A.T. Webb

Measurements beneath an Antarctic ice shelf using an autonomous underwater vehicle

The cavities beneath Antarctic ice shelves are among the least studied regions of the World Ocean, yet they are sites of globally important water mass transformations. Here we report results from a mission beneath Fimbul Ice Shelf of an autonomous underwater vehicle. The data reveal a spatially complex oceanographic environment, an ice base with widely varying roughness, and a cavity periodically exposed to water with a temperature significantly above the surface freezing point. The results of this, the briefest of glimpses of conditions in this extraordinary environment, are already reforming our view of the topographic and oceanographic conditions beneath ice shelves, holding out great promises for future missions from similar platforms.

Exploring beneath the PIG Ice Shelf with the Autosub3 AUV

On 31st January 2009, two numbers: “range and bearing” flashing up on a laptop screen, indicated that Autosub3 had returned from its last mission beneath the Pine Island Glacier (PIG) Ice Shelf in the Western Antarctic. The Autosub technical team from NOCS, Southampton, onboard the US ice breaker Nathanial B Palmer breathed a collective sigh of relief. Any significant technical failure would have resulted in total loss of the multi million Euro Autonomous Underwater Vehicle with no hope of recovery from 60 km into the ice shelf cavity. This was the last of six successful missions to investigate the shape the ice shelf, the sea bed bathymetry, the currents and the physical oceanography within the ice cavity. Each are vital to understanding the interaction between the sea water and the ice shelf, and quantifying whether the melting rate is changing. During the cruise, Autosub3 had run beneath the ice for almost 4 days and for 510 km.

First direct measurements of hydraulic jumps in an active submarine density current

For almost half a century, it has been suspected that hydraulic jumps, which consist of a sudden decrease in downstream velocity and increase in flow thickness, are an important feature of submarine density currents such as turbidity currents and debris flows. Hydraulic jumps are implicated in major seafloor processes, including changes from channel erosion to fan deposition, flow transformations from debris flow to turbidity current, and large-scale seafloor scouring. We provide the first direct evidence of hydraulic jumps in a submarine density current and show that the observed hydraulic jumps are in phase with seafloor scours. Our measurements reveal strong vertical velocities across the jumps and smaller than predicted decreases in downstream velocity. Thus, we demonstrate that hydraulic jumps need not cause instantaneous and catastrophic deposition from the flow as previously suspected. Furthermore, our unique data set highlights problems in using depth-averaged velocities to calculate densimetric Froude numbers for gravity currents.

Driven around the bend: Spatial evolution and controls on the orientation of helical bend flow in a natural submarine gravity current

Submarine channel systems transport vast amounts of terrestrial sediment into the deep sea. Understanding the dynamics of the gravity currents that create these systems, and in particular how these flows interact with and form bends, is fundamental to predicting system architecture and evolution. Bend flow is characterized by a helical structure and in rivers typically comprises inwardly directed near-bed flow and outwardly directed near-surface flow. Following a decade of debate, it is now accepted that helical flow in submarine channel bends can exhibit a variety of structures including being opposed to that observed in rivers. The new challenge is to understand what controls the orientation of helical flow cells within submarine flows and determines the conditions for reversal. We present data from the Black Sea showing, for the first time, the three-dimensional velocity and density structure of an active submarine gravity current. By calculating the forces acting on the flow we evaluate what controls the orientation of helical flow cells. We demonstrate that radial pressure gradients caused by across-channel stratification of the flow are more important than centrifugal acceleration in controlling the orientation of helical flow. We also demonstrate that non-local acceleration of the flow due to topographic forcing and downstream advection of the cross-stream flow are significant terms in the momentum balance. These findings have major implications for conceptual and numerical models of submarine channel dynamics, because they show that three-dimensional models that incorporate across-channel flow stratification are required to accurately represent curvature-induced helical flow in such systems.

Towards environmental monitoring with the Autosub autonomous underwater vehicle

In this paper we describe some of the desirable characteristics of an autonomous underwater vehicle capable of undertaking environmental surveys in the ocean. Several of these characteristics are incorporated in the 7 m long Autosub-1 vehicle which has completed over 120 missions to date in UK and US waters. We review some of the key technological innovations used within Autosub-1 and describe some results from a 110 km survey off the coast of Florida in December 1997. While the survey demonstrated many of the advantages of using an AUV for environmental monitoring the paper concludes with a discussion of technical and procedural areas that still require attention before the use of AUVs can be considered routine.

The experience and limitations of using manganese alkaline primary cells in a large operational AUV

The authors describe how the use of manganese alkaline batteries for the Autosub science programme has enabled the project to progress swiftly from being a technological demonstrator to providing new ways of gathering environmental data not possible by other methods. Throughout the programme, all areas of the battery system have undergone development. that is: specification, design and manufacture of the pack; inspection and testing; packaging, storage and physical handling; electrical and thermal insulation; estimates of remaining endurance; and disposal of depleted packs.

