Exploring the Abyss
By Graham Lester, Director Hydroid Europe
GEOMAR & theABYSS AUV Study the S. Atlantic Seafloor
For most of human history, the size and depth of the Earth’s oceans have made it nearly impossible for surveyors and scientists to collect valuable and comprehensive data on the seafloor. In recent years, this led to the question: “Why is it that we cannot map our own planet as well as we’ve mapped the Moon and Mars?” Since the development of sonar instruments at the turn of the 20th century, surface ships have been the primary means of bathymetric data collection and seafloor study. The near extinction of sunlight and electromagnetic waves in general at depths beyond 100 m, however, limits optical imaging underwater. Acoustic signals of relatively long wavelength penetrate to great depths; however, the resulting bandwidth from a surface ship is fairly small. Thus, the high-resolution of optical and acoustic images obtained in shallow water is not readily possible at abyssal ocean depths—until very recently.
Recent advances in autonomous underwater vehicle (AUV) technology have revolutionized 21st century oceanography. AUVs routinely descend to depths that manned submersibles can reach only with great difficulty, great expense and high risk to human life. AUVs allow researchers to collect high-resolution seafloor data faster and more reliably in deep water than ever before.
One organization on the forefront of scientific AUV deployment is the German research facility GEOMAR – Helmholtz Center for Ocean Research Kiel. Using a specially designed REMUS 6000 type AUV called ABYSS, scientists at GEOMAR are exploring seafloor regions at depths up to 6,000 m, and have collected geological and bathymetric data of unprecedented quality. The ABYSS has explored several deep seafloor regions with tectonic and volcanic activity that was previously unmapped.
For example, recent multidisciplinary studies of hydrothermal vents on slow spreading ocean ridges have benefited greatly from data collected with the ABYSS. The studies reveal a remarkable diversity of hydrothermal systems in the Atlantic, Indian and Arctic oceans. When hydrothermal systems were first discovered on the East Pacific Rise and other Pacific Ocean ridges in the late 1970s, the community consensus held that the magma delivery rate of intermediate to fast spreading ridges was necessary to support black smoker-type high-temperature systems, associated chemosynthetic ecosystems, and polymetallic sulfide deposits. Contrary to the consensus, data gathered by Prof. Colin Devey, Scientific Head of the AUV Team at GEOMAR, reveal that hydrothermal systems occur not only on slow spreading ocean ridges, but they are also generally larger, exhibit different chemosynthetic ecosystems and produce larger mineral deposits compared to systems on intermediate to fast spreading ridges. In brief, hydrothermal systems occur in a much greater diversity of geologic settings, and awareness of the full diversity of hydrothermal systems on slow spreading ocean ridges has been aided greatly by data gathered with the ABYSS. This knowledge opens an exciting new frontier for ocean ridge exploration.
ABYSS, like all REMUS 6000 AUVs designed by Hydroid Inc., is a modular vehicle and has several different sensor options available to meet the requirements of the mission at hand. When deployed to survey seafloor volcanoes, the AUV is typically equipped with a Seabird SBE 49 FastCat CTD, an Edgetech 2200-MP 120/410 kHz side-scan sonar and a SeaBat Reson 7125 multibeam (200/400 kHz). On a mission-to-mission basis, the on-board multibeam echo sounder can be swapped with a 4 Megapixel monochrome AVT Pike camera or an Edgetech 2200-MP sub-bottom profiler (4-24 kHz). Additionally, the AUV has an onboard turbidity sensor which is used to detect plumes of mineral-rich water generated by black smokers, hydrothermal vents and other seafloor landforms which usually occur in areas of volcanic activity.
“The ability to operate these instruments from on-board a deep-water AUV like ABYSS rather than from a surface ship gives us several advantages,” said Prof Devey. “For one, we’re able to avoid the ‘wavelength handicap’ of surface ships, which must use relatively low-frequency sound waves in order to reach depths equivalent to those explored by ABYSS. The long wavelengths of these low frequency waves produce poor resolution images. Because we deploy these sonar instruments on ABYSS, which typically flies within a few hundred meters of the seafloor, we’re able to use high-frequency sonar typically reserved for shallow-water imaging, which provides much higher resolution data.”
In addition to providing significantly higher resolution imaging, AUV-mounted instruments also improve data quality by operating in a much lower-noise environment than their surface ship counterparts. Unlike surface ships, ABYSS is able to maintain a constant altitude for the duration of a mission, and is not affected by surface phenomena like chop, wake, near-surface bubble entrainment, and swell.
While onboard the support vessel or surfaced nearby, engineers can program and control the vehicle using the AUV’s Vehicle Interface Program (VIP), which is provided by Hydroid and is standard across the REMUS AUV line. The VIP software allows users to perform quality control checks, export data from a previous mission and program the AUV for future deployments. The software runs on a waterproof, ruggedized laptop, allowing it to be moved around the support vessel and not be damaged in severe weather.
A suite of navigation tools, both onboard ABYSS and its support vessel, allow the AUV to safely operate at depth. The AUV’s on-board Kearfott T-24 Inertial Navigation System (INS) and Teledyne RDI 300kHz ADCP/DVL are the backbone of its navigation suite. ABYSS can also receive additional support during its mission from a Long Baseline (LBL) positioning system, which enables it to occasionally fix its exact position while submerged. Finally, the AUV’s standard loadout includes an Imagenex 852 forward-looking Echo Sounder for detecting unexpected obstacles and a Paroscientific 8B7000 pressure sensor, which helps the vehicle maintain a consistent depth.
GEOMAR has used ABYSS to study seafloor volcanism around the world since its first cruise in 2009; so far, the vehicle has been used on scientific expeditions to the Greenland continental slope, the Southern Pacific, the Central Indian Ridge and the south Atlantic. Currently, the AUV is being used to survey previously unexplored hydrothermal vent sites in the Atlantic south of 10°S, with the aim of testing a model proposed by Prof. Devey for the relation of hydrothermalism and volcanism. This cruise is a continuation of previous work by GEOMAR studying the Mid-Atlantic Ridge, and will focus on the section between 13° and 33°S.
“Data from our previous expeditions suggests that hydrothermal activity is nearly constant on marked within-axis volcanic highs, but relatively rare at deeper parts of the axis,” said Prof Devey. “Since the individual seafloor volcanoes in the deeper parts of the ridges in this region likely have a timescale of activity on the order of thousands of years, we’re using geography as a proxy for time on this cruise. Instead of monitoring a single area of the ridge for an eruption to study, we’re using the long dive times ABYSS makes possible to cover roughly 100 km every 36 hours. At that rate, we expect to encounter and examine nearly 2,000 volcanoes during the expedition, and, using our conservative eruption rate estimates, at least two of those will have erupted in the last year. Data from these recent eruptions will be critical to testing our model.”
In addition to the AUV’s applications in seafloor volcano research, ABYSS has also been used by GEOMAR for numerous other projects, including deep sea mineral exploration.
(As published in the May 2013 edition of Marine Technologies - www.seadiscovery.com)