Why Are Scientists Exploring Shipwrecks in Lake Huron?
A key purpose of NOAA’s Ocean Exploration Initiative is to investigate the more than 95 percent of Earth’s underwater world that until now has remained virtually unknown and unseen. Such exploration can reveal new natural and cultural resources, as well as provide new information and insights about human history on our nation’s coasts.
Lake Huron covers 23,010 square miles (59,596 square kilometers) on the border between Canada and the United States, and has been a significant focus of human activity for thousands of years. If the shorelines of its 30,000 islands are included, Lake Huron has the longest shoreline of the Great Lakes and is the second largest by surface area. The lake is also notorious for its dense fog banks, violent storms, rocky shoreline; and hazards that have brought disaster to many ships.
It is not certain when the first boats appeared on Lake Huron. Southern Michigan was probably occupied near the end of the last Ice Age (about 12,000 years ago), but northern Michigan probably was not occupied until several thousand years later. People in other parts of the world used boats since Neolithic times (the “Stone Age”; 8,500 - 5,200 years ago; see: This Old Ship, PDF, 272 Kb, for more information), and there is good evidence that boats may have been used when early inhabitants of North and South America migrated from Siberia about 13,000 years ago (see "By Land or By Sea or Both"?, PDF, 1.1 Mb). Archaic people in Michigan began to use fish sometime around 5,000 years ago, as indicated by artifacts such as bone or copper fishhooks, spears, notched pebble net-sinkers, and fish bones (especially sturgeon) found in upper Great Lakes sites. The point is, humans probably have been using boats on Lake Huron for a long time.
Physical remains of ancient voyages (i.e., shipwrecks) can provide information about trading patterns, sociopolitical networks, technological development and many other unique insights into early human cultures; but a variety of factors make it difficult to find such remnants. One factor in many coastal regions is that water levels have changed significantly since humans first arrived in North America. The level of Lake Huron, for example, has varied from 55 - 80 m above mean sea level about 9,900 - 7,500 years ago to its present level of 176 m above mean sea level. This means that artifacts from early human activity around Lake Huron may now be more than 120 m below the lake’s surface! Recently, archaeologists have discovered evidence of prehistoric hunters on a submerged ridge (20 - 40 m deep) that was above the surface of Lake Huron 9,900 - 7,500 years ago (O’Shea and Meadows, 2009).
In addition to changing water levels, a major obstacle is the very thing that makes ancient shipwrecks so valuable - their age. Plant and animal materials used in the construction of ancient boats are rapidly deteriorated by biological and chemical processes, so that only a few (if any) traces remain after thousands of years; unless something interferes with these processes.
Several things interfere with the deterioration of shipwrecks in Thunder Bay. Low temperatures tend to slow the rate of biological and chemical deterioration (which is why food keeps longer in refrigerators. Freshwater is much less corrosive to metal artifacts than saltwater. Shipwrecks buried in sediments can be amazingly well-preserved, because sediments often have low levels of oxygen which is required by many deterioration processes. The Ocean Explorer Thunder Bay Sinkholes 2008 Expedition explored areas in Lake Huron with very low oxygen levels. Similar areas may contain shipwrecks much older than those discovered to date.
The Thunder Bay National Marine Sanctuary (TBNMS) was established in 2000 to protect one of the nation’s most historically-significant collections of shipwrecks. The present boundaries of the TBNMS enclose 448 square miles that contain 40 known historic shipwrecks. Plans are well underway, however, to expand these boundaries to include 3,662 square miles. Archival records indicate that the expanded boundaries include more than 100 undiscovered shipwrecks which can provide unique opportunities for historians and archaeologists to study the maritime and cultural history of the Great Lakes region, as well as for recreational explorers. Finding the exact location of these shipwrecks is obviously essential to these kinds of uses, as well as to the protection of these important cultural resources.
