Tag Archives: AUV

Deep-water Technology: The Future of Maritime Archaeology

While maritime archaeology is a rather new discipline compared to terrestrial archaeology, deep-water archaeology (greater than 100 metres) is so recent that it is still largely in its infancy.  This is due to the extreme conditions of the deep ocean and lack of technology necessary to reach such depths.  In addition, there is the prohibitive cost of deep-water exploration.  Expeditions that use ocean-class research vessels can cost $40,000 USD or $44,697 AUD per day and easily exceed $1 million over a month-long period (Ballard 2008:x).  However, multi-disciplinary projects that foster cooperation with oceanographers, biologists, and engineers can reduce the cost of research and allow each scientist to collect much needed data.  Continuous advancements in the technology of human-operated vehicles (HOVs), remotely operated vehicles (ROVs), and autonomous underwater vehicles (AUVs) are allowing maritime archaeologists to reach greater depths and explore hidden cultural clues in this largely unexplored world.

Techniques commonly used by maritime archaeologists for shallow-water surveys, such as side-scan sonar, magnetometers, and sub-bottom profilers, are being applied to HOVs, ROVs and AUVs to explore the depths of the ocean.  Side-scan sonar emits sound waves that strike the sea floor and creates imagery by recording the timing and amplitude of those sound wave reflections.  Magnetometers are used to locate man-made objects by detecting anomalies in the normal magnitude and direction of the earth’s magnetic field.  Sub-bottom profilers are similar to side-scan sonar in that they emit sound waves towards the sea floor; however, the sub-bottom profiler’s sound waves penetrate the sea floor in order to identify different layers of sediment (Ballard 2008:263-274).  By utilising these devices in conjunction with HOVs, ROVs and AUVs, archaeologists are able to map and survey depths greater than 100 metres.

Human-Operated Vehicles

HOVs are also known as human-operated submersibles or simply submersibles.  Many submersibles are limited in their ability to survey large areas.  This is due to their reliance on a human occupant/operator, which limits the amount of time they can stay on site.  Although HOVs are limited by time, they provide an advantage over ROVs and AUVs because they can “typically lift heavier objects and carry more equipment and/or samples” (Ballard and Coleman 2008:12).  An excellent example of an HOV is Alvin, a U.S. Navy-owned Deep Submergence Vehicle built in 1964.  It is able to dive to a depth of 4,500 metres and remain below the surface for up to  10 hours (WHOI 2013).  Alvin is outfitted with video cameras, lights, and two robotic arms that allow the vessel to carry 680 kilograms of samples.  Alvin is perhaps best known for its involvement in the exploration of RMS Titanic in 1986 (WHOI 2013).

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Figure 1: Human Occupied Vehicle Alvin (Photo by Mark Spear, Woods Hole Oceanographic Institution 2013)

Remotely Operated Vehicles

ROVs are similar to HOVs except that instead of having an occupant inside the vehicle, the ROV is controlled from a support vessel on the surface.  ROVs are tethered to the surface vessel by fibre-optic cables and controlled via fibre-optic telemetry (Gregory et al. 2008:17).  These cables allow the operator to control the movement of the ROV as well other functions such as lighting, cameras and manipulator arms.  ROVs are better adapted for surveying larger areas than HOVs, but are still limited by the cables that attach them to the support vessel.  ROVs are sometimes used in tandem with a towsled that is positioned between the support vessel and ROV.  The benefit of using a towsled is that it absorbs the movement of the support vessel and prevailing sea conditions, which allows the ROV to work undisturbed.  The towsled often sits above the sea floor and provides additional lighting to reduce backscatter from particles in the water when images are being taken.  Besides surveying, ROVs can be used to excavate artefacts from the sea floor.  One example of this type of vehicle is the ROV Hercules and its towsled ArgusHercules is equipped with digital cameras and sonar for site mapping, as well as tube corers to extract samples of sediment in preparation for excavation (Webster 2008:45).  The ROV also features jets that provide a flow of water to clear sediment from artefacts, as well as a suction hose to lift material (Webster 2008:53).  In addition to this useful tool, Hercules’ manipulator arms can be fitted with various hand tools such as brushes and scrapers (Webster 2008:56).

