Welcome to "A Day in the Life of a Tomography Mooring"

These photos describe various stages of the deployment of moorings for the Hawaiian Ocean Mixing Experiment (H.O.M.E.) . The moorings were deployed from the R/V Revelle by John Kemp of the Woods Hole Oceanographic Institution (WHOI) , with Peter Worcester of the Scripps Institution of Oceanography (SIO) as Chief Scientist. The photos were taken during the deployment of an array of four moorings on the north side of the Hawaiian Ridge. The array took two weeks to complete, so you may find photos with sunny skies next to photos of rainy skies. These moorings have an acoustic transceiver package on them for ocean acoustic tomography, together with thermistors and microcats for recording temperature and salinity. Moorings of this type are fairly normal, except that, partly because of the time needed to prepare the acoustic source/receiver package, the tomography moorings take about 12 hours to deploy, rather than the 5-6 hours needed for a typical current meter mooring, say. It may be useful to see a figure of a mooring blueprint before browsing through these photos to get an overall view of what the mooring is meant to look like in the water column. These are deep-water, open-ocean moorings where the water depth is typically 4500-5500 meters. From top to bottom, the main elements of the mooring are: the subsurface buoy, tiltmeter and compass package, the hydrophone array which is immediately above the acoustic source cage, and, near the bottom, the dualed acoustic release package. The anchor consists of surplus railroad wheels welded together. The subsurface buoy at the top of the mooring keeps the entire mooring taut or stiff; the mooring can tilt over by a kilometer or more because of ocean currents, but the horizontal displacements of these moorings are typically of order 100 m. When the mooring is recovered at some later date (6 months to a year later), the acoustic releases are sent a special high-frequency acoustic signal which causes a small pin in them to pull back releasing the mooring so that it can float to the surface. Two acoustic releases are used so that if one of them should fail the mooring can still be recovered. The railroad wheels stay on the bottom until they are subducted under some continent several million years later.

Brian Dushaw April 2001

Please click on a thumbnail to view the full-size image.

