U.S. Naval Research Laboratory
Marine Physics Branch (Code 7420)
GLOBAL OCEAN MAPPING PROJECT
The Global Ocean Mapping Project (GOMaP)is a conceptual plan to map all the world's ocean floors, from shorelines to deep-sea trenches (GOMaP: A Matchless Resolution to Start the New Millennium). A complete map of our "water planet's" ocean floors would have enormous value to mankind, not only for basic science, but for geologic hazard assessment (earthquake faults, submarine landslides, volcanism, etc), mineral resources (fossil fuels, phosphate and manganese nodules, sand and gravel, etc), navigation hazards, fisheries, and archeological remains (shipwrecks etc.).
In the last few decades, increasingly detailed (high resolution) mapping projects have revealed the surfaces of the Moon, Mars, Venus, several asteroids, and the larger moons of Jupiter and Saturn to resolutions of better than 100 meters (about the size of a football field), and in some cases (for example the Jovian moon Europa, and Mars) to better than 10 meters (the size of a large automobile). Planetary and lunar mapping missions have generated numerous research publications. The Earth's oceans (comparison between earth, earth oceans, moons, etc) cover an area larger than one Mars sized planet plus three Moons. Only a small part of the ocean floor has been mapped to the 100 meter resolution the Magellan radar mapping mission achieved on Venus, and areas the size of cities have scarcely or never been crossed by a survey ship. GOMaP would bring knowledge about our own ocean floors up at least to that we now have for Venus. Seafloor mapping to date has been highly inhomogeneous, a crazy-quilt of sounding lines to and from major ports and local areas mapped in detail for purposes of basic research, hydrocarbon exploration, or other purposes (current track coverage map). The seafloors of the southern oceans are especially poorly known.
If water oceans were present on Earth’s moon, the lunar ocean floors might well have already been mapped by remote platforms.
The primary GOMaP goal is to achieve at least 100% coverage of the seafloor in bathymetry (depth contours)and in sidescan sonar imagery. Any and all additional instruments and observers would be taken on the mapping platforms (mainly ships), assuming these do not compromise the basic seafloor mapping mission. Examples of such additional "piggy-backed" measurements include the gravity and magnetic fields, and seismic reflection profiles, revealing the structure of the sediments and underlying volcanic rocks below the GOMaP vessels, which may also carry various oceanographic and meteorological sensors, and even observers conducting a marine mammal census.
GOMaP is expected to take over 200 ship-years for ocean depths exceeding 500 meters. The narrower sidescan and bathymetric swath widths (swath width figure) in shallower water mean that the shallowest 10% of the ocean will probably take several times as much ship time as the deepest 90%. In very shallow water, LIDARS (Light Detection and Ranging) deployed on aircraft may be the "platform of choice" for seafloor mapping, at least where the waters are clear.
GOMaP is envisioned as a fully international program, with data to be shared by all. The entire project might reasonably take 20 to 30 years. Various nations would opt to map certain "boxes" of greatest interest, and in whatever priorities they choose. For GOMaP purposes, the "survey boxes" need to be large, of standard size, and completely mapped.
The GOMaP concept was evaluated and endorsed at a special conference held in Bay St. Louis, Mississippi in early June 2000 (Endorsement of Global Ocean Mapping Project). Meeting attendees, from various US government agencies, universities, and industry agreed that the technology is currently mature enough to begin such a project, subject to international adoption of minimum standards for data accuracy, resolution, and navigation accuracy.
GOMaP and the Naval Research Laboratory
Early History of US Navy seafloor mapping
Why did the GOMaP concept (GOMaP: A Matchless Resolution to Start the New Millennium and Endorsement of Global Ocean Mapping Project) originate at NRL? The US Navy, and in particular the Naval Research Laboratory, has a long tradition of ocean floor research-including development of the tools to map ocean floors, and the use of such tools to make new discoveries about seafloor features and processes.
Many years before academic institutions turned their attention to the ocean floors, Lt. Matthew Fontaine Maury and his coworkers at the US Naval Observatory and Hydrographical Office (forerunner of the present Naval Oceanographic Office) collected wire soundings at various locations in the North Atlantic, to support the installation of undersea telegraph cables. The soundings and sediment samples stuck to the end of the plummets formed the basis for the first textbook on seafloor geology (Maury, 1855). Maury can be said to have discovered the Mid-Atlantic Ridge, which he labeled "Middle Ground" on his, the first deep-ocean bathymetric chart.
