Why Are Scientists Exploring Sunken History with Mine-Hunting Robots?
A key purpose of NOAA’s Ocean Exploration and Research Program is to investigate the more than 95 percent of Earth’s underwater world that until now has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on Earth, cures for human diseases, answers on how to achieve sustainable use of resources, links to our maritime history, and information to protect endangered species. Technology enables each advancement made in exploration and using advanced technologies is just one focus of the Ocean Exploration and Research Program. Often, techniques used for one type of ocean exploration can be adapted to explore other parts of Earth’s ocean. AUVfest 2008 focuses on using Autonomous Underwater Vehicles (AUVs), or underwater robots that operate without a pilot or cable to a ship or submersible, to explore archeological sites. The experience gained to explore these sites can help scientists explore many other places in the ocean that are too large or too dangerous to be investigated directly by human divers.
On August 5, 1778, Captain John Symons gave the order to burn and sink his own ship, the British frigate HMS Cerberus, in Newport Harbor on the coast of Rhode Island. This was probably a difficult order for Captain Symons, but he was not alone: several other ships also were deliberately sunk when French fleet appeared in Narragansett Bay under the command of Charles Henri Theodat, better known as Count D’Estaing. In February that same year, France had declared war on Britain as part of a treaty with America (this was the first document that officially recognizes America as an independent nation). D’Estaing left France on April 13, 1778 with a fleet of 12 ships of the line and five frigates to supplement the Continental Navy’s efforts to attack the British fleet on the North American coast. After arriving too late to confront British ships in Delaware and unable to cross a sandbar to meet them in New York, D’Estaing finally came to grips with British ships in Rhode Island. The British sank their ships to prevent the revolutionaries from capturing the vessels for their own use, as well as to obstruct navigation within the harbor; a strategy which ultimately proved to be successful, since it helped prevent D’Estaing from entering the harbor and capturing Newport.
Almost 230 years later, the Cerberus is once again receiving attention from the American Navy, this time in partnership with NOAA’s Ocean Exploration and Research Program as part of the Autonomous Underwater Vehicle Festival 2008 (AUVfest 2008). Since 1997, the U. S. Navy’s Office of Naval Research has sponsored Autonomous Underwater Vehicle Festivals (AUVfests) to demonstrate the capabilities of autonomous underwater vehicles (AUVs) for doing scientific and military work. In previous years, the emphasis has been on mine countermeasures and how AUVs can remove humans from the dangerous job of finding and destroying mines. AUVfest 2008 will expand this focus to include marine archeology using AUVs to map shipwrecks and discover long-buried artifacts.
These activities will take place at the Naval Undersea Warfare Center’s Narragansett Bay Test Range off Newport, Rhode Island. In addition to being a site where Navy torpedoes were tested for many years, Narragansett Bay is the site of many shipwrecks (if you want to get an idea about how many wrecks are in Narragansett Bay, visit http://www.wrecksite.eu/wreck.aspx?16438, click the ‘show wks’ box near the bottom of the page, then click inside the red rectangle just below). In addition to finding and mapping buried mines, mine neutralization, and other mine countermeasure operations, AUVs will explore four marine archaeological sites including two Revolutionary War-era British frigates (the Cerberus is one) and two wrecks of early 20th-century ships.
AUVfest 2008 is focused on increasing marine archaeologists’ understanding of how AUV technology can be used to discover and study underwater cultural resources. Key questions related to this goal include:
• How can AUV and mine countermeasures technology be applied to archeological investigations of selected shipwrecks?
• How can mine countermeasures (MCM) technology be used to identify materials and objects that are not normally MCM targets, such as glass, ceramics, and wood artifacts?
• How can AUV/MCM technology be extended for other purposes and benefits in addition to national defense?
Autonomous Underwater Vehicles
Not surprisingly, Autonomous Underwater Vehicles (AUVs) are the technological centerpiece of AUVfest 2008. AUVs are underwater robots that operate without a pilot or cable to a ship or submersible. This independence allows AUVs to cover large areas of the ocean floor, as well as to monitor a specific underwater area over a long period of time. Typical AUVs can follow the contours of underwater mountain ranges, fly around sheer pinnacles, dive into narrow trenches, take photographs, and collect data and samples.
Until recently, once an AUV was launched it was completely isolated from its human operators until it returned from its mission. Because there was no effective means for communicating with a submerged AUV, everything depended upon instructions programmed into the AUV’s onboard computer. Today, it is possible for AUV operators to send instructions and receive data with acoustic communication systems that use sound waves with frequencies ranging roughly between 50 hz and 50 khz. These systems allow greater interaction between AUVs and their operators, but basic functions are still controlled by the computer and software onboard the AUV.
Basic systems found on most AUVs include: propulsion, usually propellers or thrusters (water jets); power sources such as batteries or fuel cells; environmental sensors such as video and devices for measuring water chemistry; computer to control the robot’s movement and data gathering functions; and a navigation system.