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Figure 2:  ROV Hercules viewed from towsled Argus (NOAA 2013)

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Figure 3: Towsled Argus being lowered into the water (NOAA 2013)

Autonomous Underwater Vehicles
AUVs differ from the two previously mentioned vehicles in that they are not controlled by an operator but rather programmed to survey a certain area.  In addition to not requiring an operator, the major advantages of AUVs over HOVs and ROVs is that they can be deployed and left to survey large areas for between 24 and 72 hours without the need for a support vessel.  This saves thousands of dollars in operating costs (Bingham et al. 2010:703).  While AUVs tend to be used more for commercial purposes, such as surveys for natural resources, their role in archaeology is significant and growing.  AUVs have precise on-board navigation systems that make use of global positioning system (GPS) and differential global positioning system (DGPS) that link to the support vessel.  The exact position (3-5 metre accuracy) of the AUV is essential to mapping and surveying the sea floor (Warren et al. 2007:4).  Many AUVs carry chemical sensors for testing the environment in addition to multibeam sonar (similar to side-scan sonar), a sub-bottom profiler, and magnetometer.  AUVs are limited by the power supply needed to both run the vehicle and maintain its illumination lamps (Bingham et al. 2010:703).  Despite their limitations, AUVs are ideal for conducting general surveys and producing photomosaics of the sea floor with limited detail.  A great example of AUV application within deep-water archaeology is the SeaBED model used to document the Chios shipwreck site in the northeastern Aegean Sea (Bingham et al. 2010:702-715).

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Figure 4: Model of SeaBED AUV being deployed (Bingham et al. 2010:705)

The future of maritime archaeology is continually evolving as technological advances in various underwater vehicles allow for the ocean to be explored and mapped at greater depths.  Multi-disciplinary cooperation has facilitated archeologists’ access to these forms of technology and increased the amount of data they can collect.  This in turn has enabled the discovery and documentation of ancient shipwrecks and landscapes previously unknown to modern archaeology.

References

Ballard, R. and D.Coleman

2008 Oceanographic Methods for Under Archaeological Surveys. Archaeological  Oceanography, edited by Robert Ballard, pp. 3-14. Princeton University Press, Princeton, New Jersey.

Ballard, R.

2008 Glossary. Archaeological Oceanography, edited by Robert D. Ballard, pp. 263-274. Princeton University Press, Princeton, New Jersey.

Ballard, R.

2008 Introduction. Archaeological Oceanography, edited by Robert Ballard, pp. ix – x. Princeton University Press, Princeton, New Jersey.

Bingham, B., B. Foley, H. Singh, R. Camilli, K. Delaporta, R. Eustice, A. Mallios, D. Mindell, C. Roman, and D. Sakellariou

2010 Robotic tools for deep water archaeology: Surveying an ancient shipwreck with an autonomous underwater vehicle. Journal of Field Robotics 27(6): 702-717.

Gregory, T., J. Newman, and J. Howland

2008 The Development of Towed Optical and Acoustical Vehicle Systems and Remotely Operated Vehicles in Support of Archaeological Oceanography. Archaeological Oceanography, edited by Robert Ballard, pp. 15-29.  Princeton University Press, Princeton, New Jersey.

National Oceanic and Atmospheric Administration

2013 Hercules (ROV) and Friends, Electronic document, http://oceanexplorer.noaa.gov/technology/subs/hercules/hercules.html, accessed 10/9/13.

Warren, D., R. Church, and K. Eslinger

2007 Deepwater Archaeology with Autonomous Underwater Vehicle Technology. In Offshore Technology Conference. Houston Texas Electronic Document, e-book.lib.sjtu.edu.cn/otc-2007/pdfs/otc18841.pdf, accessed 10/9/13

Webster, S.

2008 The Development of Excavation Technology for Remotely Operated Vehicles. Archaeological Oceanography, edited by Robert Ballard, pp. 41-64 Princeton University Press, Princeton, New Jersey.