Acoustic depth sounding began with the first operational echo sounder, in 1921, by Harvey Hayes, who would become the first director of the Acoustics Division at the US Naval Research Laboratory, created in 1923. The sound-speed structure of the oceans in space and time (e.g., Hurdle, 1986) was a subject of NRL acoustic research, one which would become vital to the accurate conversion from round-trip echo time to water depth. Accurate seafloor mapping such as proposed under GOMaP continues to depend on knowledge of this sound speed structure.
Swath bathymetric and sidescan sonar seafloor mapping by Naval Oceanographic Office (NAVOCEANO) and Naval Research Laboratory (NRL)
GOMaP-level bathymetry can only be accomplished with multi-narrow beam ("multibeam") bathymetry, which was first developed and deployed by the US Naval Oceanographic Office in the 1960s (Glenn, 1970; 1976). In later decades, this "SASS" (Sonar Array Sounding System) would become the prototype for the many commercial variants now in use in the world's oceanographic fleet.
Extensive areas mapped with the SASS system profoundly revised ocean floor morphology, although only somewhat smoothed versions of the contours have been placed in the public domain. Features mapped with SASS include seamounts in the far western Pacific (Hollister et al., 1978), eastern Pacific guyots (Vogt and Smoot, 1984), and the northern Mid-Atlantic Ridge eastern flanks ca. 55 to 60 N (Johnson et al., 1971) and from 49 to 51 N (Johnson and Vogt, 1973). A special SASS survey of the FAMOUS area of the Mid-Atlantic Ridge rift zone (Phillips and Fleming, 1978) was done to prepare for the first ALVIN dives there: Submersible pilots in their stocking feet walked over multibeam bathymetry and NRL's LIBEC seafloor photomosaics (Brundage and Patterson, 1976) laid out on the NRL gymnasium floor. Magnetic data routinely collected with the SASS data broke new ground in plate tectonic understanding (e.g., Vogt and Avery, 1974), and the same will no doubt be true for magnetic data collected on future GOMaP cruises in remote ocean areas.
While SASS surveys upgraded bathymetry in parts of the North Atlantic, the new paradigm of plate tectonics also influenced the way cartographers interpreted sparse bathymetric data. Together, these two developments explain much of the difference in two relief models of the North Atlantic issued by Navy institutions more than 15 years apart (Anonymous, 1963; Anonymous, 1979).
In more recent years, NRL has partnered with academic institutions in various deep-sea mapping campaigns. For example, SEABEAM mapping cruises to the southern Mid-Atlantic Ridge axial region in the middle to late 1980s produced much new morphologic knowledge and a spate of publications (e.g., Grindlay et al., 1992). As NRL has done in several other ocean areas, these and other multibeam bathymetric data were incorporated, for practical purposes somewhat smoothed, into regional bathymetric charts (e.g., Cherkis et al., 1989).
The Nordic Basin is another ocean area investigated by NRL and collaborators using swath mapping techniques, in some areas achieving 100% coverage with both sidescan sonar and swath bathymetry. The 1990 NRL investigation of the extinct Aegir Ridge (Norway Basin), on board the LDEO vessel R/V Maurice Ewing (Jung and Vogt, 1997;Vogt, 1997) was the world's first expedition with all possible parameters measured simultaneously: Multibeam bathymetry (Hydrosweep) with tracks close enough for 100% coverage of the seafloor; SeaMARC II sidescan sonar and swath bathymetry (almost 200% sidescan coverage); 3.5 kHz profiler; three-channel airgun seismic reflection; magnetics and gravity. This 1990 NRL seafloor mapping expedition can be considered a GOMaP prototype, assuming incorporation of recent improvements in multibeam technology, acquisition of sonar imagery from the multibeam data (vs. collection by a separate towed sidescan system), and pixel navigation to GOMaP standards.
Farther north in the Nordic Basin, NRL cooperated with the University of Bergen to map parts of the Knipovich Ridge, a slow-spreading northern part of the Mid-Atlantic accreting plate boundary. Hull-mounted multibeam was not available on the survey vessel (F/S Haakon Mosby), so both bathymetry and sidescan imagery were acquired by the towed SeaMARC II system. The mapping results (Crane et al., 1995) illustrate both the merits and limitations of such towed systems. Although the coverage of much of the area approached 100% in sidescan imagery, and much new was discovered (e.g., Vogt et al., 1999) neither the resolution nor the swath width of the SeaMARC II-derived bathymetry would meet proposed GOMaP standards.
The superior spatial resolution of sidescan sonar imagery, compared to coregistered swath bathymetry (even with hull-mounted multibeam systems) is illustrated by several kinds of features discovered in the sidescan, but not apparent in the swath bathymetry, on both the 1989-90 Haakon Mosby and 1990 Maurice Ewing expeditions. The features include widespread mounds (Vogt, 1997), as well as pockmarks, glacigenic mudflows, and a 1-km diameter, 5-10 m relief mud volcano (Vogt et al., 1999). Once discovered in the sonar imagery, however, these small seafloor features (25-1000 m horizontal spatial scales) could easily be detected on nadir bathymetric profiles, and a contour map constructed with close-spaced, GPS-navigated narrow single-beam tracks.