To help meet this need, in 2008 a remote sensing survey was undertaken in the northern portion of the proposed expansion area. This survey used a side scan sonar towed from a research vessel, as well as a conventional sonar system mounted on an autonomous underwater vehicle (AUV; sonar and AUVs are discussed below in greater detail). The towed side scan sonar was found to be most useful for providing high quality sonar images over moderately large areas, while the AUV was most effective for surveying areas that are too shallow for survey vessels to operate.
The 2008 survey covered an area of about 100 square miles and located two new shipwrecks. The total proposed expansion area is much larger, though, so a third survey strategy is needed to efficiently cover large areas of deep water. As its name suggests, the Thunder Bay 2010: Cutting Edge Technology and the Hunt for Lake Huron’s Lost Ships Expedition will use state-of-the-art technology that includes a sophisticated AUV carrying a one-of-a-kind precision sonar system to survey up to 200 square nautical miles in the proposed expansion area.Expedition Questions
The Thunder Bay 2010: Cutting Edge Technology and the Hunt for Lake Huron’s Lost Ships Expedition is directed toward three key questions:
- What are the locations of shipwreck sites within the potential expanded boundaries of the Thunder Bay National Marine Sanctuary (TBNMS)?
- How can new sonar and AUV technologies be most effectively used to survey large, deep areas to best serve TBNMS and the archaeological community?
- How can activities of the Expedition be used to foster among the public an increased appreciation for America’s maritime heritage and advanced marine technology?
Forward-Looking Sonar - 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 time elapsed between the emission of the sound pulse and the reception of the echo is used to calculate the distance of the object. Some sonar systems also measure the strength of the echo, and this information can be used to make inferences about some of the reflecting 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.
There are many specialized varieties of sonar. Two of the most widely used are side-scan sonar and multibeam sonar. Side-scan sonar systems 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 seafloor 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).
Side-scan sonar systems do not provide bathymetric data. When this information is needed, multibeam sonar systems are used. A multibeam system uses multiple transducers pointing at different angles on either side of a ship to create a swath of signals. The time interval between signal transmission and return echo arrival is used to estimate depth over the area of the swath. In some systems, the intensity of the return echo is also used to infer bottom characteristics that can be used for habitat mapping.
The Thunder Bay 2010: Cutting Edge Technology and the Hunt for Lake Huron’s Lost Ships Expedition will use another type of sonar called ATLAS, which stands for Autonomous Topographic Large Area Survey. This is a type of forward-looking sonar that provides an image of objects in front of the vessel on which the sonar is mounted. The ATLAS system, developed by the Applied Research Laboratory at the University of Texas, uses cell phone technology to make the system smaller than earlier types of forward-looking sonar. In addition, the ATLAS system provides intelligent control of the AUV that carries the system using software currently in use on Mars Rover robots. Mounted on a REMUS 600 AUV, the ATLAS system can efficiently search large deep-water areas with approximately ten (10) times the coverage of traditional sonar.
Autonomous Underwater Vehicles - Autonomous Underwater Vehicles (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.
The REMUS (which stands for Remote Environmental Measuring UnitS) 600 AUV used to carry the ATLAS sonar system was designed by Woods Hole Oceanographic Institution and built by Hydroid Inc. [NOTE: Mention of proprietary names does not imply endorsement by NOAA]. REMUS 600 operates to depths of 600 meters (with modifications its depth range can be extended 1500 meters), and can operate for missions lasting up to 70 hours. REMUS AUVs were designed as low-cost vehicles that can be operated with a laptop computer. Instruments typically include an Acoustic Doppler Current Profiler, side scan sonar, conductivity and temperature profiler, and a light scattering sensor. Many other instruments can be carried depending upon mission needs, including fluorometers, bioluminescence sensors, radiometers, acoustic modems, forward-looking sonar, altimeters, and Acoustic Doppler Velocimeters, video plankton recorder, and a variety of digital cameras. For more information, see http://www.whoi.edu/page.do?pid=29856.
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Director, Education Programs
NOAA Office of Ocean Exploration and Research