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Mid-Ocean Ridge Exploration with an Autonomous Underwater Vehicle

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Mid-Ocean Ridge Exploration with an Autonomous Underwater Vehicle or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: [email protected] ofor Th e The to: [email protected] Oceanography approval Oceanography correspondence all portionthe Send Society. ofwith any permitted articleonly photocopy by Society, is of machine, reposting, this means or collective or other redistirbution This article has This been published in S P E C I A L Iss U E O N O C E A N E XPL O RATI on B Y D A N A R . Yo E R G E R , A L B E R T M . B R A D L E Y, MICHAEL JAKU B A , Oceanography M A U R I C E A . T I V E Y, C HRI S TO P H E R R . G ER M A N , journal of The 20, NumberOceanography 4, a quarterly Society. , Volume T I mo T H Y M . S H A N K , A nd Rob E R T W. Emb LEY Mid-Ocean Ridge C Exploration with an The 2007 by opyright Oceanography Society. Autonomous Underwater A Vehicle ll rights reserved. Human-occupied submersibles, towed sors, and cameras in the rugged deep-sea preclude acoustic contact with the sup- vehicles, and tethered remotely operated terrain that has been the focus of numer- port vessel. Unlike towed assets, ABE P ermission is granted to copy this article for use in teaching and research. article use for research. and this copy in teaching to granted is ermission vehicles (ROVs) have traditionally been ous scientific expeditions (e.g., those to can travel close to the seafloor along used to study the deep seafloor. In recent mid-ocean ridges and ocean margin set- well-controlled tracklines, enabling high- years, however, autonomous underwater tings). The Autonomous Benthic Explorer resolution seafloor imaging through a vehicles (AUVs) have begun to replace (ABE) is an example of an AUV that has variety of modalities. ABE generally col- these other vehicles for mapping and been used for over 20 cruises sponsored lects co-registered bathymetric and mag- survey missions. AUVs complement the by the National Science Foundation netic data while determining its position capabilities of these pre-existing systems, (NSF), the National Oceanic and through a combination of long-baseline offering superior mapping capabilities, Atmospheric Administration (NOAA) acoustic transponders, a Doppler veloc- improved logistics, and better utilization Office of Ocean Exploration (OE), and ity log, and a fluxgate magnetic compass P of the surface support vessel by allowing international and private sources. This (Yoerger et al., 2007). Equally important, O Box 1931, other tasks such as submersible opera- paper summarizes NOAA OE-sponsored ABE also measures water-column prop- tions, ROV work, CTD stations, or mul- cruises made to date using ABE. erties using dual conductivity/tempera- R ockville, MD 20849-1931, R epublication, systemmatic reproduction, reproduction, systemmatic epublication, tibeam surveys to be performed while the ABE commonly operates with full ture probes, an optical backscatter sen- AUV does its work. AUVs are particularly autonomy; after launch, the vehicle sor, and a redox potential probe. well suited to systematic preplanned sur- completes its mission without human Since its initial trials in 1994, ABE has veys using sonars, in situ chemical sen- intervention, often at distances that completed 210 deep-ocean dives, cover- U S A . 52 Oceanography Vol. 20, No. 4 ing over 3600 km of bottom tracks at an 2003). Alvin safety considerations pre- and extinct hydrothermal sites (Tivey average depth exceeding 2000 m (Yoerger cluded simultaneous operations, so ABE and Johnson, 2002). et al., 2007). The mid-ocean ridge has averaged about 7.5 hours of bottom Near 86°W, ABE made four dives at been the focus of most of these efforts, time over seven dives, covering a total of a nominal survey height of 40 m and particularly sites with hydrothermal 107 km of bottom tracks. During these a trackline spacing of 60 m to produce activity. Some cruises featured detailed dives, ABE collected high-resolution a bathymetric map of the rift valley. study of previously discovered sites, while bathymetric data using a 675-kHz scan- This survey included the Rosebud site other cruises focused on unexplored ning sonar with a nominal pixel resolu- discovered by scientists in Alvin on the areas. NSF funded initial ABE develop- tion of 2–5 m with concomitant record- same cruise. ABE also identified sev- ment and field deployments. Since 2002, ing of conductivity and temperature data. eral smaller vents, primarily through five cruises have been sponsored by the One key objective of our 2002 cruise temperature anomalies on the order of NOAA OE program, accounting for was to revisit the Rose