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Parks Canada 2012 Franklin Expedition
Secrets of the Seabed Revealed
Cutting-edge tools for bathymetric surveying allow the Canadian Hydrographic Service to visualize spectacular seabed detail and expose long-hidden secrets.
By Paul Holroyd
For hundreds of years, explorers used simple lead-weighted lines lowered from survey launches or sailing vessels to measure water depths. In all likelihood the Arctic explorers of the 1845 Franklin Expedition, on the Terror and the Erebus used such methods. This year and last, Canada's Hydrographic Service put modern seabed mapping technology into service in Victoria Strait, Nunavut, an area of Arctic waters where it is suspected those ships were lost. This technology has already proven successful for the location of other historic wrecks in Canadian waters.
Measurements in the old days were taken by dropping the leadline into the water and reading the depth from markings on the line once the lead at the end of the line reached the seabed, then pulling up the line and moving to another position. The locations of the depths were determined by taking astronomic observations or by measuring angles from known points on land to and sometimes from the vessel using a survey technique referred to as triangulation. Few depth measurements could be recorded this way - perhaps just 1,000 to 2,000 per survey - and the sparse nature of the soundings revealed little about the nature of the seabed or the secrets it may hide.
During the 20th century, survey launches became motorized and advances in electronics set in motion a series of revolutionary changes, such as today's satellite-based Global Positioning System (GPS). Sound became the new tool for measuring water depths, evolving to the point where a continuous line of soundings could be recorded. Collecting 500,000 to 750,000 soundings became typical for a hydrographic survey; however, little was known about the seabed between the sounding lines.
As the century drew to a close, multibeam sounders started to appear, able to use sound waves to image 100% of the seabed and generate 4-to-100 million soundings for a typical hydrographic survey. This meant, for the first time, hydrographers could generate three-dimensional images of the seafloor equal to what land-based mapping using aircraft and satellite imaging had been able to do for decades. This technological leap means that today the Canadian Hydrographic Service can not only map the seabed, but can also produce information about the seabed's characteristics (sand, clay, rock, etc.), a dramatic step forward in surveying and charting.
The remarkable changes in hydrographic surveying technology over the past 150 years, and what hydrographers can detect on the seabed are best illustrated in the following images.
The latest technological innovation uses lasers to measure water depth from an aircraft. LiDAR (Light Detection and Ranging), is showing promise as a method for quickly acquiring the same bottom coverage as multibeam, but at the speed of an aircraft, in water up to 30 metres in depth. Airborne bathymetric LiDAR is well suited for collecting data in the shallower waters of the near-shore area, where the highest number of underwater hazards are found.
Today's bathymetric data collected by the Canadian Hydrographic Service does more than keeping ships safe in Canada's waters. This comprehensive data has become an invaluable resource for ocean exploration and exploitation, marine science, engineering, ocean management, cable laying, fisheries management, military applications and now, marine archeology.
In fact, since multibeam sounders have become a common part of hydrographic surveys, numerous wrecks have been detected, such as the 98-year-old wreck of the Montmagny, shown below. The wreck was located lying in the St. Lawrence River, just east of Quebec City and discovered by Canadian Hydrographic Service hydrographers in November 2010.
In August 2011, the Canadian Hydrographic Service conducted survey operations in Victoria Strait, deploying two launches equipped with multibeam systems and an airborne bathymetric LiDAR system. While the primary purpose of the LiDAR surveys was to gather seabed information for nautical charting, the information could also be used to identify usual bottom features - like sunken ships - making it equally valuable to marine archeology. Over a six-day-period, the launches covered 140 square kilometres of new search area, while the LiDAR survey targeted an adjacent area of 380 square kilometres. The 2012 expedition will expand on that search.
While previous surveys and multiple expeditions that followed the 1845 voyage of Sir John Franklin and his ships have not yet revealed the whereabouts of the HMS Terror or HMS Erebus, many square kilometres of ocean have been ruled out.
The new, high-resolution bathymetric data collected during each mission will contribute to improved navigation safety in Arctic waters, through the production of enhanced nautical charts, directly reducing risks to marine traffic and the environment. These efforts make a direct long-term contribution towards economic prosperity in Canada's north.
For more information on the work of the Canadian Hydrographic Service, please visit www.charts.gc.ca
* Paul Holroyd is the Manager of Intellectual Property and Licencing for the Canadian Hydrographic Service in Ottawa, Ontario.
Avoiding a maritime accident is always far less costly than responding to one. The Canadian Hydrographic Service produces modern, accurate hydrographic surveys and nautical charts, which are the cornerstone of Canada's extensive maritime highway system, and crucial to Canada's economic prosperity.
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