Navigation has been one of the biggest challenges for AUV engineers. Today, everyone from backpackers to ocean freighters use global positioning systems (GPS) to find their location on Earth’s surface. But GPS signals do not penetrate into the ocean. One way to overcome this problem is to estimate an AUV’s position from its compass course, speed through the water, and depth. This method of navigation is called ‘dead reckoning,’ and was used for centuries before GPS was available. Dead reckoning positions are only estimates however, and are subject to a variety of errors that can become serious over long distances and extended time periods.
If an AUV is operating in a confined area, its position can be determined using acoustic transmitters that are set around the perimeter of the operating area. These transmitters may be moored to the seafloor, or installed in buoys. Some buoy systems also include GPS receivers, so the buoys’ positions are constantly updated. Signals from at least three appropriately positioned transmitters can be used to accurately calculate the AUV’s position. Although this approach can be very accurate, AUV operators must install the transmitters, and the AUV must remain within a rather small area.
A more sophisticated approach uses Inertial Navigation Systems (INS) that measure the AUV’s acceleration in all directions. These systems provide highly accurate position estimates, but require periodic position data from another source for greatest accuracy. On surface vessels and aircraft equipped with INS, additional position data are often obtained from GPS. On underwater vessels, the accuracy of INS position estimates is greatly improved by using a Doppler Velocity Logger (DVL) to measure velocity of the vessel’s speed. On some AUVs, several of these systems are combined to improve the overall accuracy of onboard navigation. For more information about INS and DVL systems, visit http://www.oceanexplorer.noaa.gov/explorations/08auvfest.
Sonar (which is short for SOund NAvigation and Ranging) systems are used to determine water depth, as well as to locate and identify underwater objects. In use, an acoustic signal or pulse of sound is transmitted into the water by a sort of underwater speaker known as a ‘transducer.’ The transducer may be mounted on the hull of a ship, or may be towed in a container called a ‘towfish.’ If the seafloor or other object is in the path of the sound pulse, the sound bounces off the object and returns an ‘echo’ to the sonar transducer. The system measures the strength of the signal and the time elapsed between the emission of the sound pulse and the reception of the echo. This information is used to calculate the distance of the object, and an experienced operator can use the strength of the echo to make inferences about some of the object’s characteristics. Hard objects, for example, produce stronger echoes that softer objects. This is a general description of ‘active sonar.’ ‘Passive sonar’ systems do not transmit sound pulses. Instead, they ‘listen’ to sounds emitted from marine animals, ships, and other sources. Sub-bottom profiler systems are another type of sonar system that emits low frequency sound waves that can penetrate up to 50 meters into the seafloor. Visit http://ocean.noaa.gov/technology/tools/sonar/sonar.html for more information about sonar systems.
Side-scan sonar systems use transducers housed in a towfish, usually towed near the sea floor, to transmit sound pulses directed sideways, rather than straight down. Return echoes are continuously recorded and analyzed by a processing computer. These data are used to construct images of the sea floor made up of dark and light areas. These images can be used to locate seafloor features and possible obstructions to navigators, including shipwrecks (visit http://oceanexplorer.noaa.gov/technology/tools/sonar/sonar.html for more information). Multibeam sonar system are used to make bathymetric maps and create three-dimensional images of the seafloor. Multibeam sonars send out multiple, simultaneous sonar beams in a fan-shaped pattern that is perpendicular to the ship's track. This allows the seafloor on either side of the ship to be mapped at the same time as well as the area directly below (visit http://oceanexplorer.noaa.gov/technology/tools/sonar/sonar.html for more information).
A magnetometer is an instrument that measures the strength and/or direction of magnetic fields. Magnetometers used in ocean exploration are usually towed behind a research vessel, or in some cases may be carried aboard aircraft or satellites. In marine archeology, a magnetometer is used to measure Earth’s magnetic field near the seafloor. This magnetic field will be changed by large masses of iron or some ceramic materials (such as bricks, or pottery storage containers that were used for many centuries on cargo ships), so changes (called ‘anomalies’) in Earth’s magnetic field can indicate the presence of buildings, shipwrecks or cargo from wrecked vessels.
A gradiometer is an instrument that measures gradients. Magnetic gradiometers are widely used by archaeologists to detect variations in Earth’s magnetic field that may be caused by buried artifacts.
More About Marine Archeology
Archeology is the study of past civilizations and ways of life. Like other sciences, the goal of archeology is to understand these ways of life, not merely to describe their remains. Marine Archeology is archeology that takes place underwater. Similar terms include nautical archeology, underwater archeology, and maritime archeology. Shipwrecks are often the focus of marine archeological investigations, but the same scientific techniques are also applied to other submerged structures such as ports and cities. Regardless of specific focus and terminology, these investigations share one important feature: their primary goal is collecting information that contributes to understanding the lives of people who built and used the things being investigated. Archeological activities are fundamentally different from salvage or treasure hunting activities whose primary goal is collecting objects and artifacts. In some cases, archeology may include artifact retrieval; but when objects are recovered primarily for their commercial or souvenir value, important archeological evidence is almost always destroyed.