Garden site that 50 millidegrees that were later investi- about one-third of ABE’s at-sea time. was first discovered in 1979 by scientists gated using Alvin. CTD casts and tow- During these cruises, we made many in Alvin and revisited in 1988 and 1990 yo runs showed the water column to be important additions to ABE’s technical (Hessler et al., 1988). This site’s lush well stratified near the seafloor, which Mid-Ocean Ridge capabilities. Those funded by NOAA OE faunal communities that featured dense made the small temperature anomalies include the addition of a multibeam tubeworm clusters made it an icon of caused by the vent plumes conspicuous. mapping sonar, installation of a Doppler hydrothermal vent research. We found, In each case, the combination of ABE’s Exploration with an velocity log, and development of an however, that the Rose Garden site had bathymetric mapping and delineation anchoring system to allow ABE to “park” been paved over by lava flows since the of water-column anomalies enabled sci- itself on the seafloor at the end of a dive last visit by Alvin. No sign of the vent entists in Alvin to find the vent. These Autonomous Underwater or in the event of a serious fault. This site was found by either ABE or Alvin, exercises illustrated the value of an AUV- capability greatly improves our ability nor did we find any signs of previous based vent search, and showed that tem- to operate ABE unattended, freeing up dives such as markers or dive weights. perature anomalies encountered at our Vehicle the vessel for other work. It also greatly The absence of temperature anomalies in nominal survey height could be used to improves ABE’s ability to work simulta- ABE data helped confirm that our inabil- find vents, at least under favorable con- neously with ROVs. GALÁPAGos 2002 EXPEDITIon Unlike towed assets, ABE can travel close The goals of our Galápagos expedi- tion were to revisit the sites where to the seafloor along well-controlled hydrothermal vents were first discov- tracklines, enabling high-resolution seafloor ered in 1977 and 1979 in the vicinity of 0°48´N, 86°13´W and to study how imaging through a variety of modalities. those sites changed in the ensuing years. We also surveyed unexplored sites near 89°W. This cruise was supported by ity to detect hydrothermal vent features ditions. Figure 1 shows the assembled the NOAA OE program, NSF, and the from Alvin was not due to navigational bathymetric map, the locations of the Woods Hole Oceanographic Institution. error. These conclusions were also sup- measured temperature anomalies, and We operated ABE cooperatively with ported by ABE magnetic data, which the locations of the vent sites. DSV Alvin, with ABE performing sys- showed a reduced zone of magnetism at The second site we visited on the tematic mapping at night while the Alvin the paved-over Rose Garden site (Shank Galápagos Spreading Center, near 89°W, dives focused on detailed inspection and et al., 2003). Such reduced zones of mag- taught us more important lessons. Three sampling during the day (Shank et al., netism have been associated with active dives in this area enabled production Oceanography December 2007 53 Figure 1. A bathymetric view of the floor of the Galápagos Spreading Center rift valley at 86°W obtained with a scanning sonar and gridded at 5 m. The red crosses indicate locations where temperature anomalies were detected byABE while surveying ~ 40 m above the seafloor, including the nascent Rosebud vent and the small ALR vent. The highly stratified water column made tempera- ture anomalies as small as ~20 millidegrees conspicuous. No sign of hydrothermal activity was found near the Rose Garden site. ABE data showed a magnetic low at the reported Rose Garden location, a result consistent with previous hydrothermal activity. of another bathymetric map, which While the ~ 2450-m deep 86°W site is 86°W, and we could find no correlation included the Calyfield vent site that located in the rift valley, the 89°W area between temperature anomalies seen in had been found using Alvin. Unlike the lies near a topographic high in less than the ABE data and the Calyfield site. 86°W site, the Calyfield site could not 1700 m of water on the flank of the rift. We concluded that our vent-searching be clearly distinguished through tem- CTD data showed that the near-bottom technique required additional sens- perature anomalies in the ABE data. hydrography was more complex than at ing, and fortunately our collaboration with Ko-ichi Nakamura of the Japanese DANA R. YoERGER ([email protected]) is Associate Scientist, Department of Applied National Institute of Advanced Industrial Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA, Science and Technology provided USA.
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