Woods Hole Oceanographic Institution

2013 Human Occupied Vehicle Alvin. Electronic document, http://www.whoi.edu/alvin/, accessed 9/9/13.


Let’s get Geophysical! Non-invasive Underwater Archaeological Survey Methods

Trends are not a new concept to archaeology. The patterns found in the archaeological record are what lead to the wider inferences made about past cultures or behaviours. However, the latest trend in archaeology isn’t about similarities in information sets or assemblages,but rather the movement towards in situ (in place) preservation of archaeological sites, especially in underwater archaeology. I use the term ‘trend’ loosely, as it implies that in situ preservation is a ‘fad’ that will become obsolete given enough time or with the arrival of a newer, en vogue concept. I actually believe the opposite is true, that in situ preservation is here to stay and that it is the future of archaeology, above or below the water. This is not so much my opinion, but more of an observation. Looking at the international legislation that surrounds underwater cultural heritage (UCH), one cannot help but see that in situ preservation is pressed as the primary approach (UNESCO, 2001: Article 2,5; UNESCO Annex, 2001:Rule 1) and in many introductory texts, non-invasive survey methods are considered the future (Bowens, 2009:5). We need to know what is under the seabed in order to know if archaeological sites lie beneath, but we are trending away from invasive methods of surveying like subsurface testing. This leaves non-invasive approaches like geophysical surveys and remote sensing.

Geophysics in underwater archaeology is the scientific study of features below underwater and under the seabed using a range of specialized instruments while remote sensing is obtaining images of a phenomena from a distance (Bowens 2009: 217). It is common for these two methods to be grouped together, as they both deal with the ability to collect large amounts of data quickly and understand the scale of the surveyed site without having to be directly on or necessarily near it. In the past, geophysics was used primarily for site prospection but has been applied more recently to research and site management (Bowens 2009: 103). Geophysical and remote sensing surveys allow for the coverage of large areas relatively quickly and economically. They are not meant to replace divers on a site, but aid in timely identification of site locations, site distribution, site boundaries, and sub-seabed phenomena and are particularly useful in environments with poor underwater visibility, strong currents, or any other environmental hazards. Geophysical and remote sensing surveying methods will be discussed and can be grouped into three types: acoustic systems, magnetometers, and submersibles. These methods are used over a large area to ensure complete coverage of the site and its environmental context and are very accurate when used with global positioning system (GPS) satellites and differential global positioning system (DGPS) land-based reference stations. Using both will increase site position fixing as DGPS makes range corrections for GPS satellites; the addition of an on-boat GPS antenna increases accuracy (Bowens 2009:94).

Acoustic Systems:

These systems are the most commonly used geophysical method for underwater archaeological surveying. Sonar,or sound waves, are used in order to obtain the desired information. Some forms of acoustic surveying systems are: echo-sounders, multibeam sonars, side scan sonars, and sub-bottom profilers (Bowens 2009:104). The general idea behind these types of non-invasive systems is to use reflected sound waves (echoes) to construct a picture of what the underwater site and bathymetry, or depth over seabed, looks like.  Figure 1 shows the different components and general setup for using side scan sonar. Side scan sonar uses a wide-angle pulse of sound (emitted from the towfish) and the strength of the reflected scattered sound to display an image (Figure 2). The coverage of the side scan sonar can reach over 100m on either side of the track line. The track line is a gap in between the two sides; its size varies by size of coverage and depth. It is a ‘dead space’ of sorts where there is too much interference between the two sides to get an accurate image. This problem can be countered by overlapping boat runs to ensure full coverage. Acoustic shadows are also important as they can give a general description of objects that sit proud (vertical) to the seabed (Bowens 2009:108), see Figure 2.

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Figure 1.  The components and set up of a side scan sonar (image created by author)

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Figure 2. The results of a side scan sonar survey (after Kainic 2012)

Echo-sounders and multibeam sonars are generally used to gauge vertical measurements or depth. Echo-sounders were first to be applied to maritime archaeology and used a single transceiver to send an acoustic pulse straight down to the seabed and read the reflection or echo on a single prescribed spot. Multibeam sonar (also known as swath bathymetry) records a continuous thin strip of depth directly below and to the side of the boat (Figure 3), effectively scans the surface of the seabed, and creates a 3D image via colour gradations to highlight depressions and outcrops, as represented in Figure 4 (Bowens 2009:106). Sub-bottom profiling is the only means to locate buried wooden material culture underwater; metal material culture will be discussed in the next section. Strong short pulses of sound are shot into the seabed sediment and ‘reflect’ anything that sends the echo back earlier than the rest. The two forms of sub-bottom profiler are single-frequency pulse (also known as ‘pingers’ and ‘boomers’) and swept-frequency pulse (‘chirp’) (Bowens 2009: 109). Using both devices ensures the best coverage and penetration of the seabed.

