NRL Scientists Host Workshop on Systematic Global-Ocean Sea Floor Imaging Mission

8/8/2000 - 11-00r
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A systematic international effort to map the entire global ocean floor to at least 100 meters spatial resolution is a worthwhile and feasible project to begin the new millennium, say scientists from the Naval Research Laboratory's Marine Geosciences Division. Dr. Peter Vogt and his colleagues laid out the research issues of such a project at the 1999 American Geophysical Union (AGU) meetings in Boston, MA, and further developed the concept in the June 6 2000 issue of the AGU publication, Eos. Informal dialogues with academic and other government scientists have consolidated thoughts on the concept, dubbed GOMaP (Global Ocean-floor Mapping Project). Open public access to all GOMaP data would be an essential ingredient for this project.

The "resolution" of an image, whether a photo or a geophysical map, is its sharpness. In an image with a spatial resolution of 100m, objects of about this diameter or width are barely detectible. Dr. Vogt and his collaborators note that most or all of the Moon, Mars, Venus, and several of the Jovian moons have been imaged to 100m or better resolution in recent years, while our own planet's ocean floor continues to be mapped piecemeal by many institutions for many different purposes, using a variety of instruments of various accuracies and resolutions. Only a few percent of the sea floor has been mapped -- by acoustic backscatter imagery (sidescan sonar) and swath bathymetry (echo-sounding) -- to the 100m resolution returned from our Moon by the NRL-engineered Clementine mission. Even this small percentage forms an inhomogeneous crazy-quilt coverage concentrated near industrialized nations and a few sections of the Mid-Oceanic Ridge.

Sea floor imaging in the visual spectrum -- the basis for our new views of the Moon and Mars -- can be achieved from above only in very shallow, clear water. Several operational systems can map sea floor topography from aircraft, using scanning lasers. NRL's Remote Sensing Division has demonstrated shallow water sea floor imaging by hyperspectral scanning, using passive illumination by sunlight. In most of the oceans, where the water depths are generally 3,000 to 6,000 meters, visual-spectrum imaging can only be done from manned submersibles or unmanned, remotely operated or towed instruments deployed close to the ocean bottom. High frequency sonars can also be deployed on such platforms, complementing the visual-spectrum techniques.

To date, the best homogeneous image of the global ocean floor is actually the ocean surface shape, called the geoid, which largely reflects sea floor topography. The ocean geoid has been mapped using satellite radar altimetry, as first proposed by NRL's Dr. Benjamin Yaplee in 1969. Although of great scientific value ­ as demonstrated by NRL and other geoscientists ­ this sea floor "image" is some 100 times poorer in spatial resolution than images returned from the Moon by the Clementine mission.

Dr. Vogt and his colleagues at NRL's Marine Geosciences Division and elsewhere had been studying the Norwegian-Greenland Sea geology and geophysics for more than a decade using 2 to 20 km-resolution techniques. While much was learned at these spatial scales, only subsequent 10 to 100m (and better) resolution sea floor acoustic imagery collected by NRL revealed the wealth of previously unknown features and processes, including an active sea floor mud volcano. Similar revelations by other scientists in other oceans only hint at the new discoveries that GOMaP could return.

Imaging the entire global ocean floor at 100m spatial resolution is no mean feat; at about 370 million square km, the area of our planet's water-covered lands exceeds the area of Mars plus three Moons. However, the task can be done now with state-of-art surface-ship-based technology, or perhaps, in some areas, by small fleets of autonomous underwater vehicles at higher resolution, with appropriate technological advances in the near future. However, platforms move through the water at speeds of only meters per second, not kilometers per second like orbiting spacecraft.

Although only a small percentage of the ocean floor lies at very shallow depths (less than 50m), the narrow sonar swath widths in shallow water translates into large amounts of ship time. However, the resolution of imagery is progressively better as water shoals , and may be better than 1m in waters less than 50m deep.

With support from the Office of Naval Research (ONR), NRL hosted a small pilot workshop in Bay St. Louis, Mississippi, June 12-14 2000. A small group of experts from academia, the commercial sector and U.S. government agencies assembled to examine critically the technological and data-handling/data-analysis issues of GOMaP. The workshop considered the Juan de Fuca tectonic plate (off Washington and Oregon) and a large part of the Gulf of Mexico as "pilot" areas where GOMaP technology can be demonstrated and evaluated. Attendees agreed that GOMaP was feasible and of great potential value for mankind. A larger, more international conference will be planned for 2001.

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