Oceanographic Surveys with a 50 hour Endurance Autonomous Underwater Vehicle

Autonomous Underwater Vehicles (AUVs) are becoming accepted data-gathering tools within the marine science community in Europe, the US and elsewhere. Technology can now provide vehicles with a useful range and depth envelope. For example, the Southampton Oceanography Centre’s Autosub-1 vehicle has already covered 263 km in a single oceanographic survey mission, reaching depths of 500 m off Bermuda in September 1998. The challenge now is to ‘free the technology from the research community. Potential direct and indirect benefits from the offshore energy industry’s use of AUVs have been quantified. For one company, the benefits of using survey class AUVs has been estimated at

Observations beneath Pine Island Glacier in West Antarctica and implications for its retreat

60m over 5 years. The economics become more attractive as the industry moves to deeper water. While today’s AUVs of long endurance are limited in depth, developments on the horizon in composite materials and high capacity, lightweight secondary batteries will enable 1000 km range, eight-day endurance and at least 1600 m diving depth to be in active service within the next two years. This paper will review the oceanographic survey achievements of the Autosub AUV over its 216 missions (to December 1999), working in UK, US and Bermudan waters. Moving to the future, the paper will illustrate how such an AUV could be used in the offshore industry for survey, monitoring and emergency response. Finally, we will describe the present Autosub development programme, scheduled to deliver an operational 1000 km, 1600 m vehicle by mid 2000. Introduction The Autosub AUV was conceived as a survey vehicle to complement research ship programs by the provision of a cost effective tool to collect water column and seafloor information. Included within the design brief was the aim to address the widest possible scientific community hence allowing varied missions and payloads. This requirement ensured that flexibility was fundamental to the technology design. Hence for Autosub-1 there was no fixed sensor suite, the vehicle sub-systems were modular and thus offered a simple upgrade path as new technologies became available. One other by-product which is envisaged but not yet proven is the opportunity to change the vehicle characteristics i.e. size and shape, without a major re-design program. (Fig 1) The first vehicle, Autosub -1 was constructed in 8 months, being completed in May 1996. The short production cycle was only possible because of the extensive development programme which had been underway for the previous 5 years. The first phase saw Autosub-1 progress from its first ‘in water’ missions in Empress Dock, Southampton, (June 1996) to completion of the acceptance trials in just under one year (April 1997) Oban 97 The Autosub performance and acceptance trial criteria have been described by Millard et al (1), the main objectives were: • Profiling autonomously from close to the surface to near the seabed in at least 100m of water; • Mission lengths of at least 6 hours, with satellite fixes updating the dead reckoning(DR) vehicle navigation; and • Collection of meaningful scientific data. After completion of the engineering acceptance phase, the vehicle program has been strongly focussed on the customers’ scientific objectives, which has taken the vehicle into a multirole capability, via a series of campaigns worldwide. Florida 97 Florida in December 1997 was a significant step, as it moved Autosub from missions in the sheltered coastal waters off Oban, Scotland to the open seas of the Atlantic Ocean. In a joint collaboration with Florida Atlantic University (FAU), co-funded by the US Office of Naval Research (ONR), a campaign saw the vehicle deployed on the edge of the Gulf Stream. Local conditions, with surface current speeds of up to 2m/s, provided significant challenges for the vehicle operating a water column regime of above average current shear and sound speed gradients. These environmental features also OTC 12003 Oceanographic surveys with a 50 hour endurance autonomous underwater vehicle G Griffiths, K G Birch & The Autosub Technical Team :N.W. Millard, S.D. McPhail, P. Stevenson, M. Pebody, J. R. Perrett, A.T. Webb, M Squires & A Harris, Southampton Oceanography Centre 2 G Griffiths, K G Birch & the Autosub Technical Team OTC 12003 provided the support team with a challenge to keep track of the vehicle’s progress throughout individual missions. The results of the campaign have been described by Griffiths et al (2), the main achievements were: • 72 hour mobilisation from unpacking the container to operational deployment of the vehicle into the water; • completion of a 110km CTD & ADCP survey; • terrain following mission 10m above seabed in water depths varying between 12-204m; and • due to high surface current the vehicle was out of acoustic contact with the support boat – thus demonstrating unsupervised operations. Bermuda 98 With deployment and recovery via the Bermuda Biological Station vessel, the Weatherbird, Autosub was a key component in the Autonomous Vehicle Validation Experiment (AVVEX). The overall aim of the experiment was to demonstrate the synergy between a moored times series dataset and an AUV spatial survey. Amongst the challenges for Autosub were to: • increase the range and depth capability; • demonstrate the easy of integration for commercial off the shelf (COTS) sensors; • gather multidisciplinary data in a number of survey modes; and • verify that operations could become routine and cost effective. The campaign had proved once again the viability of transporting the vehicle, plus the support team, and the ability to work on different vessels without significant time penalties to mobilise. During AVVEX the vehicle completed the longest mission to date, covering 263km and routinely profiling to a depth of 400m. The mission lasted approximately for 53 hours, during which the recovery waypoint was altered, by radio link, to provide safer operational conditions for the Weatherbird recovery as the sea state had worsened during the mission time.(Sea Technology Feb 2000) The vehicle also achieved a record descent to 504m. As a result of the 263km mission it was possible for the first time to establish a the vehicle battery cost as 43

Spatially complex distribution of dissolved manganese in a fjord as revealed by high-resolution in situ sensing using the autonomous underwater vehicle autosub

per MJ, when using primary manganese alkaline batteries. This equates to about 15

Deep‐sea, high‐resolution, hydrography and current measurements using an autonomous underwater vehicle: The overflow from the Strait of Sicily

per kilometre, and compares well with silver zinc re-chargeable cells at 30