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Figure 3. The setup of a multibeam sonar survey (image created by author)

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Figure 4. The results from a multibeam sonar survey, red are closer to the surface while blue is deeper (after Cox 2012)

Magnetometers:

Magnetometers measure the strength of the earth’s magnetic field and are used to detect the presence of ferrous material (iron) by the variations they cause in said field (Bowens 2009:111). This may include both man-made objects, like the cannon in Figure 5, or geological formations. They are usually deployed in a towing array to inhibit interference from the tow boat and the data they collect are plotted (or ‘contoured’) according to varying magnetic intensities (Figure 6).

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Figure 5. The setup of an underwater magnetometer survey (image created by author)

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Figure 6. The results of a magnetometer survey (Spirek 2001: Figure 2)

Submersibles:

Submersibles for archaeological surveying come in three forms: remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs), and manned submersibles. They can perform many tasks including visual assessments or searches, and photography, thereby negating the need for divers in the water (Bowens 2009:112). ROVs are piloted from the boat and can be outfitted with an array of data-collection devices like acoustic systems or video recorders (Figure 6). AUVs can be outfitted with these devices as well, but are not piloted nor are they attached to a vessel. Manned submersibles can complete the same aforementioned tasks but with an on-board pilot for more control and precision. manned submersibles fall into three categories; commercial, tourism, and research (Kohnen 2005:121).  (Figure 7).

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Figure 6. The setup of a remotely operated vehicle (ROV) (image created by author)

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Figure 7. The Institute of Nautical Archaeology’s (INA) manned submersible Carolyn in operation in the Aegean Sea (Kohnen 2011)

We know that the goal is to try to leave the archaeological site in its original context, as well as the best non-invasive ways to survey it, but why go through all the trouble? As archaeology is an ever changing field that progresses in parallel with new technology, it is undeniable that the information we gather ten years from now will be of a higher quality and degree of accuracy than what we collect today. This means that whatever we choose not to disturb today may never need to be disturbed in the future. Yet we must still yield a high degree of archaeological data,and therefore non-invasive survey methods, like those mentioned above, are an investment for our future AND our past.

References

Bowens, Amanda (editor)

2009 Underwater Archaeology: The NAS Guide to Principles and Practice. 2nd ed. Blackwell Publishing, West Sussex.

Cox, Marijke

2012 Building an Estuary Airport Close to Sunken Warship Branded ‘Bonkers’. Electronic document, http://www.kentnews.co.uk/news/building_an_estuary_airport_close_to_sunken_warship_branded_bonkers_1_1416102, accessed 30/08/2013.

Kainic, Pascal

2012 Search and Recovery Side Scan Sonar. Electronic document, http://www.yousaytoo.com/search-and-recovery-side-scan-sonar/1924787#:image:2729577, accessed 30/08/2013.

Kohnen, William

2005 Manned research submersibles: State of technology 2004/2005. Marine Technology Society Journal, 39(3): 121-126.

Kohnen, William

2011 Carolyn‘s 10-year Aegean voyage for INA. Electronic document, http://nauticalarch.org/news_events/news_events_archives/prior_to_2011/carolyn_takes_a_break/, accessed 01/09/2013.

Schott, Becky K.

2013 The Wrecks of Thunder Bay: A Photo Essay. Electronic document, http://www.alertdiver.com/m/?a=art&id=780, accessed 30/08/2013.

Spirek, James

2001 Port Royal Sound Survey: Search Begins for Le Prince. Legacy, 6(2):28-30.

UNESCO 2001 Convention for protection of underwater cultural heritage.