Most marine archeological investigations involve six major steps:
1. Research - This provides the information needed for an overall project plan or research design, and describes the project’s objectives, background information, and methods that will be used to accomplish the remaining steps. Archival research is an important part of this step, particularly for investigations that involve finding shipwrecks whose location is not precisely known. Archives are basically collections of old records and publications, often in libraries and government offices. Searching archives can be very time consuming, but can provide important clues that save valuable field time during the search and investigation steps, as well as help understand the cultural context of the study site.
2. Search - Obviously, the location of an archeological investigation’s target site or shipwreck must be known before the investigation can begin. Searching for these targets can involve a variety of techniques. Specific search strategies are usually based on the results of background research, and may include:
- Visual techniques using divers, submersibles (manned submarines, ROVs, or AUVs), or aerial photography -- The most appropriate method depends upon the size of the area to be searched, depth, and environmental conditions. Aerial photography provides coverage of large areas and suitable images of the search area may already be available, but is limited by water depth and does not provide the visual detail available to divers or video cameras on submersibles. Divers are limited by water depth and environmental conditions, and are most effective for searching relatively small areas. Submersibles can cover much larger areas, provide good visual detail, and can operate in conditions that are hostile to free divers. But submersibles (especially manned submersibles) are expensive.
- Electronic techniques using magnetometers and gradiometers (discussed above)
- Acoustic searches using sonar or sub-bottom profilers (discussed above)
The two most common search techniques are sonar and magnetometer, because they cover ground very quickly and by water clarity which is often bad in places where ships wreck.
3. Investigation - This is the step that most people imagine when they think of ‘archeology,’ and usually involves several activities. Typically, a ‘first-order survey’ is conducted to record the site as it exists before anything else is done. At a minimum, this includes accurate mapping of the perimeter of the site and the location of major features within that perimeter. Traditionally, this mapping is accomplished by divers who make precise linear and angular measurements relative to a fixed baseline. For some sites, new ROV and AUV technologies are able to provide large amounts of information in much less time than would be needed for divers to collect the same data. After the first-order survey, a site plan is prepared by adding more details from photographic and possibly sub-bottom profiling data. Computer techniques such as geographic information systems are making it possible to analyze large amounts of information relatively quickly. ‘Site Surveyor’ is a widely-used computer program designed specifically for underwater archeological investigations. Completing a detailed site plan may be the end of fieldwork for some projects. Other investigations, though, may also include activities that physically alter the site. These activities include probing, collecting samples, and excavation. Probing with thin metal rods or other devices is used to locate structures beneath surface sediments. Samples of artifacts or other materials from an archeological site are collected for various analyses (such as radiocarbon dating) at a later time. Archeological excavations involve removing sediment or other material that obscures archeological material, finding and identifying the archeological material, and recording and recovering the material. The primary purpose of archeological excavations is to obtain useful information, so it isn’t enough to just collect artifacts. It is also essential to know where the artifacts are found on the site and their relationship to other features of the site. For this reason, archeological excavations are usually conducted layer by layer, either over a large area one layer at a time, or in small sections (such as trenches) one layer at a time. A variety of mechanical devices have been used for underwater excavations, including airlifts, water dredges, and water jets. Because these inevitably disturb the site and have the potential to destroy important archeological evidence, excavations require discipline, skill, and experience.
4. Post-Survey Research - Because the goal of marine archeology is to use physical evidence to understand past societies and cultures, every field survey must be followed by work that interprets the physical evidence produced by the survey. ‘Decoding the clues’ provided by this evidence is the step that justifies an archeological project. If artifacts have been removed from an underwater environment, they must be stabilized and preserved. This process often begins as soon as the artifacts are collected, since deterioration can set in rapidly once they are exposed to air and begin to dry. The stablization/preservation process may be the first time that artifacts are closely examined, and major discoveries are often made at this stage when archaeologists discover inscriptions or other important features. Other key post-survey research activities may include analysis of artifacts, scientific analyses (such as dating artifacts and samples, determining their chemical composition, anatomical studies of human remains), and additional research in historical records.
5. Cultural Resource Management - In many cases, past societies and cultures that are the focus of an archeological investigation are also part of the cultural history of present-day countries and cultures. The physical remains studied by archeologists are also part of this history, and many people want their history to be preserved and protected. If artifacts have been collected during the Investigation step, they must be properly preserved and stored for future study. In addition, investigation sites often require protection from looters and souvenir hunters. Cultural Resource Management plans may be developed to achieve this kind of protection. These plans typically include statements of the specific cultural resources to be protected, define the issues related to their protection, describe various interest groups who are concerned with protection for one reason or another (such as recreational divers, academic researchers, government agencies, non-governmental organizations, commercial interests, and/or the general public), identify ways in which the resources can be protected, and describe appropriate information and education activities.
6. Communication - An archeological investigation has very little value unless the findings of the investigation are made available to other archeologists and the general public. As in many other sciences, the evidence gathered by an investigation can often be interpreted several ways, and the interpretations can change dramatically as new evidence is produced by additional investigations. Archeological knowledge is almost always the sum of many investigations, and each one is important to assembling the overall picture. Publication of results and interpretations is an essential part of every archeological project.
For More Information
Contact Paula Keener-Chavis, national education coordinator for the NOAA Office of Ocean Exploration, for more information.
Other lesson plans developed for this Web site are available in the Education Section.