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Maintenance of the Channel Islands Acoustic Receiver Array
PIER diver attached to an acoustic receiver. Photo: Robert Schwimmer
The servicing of the 96 acoustic receivers in the array is no simple task. Each receiver must be retrieved three times per year to be downloaded, cleaned, and get a fresh set of batteries. This regular maintenance is also how PIER is able to replace missing or broken equipment, making sure that the array is always functioning as efficiently as possible. Stored in computers on board PIERís research vessel Malolo, are precise GPS coordinates of each receiver in the array. The boat is positioned above the receiver and either SCUBA divers are sent down or a device called a hydrophone is lowered over the side. About half of the receivers are located at SCUBA depth and the other half are below 125 feet.
For the diveable sites, a buddy team of divers swim down with a clean, blank receiver loaded with fresh batteries. They locate the old receiver, which is held in place by a sand screw or by five 30-pound sandbags, depending on the bottom substrate. The old receiver is removed and the new one is attached. Once on board, the data on the receiver is downloaded to a computer and its memory cleared. It receives new batteries and a scrub-down and is ready for deployment at another station.
ARC-1 Acoustic release device. Photo: Robert Schwimmer For the deep sites, an acoustic release (called an “ARC-1”) developed by Desert Star Systems and PIER is added to the same setup used in the shallower stations. Once Malolo is positioned above the GPS coordinates, a hydrophone is deployed over the side of the boat at a depth shallower than the seafloor. This hydrophone, attached to a computer onboard sends a signal to the ARC-1, instructing it to release from the sandbags that are holding it down. The two sub-surface buoys that hold the receiver upright send the whole setup rocketing to the surface, where it is brought on board to be downloaded and cleaned. Malolo is then repositioned over the coordinates and a new receiver setup is deployed overboard.
As with many types of scientific field work, things don’t always go as planned. With the acoustic releases, a number of things can happen. For example, aggressive fouling on the release mechanism prevents the moveable arm on the ARC-1 from releasing the sand bags, and there is a variety of other strange occurrences of that have stranded the equipment on the seafloor.
When this happens, Tobiko, PIER’s remotely operated vehicle (ROV) is deployed. Tobiko, controlled from the surface by PIER’s research staff, locates the receiver setup. The ROV has an mechanical arm equipped with a razor blade which cuts the rope tied to the sandbags, sending the equipment shooting to the surface, where it is brought on board.
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