a UCH site but these interferences will most commonly fail to create a shadow that matches the object present in the side scan image (Singh et al., 2008).
When do we Use Side Scan Sonar?
In the field of maritime archaeology, side scan sonar has become the choice technology for the locating and determining the size of sites. When locating sites, the side scan sonar will typically be placed higher in the water and set to a low frequency to maximize seafloor coverage (Warren, 2008). The search area of side scan sonar is a direct result of the distance between the fish and the seafloor, the height being 10% of the width of the swath (Singh et al, 2008). So a broad-area search at a height of 40m would presumably cover 400m of sea floor. When targets have been located, side scan sonar will be placed between 10 to 20 meters above the seabed with a high frequency (410kHz) pulse to gather more data. In these site-specific surveys the survey line spacing is greatly reduced, reaching intervals of 5 meters or less (Warren, 2008). Field redundancy is also a consideration, whether one requires continuous coverage with no gaps or possibly sites from multiple angles. A side scan sonar operating at hundreds of kHz may vary its overlap from 25% to 100% (Singh et al., 2008). After sites have been located and surveyed in a site-specific manor, the survey will progress to optical sensing equipment (Singh et al., 2008). However, in low visibility environments high frequency images from side scan sonar can be used to supplement an optical site map and assist divers in identifying various aspects of a wreck.
Like any technology, side scan sonar is not without its imperfections. Interference with sonar images can be caused by poor sea conditions during use, the pitch, roll, and yaw of the towboat will all conceivably have an effect on the accuracy of the image produced by side scan sonar (Singh et al., 2008). The seabed may also present problems in sonar imaging; rapidly undulating terrain and rocky seabed present a variety of potential “noise” that will deteriorate the resolution of the final image. Therefore, ideal conditions for the operation of side scan sonar surveys is in calm, flat waters with a sandy seabed (Green, 2001). The interfacing of side scan sonar with GPS has eliminated problems caused by inconsistent tow speeds through the water, ensuring that images are continually created to scale. This effectively means that sonar images can be compared to known plans of vessels to compare dimensions of the target with known ships, a principle applied on the discovery of the Sapporo Maru in Truk Lagoon (Green, 2001).
What is next for Side Scan Sonar?
Fig. 3 The Autonomous Underwater Vehicle, or AUV, currently in use by C & C Technologies.
Side scan sonar systems have become greatly varied since their inception, with various models ranging from simple and cheap to complex, sophisticated systems capable of working at extreme depths. C & C technology for example, operate an autonomous underwater vessel (AUV) which is equipped with swath bathymetry systems, chirp sub-bottom profiler systems, and dual-frequency side scan sonar in either 120/410 kHz or 230 dynamically focused/410 kHz configurations (Warren, 2008). Internal guidance systems and GPS ensure the AUV’s positioning at all times with a much higher degree of accuracy than deep-tow side scan sonar.
Fig. 4 Side scan sonar images can be overlaid on satellite photography using GIS software to create comprehensive imaging of the seabed.
Finally, the combination of side scan sonar with other technologies has allowed for the ability to sonar mosaic. The incorporation of GPS means that each point on a graphic image produced by side scan sonar is known, and therefore using software these images can be georeferenced and mapped in GIS programs, making it possible to overlay the images on maps and aerial photography (Green, 2001). The final product of all this technology will be a high resolution image of a correctly scaled and geographically located image of UCH sites, from ancient shipwrecks (Ballard et al., 2001) to WWII wreck sites. No longer are the depths of the ocean capable of keeping some of the world’s most culturally significant wrecks from those who wish to learn from the secrets they hold in the deep.
References Cited:
Warren, D., 2008. “Using AUVs to Investigate Shipwrecks: Deepwater Archaeology in the Gulf”. Sea Technology. Vol. 49, no. 10, pp. 15-18.
Attallah L., Shang, C., Bates, R., 2005. “Object Detection at Different Resolution in Archaeological Side-scan Sonar Images”. Proceedings of Oceans ’05 Brest, France.
Ballard, R. et al., 2001. “Deepwater Archaeology of the Black Sea: The 2000 Season at Sinop, Turkey”. American Journal of Archaeology. Vol. 105, no. 4, pp. 607-623.
Green, J., 2001. “Maritime Archaeology: A Technical Handbook”. Second edn., Elseiver Academic Press, U.S.A, pp. 77-83.
Singh, H. et al., 2000. “Imaging Underwater for Archaeology”. Journal of Field Archaeology. Vol. 27, no. 3, pp. 319-328.
Images Cited:
Figure 1: Side Scan Sonar in Use. Retrieved October 4th, 2011 at http://gralston1.home.mindspring.com/Sidescan.html
Figure 2: Side Scan Sonar Image, Tugboat. Retrieved October 4th, 2011 at http://njscuba.net/reefs/chart_nj04xc_axel_carlson.html
Figure 3: C & C Technologies AUV Unit. Retrieved October 4th, 2011 at http://xpda.com/junkmail/junk187/junk187.htm
Figure 4: Side Scan Sonar Overlay. Retrieved October 4th, 2011 at http://geosolutions.blogspot.com/
