Chapter 7: Bathymetric Mapping

The Southern Indian Ocean west of Perth, Australia is largely uncharted, even now, a year after the search for MH370 began. A few small-scale efforts to scan the ocean floor in that area date back to the 1960s (e.g., National Geographic in 1967), a few more before and after 2000; but none provided a comprehensive picture of the underwater terrain that search vessels and crews had to deal with to search the ocean floor.

The extent of what was known initially was that it is deep everywhere, and rugged in places. The best example of rugged was 1,900 kilometers west of Perth along an east-west gash or rift in the seafloor known as Broken Ridge to the west and the Diamantina Trench to the east. The name “Broken Ridge” matches its features: cascading Yosemite-style 1,500 to 2,000 meters into a nameless basin to the south of the area MH370 was believed at the time to have ended its flight.

It was south of Broken Ridge that Go Phoenix was eventually tasked. The minimum ocean depth in the area is -4,000 meters. That extends about 580 kilometers (360 miles) southwest of Broken Ridge along the 7th Arc before seafloor depth moderates a bit to -3,900 meters. Farther south still from Broken Ridge, an under-sea feature known as Southeast Indian Ridge, averages about -2,500 meters in depth. Depths such as the latter are more conducive to underwater sonar work, but crews had to follow the most likely flightpath, not simply pick relatively shallow underwater locations.

Satellite bathymetry exists, of course, and it is rapidly getting better, but it is not yet good enough to identify all obstructions that could damage or destroy sidescan sonar equipment. Vessels do not always carry backup sonar units, and it was deemed important to avoid delays if at all possible. Towfish, as the sonar units that were to be used are called, are expensive. Upwards of $1 million US each. Moreover, replacing one would require a 5,000 kilometer round-trip to Fremantle.

The images below help illustrate the progress Fugro Equator made between June 2014 and December 2014. Equator was able to work at a relatively rapid pace during the bathymetric phase. The objective was to map underwater terrain, not identify distinctive features that might have been the plane. Equator consistently scanned at 6 knots or more during that phase, about 3 times faster than any of the vessels would be able to work once they commenced sidescan sonar scanning in October 2014.

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The following images are from satellite bathymetry and show part of the 2014 General Bathymetric Chart of the Oceans (GEBCO). It is annotated and detailed. Satellite bathymetry has come a long way in the last ten years, and will continue to improve dramatically during the next five years. Enough to find sunken vessels and planes? Probably not; but if it can replace all or part of the tedious vessel-based effort required now, it will be enormously valuable.

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Bathymetric Video
This 36 second visualization of the Seventh Arc seafloor was prepared by Geoscience Australia to help next of kin and others understand what the bathymetric task entailed.

 

Actual Broken Ridge Seafloor Images
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Indian Ocean Depths on the Seventh Arc
The third-deepest part of the Indian Ocean is also sometimes said to be “the deepest”, but it isn’t. That is “Java” or “Sunda” Trench. NOAA has measured it at -7,291 meters below the surface. (Wikipedia currently indicates that the deepest point in Sunda Trench is -7,725 meters below the surface; but does not cite a source and is flagged for correction.)

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At -7,431 meters, the Diamantina Trench, or Diamantina Deep, is the second-deepest point in the Indian Ocean. It is often said to be the deepest part of the Indian Ocean, but it isn’t.

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Even if NOAA’s -7,291 finding is a little light, the deepest water is 1,951 kilometers southwest of Sunda Trench at the base of the highest Seamount in the area: Zenith Plateau. Zenith rises to within -1,753 meters of the surface of the Indian Ocean; on its southwest side is a nameless escarpment with a rugged, irregular valley floor that separates Zenith Plateau from Lost Dutchman Ridge. The deepest part of the valley is -7,883 meters below the surface, and is the deepest point on the Seventh Arc. It is also the deepest point in the entire Indian Ocean.

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The Big Picture

This graphic shows where each of these important seafloor features are in relation to the all-important Seventh Arc. While average seafloor depth south of Broken Ridge is a respectable -4,000 meters, it is half again that much between Zenith Plateau and the Sunda Trench.

 

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