Alice: an Instrumented Tripod
Alice is an instrument package for measuring bottom-boundary-layer processes in estuaries and the ocean. It consists of a self-ballasted tripod upon which is mounted a "core" sensor package for measuring boundary-layer currents, turbulence and waves.
Alice is principally rigged for sediment-transport studies (with optical and acoustic backscatter sensors, sediment traps and a pump sampler) but has also been used extensively in studies of boundary-layer mechanics, animal–flow–sediment interactions, wave dynamics and nutrient/gas fluxes. Onboard computers take care of instrument control and data logging, and allow the user to design sampling schemes that suit the experiment in hand. All power – enough for 30 to 90 days' deployment, depending on sampling regime – is provided by alkaline batteries stacked inside a pressure canister, which is also mounted on the tripod. "Guest" sensor packages can simply be mounted on the frame and run independently of other sensors, or they can be interfaced with the core package to enable control and precise synchronisation with other datastreams.
Examples of guest packages used to date include a sector-scanning sonar for imaging the seabed and resolving bedload sediment transport, an upward-looking ADCP current profiler, and CTD/DO sensors.
The tripod sits on the seabed, unattended for the duration of the experiment. Alice is typically deployed and retrieved by helicopter, which has proven to be safe, cost-effective and fast. Once in the water, she can be moved between sites underneath a small boat and precisely oriented on the seabed by divers.
Alice has been used successfully in environments ranging from intertidal mudflats to 40-m water depth on the open continental shelf.