Whether discovered only in sidescan imagery or also in the swath bathymetry, new or special seafloor features and processes invited subsequent investigations by deep tow video, submersible dives, coring and dredging, biological and oceanographic sampling, and the many types of shore-lab analyses that have been and are still being conducted on samples retrieved from selected features first discovered by systematic mapping in the Nordic Basin (e.g., Vogt et al., 1999, and other articles in vol.19 of Geo-Marine Letters). This pattern of discovery and subsequent close-up investigations will no doubt be repeated many times once GOMaP gets underway. However, we do not include this follow-up research under GOMaP, which is envisioned as the "road map" to detailed, process-oriented seafloor research.
Satellite Radar Altimetry maps seafloor topography: NRL and NAVOCEANO roles
The world oceans are now reasonably mapped to spatial scales of ca. 10-20 km with the help of satellite altimetry, and the geoid perturbations represented by ocean surface undulations of from a few centimeters to several meters in relief. Today, the best grid of global ocean depth to ca. 20 km spatial resolution are those by Smith and Sandwell (e.g., 1997), which combine available bathymetry with predictions from geoid perturbations, reflecting seafloor mass distributions mainly in the form of bathymetry. The new global bathymetry is in the form of a grid, basically an upgrade of the 5'x 5' global bathymetry database created at the Naval Oceanographic Office (DBDB-5, later incorporated into a global land-and-sea database called ETOPO-5 [Earth Topographic Database at 5’ latitude, 5’ longitude grid spacing]). DBDB-5 (Digital Bathymetric Data Base) itself was descended from the first global digital database, also developed at the Naval Oceanographic Office, under the name SYNBAPS (Van Wyckhouse, 1973).
The use of a microwave (radar) altimeter to map the ocean geoid was first proposed by NRL researcher Benjamin Yaplee (Shapiro and Yaplee, 1970). In the 1970s and 80s, the SEASAT and GEOSAT radar altimetry missions (e.g., Anderle, 1986) were in large measure Navy-supported and justified; the use of radar altimetry to predict seafloor topography in poorly mapped ocean areas (e.g., Vogt and Jung, 1989) and to understand and model ocean floor tectonic processes (e.g., Jung and Vogt, 1992, 1997a) were incidental research applications for those missions. NRL was the first to run subsatellite "ground-truthing" research ship tracks "under" representative SEASAT and GEOSAT satellite tracks (e.g., Vogt et al., 1984; 1992); the bathymetry collected along these tracks continue to be important for "calibrating" the seafloor topographic prediction schemes of e.g. Smith and Sandwell (1997). At the same time, NRL’s P-3 based aerogeophysics capability (Brozena, 1984 and subsequent publications), considered the best for long range missions, can supplement and improve satellite altimetry-based predictions in three ways: 1) The ocean surface and gravity field can be measured in coastal, lacustrine and riverine water areas, where the satellite altimeter's footprint is corrupted by land returns; 2) Aerogravity and aircraft-based altimetry can separate the oceanographic and meteorological effects on the ocean surface from the geoidal ones, which satellite altimetry cannot completely accomplish; 3) The magnetic anomaly field can be measured at the same time as the gravity, at scales suitable for plate tectonic applications. This cannot be done at the elevation of orbiting earth satellites.
While satellite altimetry has played a profound role in upgrading maps of ocean floor topography at spatial scales greater than ca. 10km, GOMaP seafloor mapping will return bathymetry at spatial scales up to two orders of magnitude better, and sidescan sonar imagery up to three orders of magnitude better, even at normal ocean depths. Seafloor topography and geology is not, in general, fractal in nature, so that short wavelength bottom characteristics in general cannot be predicted from longer wavelengths. Altimetry-derived bathymetry is excellent for resolving the plate kinematic " big picture" fabric-major transform faults, seamount chains, extinct and active rift valleys, etc. However, many seafloor processes, including most of the active ones, cannot be detected by satellite altimetry.
Examples include the mounds, pockmarks, mud volcanoes, glacigenic mudflows mentioned above, as well as hot vents, fault traces, small volcanic cones, and fissures along the active rift valleys, and probably many other features not yet imagined.
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Anonymous, 1963, North Atlantic Ocean: Basin Relief Model, USN PIC 910/60 U, US Naval Photographic Interpretation Center, Washington, DC (relief chart)
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