The R/V Roger Revelle prepares to go to sea.
Acoustic source packages on deck. The white cylinders are electronics, pressure compensation, and battery packs. The big white cylindrical thing in the middle of the cage is the 250 Hz acoustic source. Note the miles and miles of wire rope and kevlar cable.
The R/V Revelle departs Honolulu.
Peter and Lisa attach a light and a radio beacon to the top of the buoy to help locate it when the mooring is recovered. It is hard to find things on the ocean's surface.
Surface buoy and glass balls are ready to go at first light.
The surface buoy is raised over the ship's side by the crane and a mulititude of tag lines. The wire rope of the mooring goes through the traveling block at far right, and around the ship's stern to the buoy, with various people keeping it from hanging up on anything. A thermistor destined for 150 m depth is visible on the cable.
Once the buoy is released, it drifts behind the ship. The ship steams at a steady 1-1.5 kts upwind to keep tension on the wire. Since the mooring takes 12 hrs to deploy, the deployment starts 12 nm away from the intended anchor site. Hopefully, the wind direction does not change during the deployment...
The anchor seen in the foreground will be needed about 12 hours later.
The wire rope is fed out using the Lebus winch. Sometimes it rains at sea.
Glass balls (the small yellow things dragging behind the ship) are inserted into the mooring to account for the weight of the wire rope, thus keeping near uniform tension at every depth of the mooring. A microcat, which records temperature and salinity, is clamped onto the cable here.
The source package cage is laid down and dragged to the stern of the ship to prepare its deployment. The white, 4-element hydrophone array cable is zig-zagged up and down the deck for its deployment and to be attached to the electronics package in the cage. The bitter end of the mooring line is held to the cleat at lower left. The black thing on the hydrophone cable near the lower left corner of the cage is an acoustic interrogator which is used to track the position of the source cage.
Great care is taken to make waterproof connections between the hydrophone array and the electronics package, which is the silver pressure case directly behind Lisa.
This particular cruise was well documented - to appear in a video edition of SIO's Explorations.
The tilt meter/compass package, used to measure the tilt and orientation of the mooring line, is attached to the mooring line and to the top of the hydrophone array. The plastic bin holds the slip line that will be used in a moment.
The tilt meter/compass package and the hydrophone array is slowly fed out to sea using a slip line - a loop of line that will be pulled back in once the tension of the mooring has been transferred to the source cage. Lloyd holds a hydrophone element to be sure it gets over the side safely.
The tension of the mooring line is on the cage now. The hydrophone cable is to be carefully secured so that it safely crosses from the wire rope to the cage. When the cage goes over the side, these cables can take quite a beating. The hydrophone cable connection to the electronics package is in the foreground; the acoustic interrogator is 20' up the mooring line+hydrophone cable.
The 3600 lb cage is ready to go over. The ship's crane lifts it over the stern, with all hands holding one tag line or another. Generally a tense moment, so there are no photos here of the source actually going over...
Another view. While putting the source over the side may be stressful and even dangerous, fortunately John Kemp has taken stress management classes so he knows how to count to 10 properly before getting upset.
Another view of the Lebus winch that is used to pay out the wire rope (jacketted with plastic) and kevlar cable.
Once the source package goes over the side, there is not much else to put out but about 3500 m (about 2 miles) of wire rope and kevlar cable. The day grows long, the people grow weary, while the kevlar continues to pay out.
A central North Pacific sunset.
A final burst of activity near the end of the mooring. The acoustic releases are on deck and attached to the mooring line. The releases will be about 30 m above the anchor (see the mooring blueprint). The chains and nylon rope connecting the acoustic releases and the anchor are being prepared here. There is a set of glass balls (already over the stern at the time this picture was taken) directly above the releases to bring them to the surface when the mooring is recovered.
The acoustic releases have been put over the stern and the mooring is presently held by another slip line at center. If the tension of the mooring is transferred too soon to the anchor, the mooring may deploy prematurely.... The location of a mooring needs to be fairly accurate - the actual and desired positions are typically within 100 m or so.
Once the proper drop point has been reached, the slip line is let out to transfer the tension of the mooring to the anchor, and the ship's crane lifts the plate on which the anchor sits to encourage it to leave the ship.
Another view of an anchor drop; photo taken from a different mooring. The anchor will fall about 45 minutes (literally) before reaching the ocean bottom.
Bye anchor!! The anchor has the entire mooring attached to it as it falls, of course, and so the anchor falls back towards the mooring about 750 m before it lands on the bottom. Because of the momentum it carries in this process, the source cage will briefly overshoot its intended 750-m depth by 150 m. Rare frontal photo of John Kemp.
The orange balls are acoustic transponders. These instruments are deployed in a square about 7 km on a side on the ocean bottom around the mooring. The acoustic interrogator (mentioned above) sends out acoustic pings that these instruments detect and reply with their own pings. The acoustic travel times of these pings are recorded by the electronics in the source package and these data are used to correct the tomographic data for the motion of the moorings. Typical positioning accuracy is about 1 m.
After the mooring is deployed, an expendable sonobuoy is tossed over the side to listen for the transmissions of the acoustic source to be sure it is working. The sonobuoy deploys a small hydrophone to about 60' depth, and radios what it detects on an FM frequency. On the ship, some distance away, we can then listen to the sounds detected by the sonobuoy. This is a *.wav file (1.3 MB) recording of an actual source transmission on this cruise - it begins with several seconds of noise and the transmission lasts 67 seconds (garbled near the end). The source signal is a "phase-coded linear maximal shift register sequence with 250 Hz center frequency".
With the mooring deployed, we depart leaving the ocean to the albatrosses, the mahi-mahi, the giant squid, and the fin whales. We'll be back in six months to recover these moorings. To view pictures of the mooring recovery, view these pictures in reverse order.

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Created on Tue Apr 24 21:00:19 2001
Brian Dushaw