August 10, 1999
Japan Marine Science and Technology Center (JAMSTEC)

Ocean Acoustic Tomography System To Be Recovered and Repaired

1. The facts
In December 1998, JAMSTEC deployed five 200 Hz transceiver moorings (ocean acoustic tomography system [Note 1]) in the central Equatorial Pacific Ocean. Each transceiver mooring was scheduled to be recovered two years later in December 2000.

Between mid-February and late-April of 1999, however, JAMSTEC lost radio contact with surface buoys #2, #3, #4, and #5 [Note 2]. The submersed transceivers connected to these buoys were still functioning normally at that time, so JAMSTEC decided to continue the experiment.

On June 23, however, the transceiver [Note 3] connected to the active buoy #1 did not work properly , and it stopped receiving and sending acoustic signals. After the transceiver stopped working, JAMSTEC was no longer able to pick up signals from transceivers #2-5. Furthermore, it appeared to be just a matter of time before transceivers #2-5 would stop functioning altogether.

A variety of simulation tests were conducted on land, and the results appeared to indicate a problem with the lithium batteries [Note 4]. In order to determine the cause with certainty, and to carry out complete inspections, JAMSTEC intends to recover all of the five 200 Hz transceiver moorings from the Equatorial Pacific.

Duration: Thursday, August 12 - Thursday, September 16, 1999
Schedule: Aug. 12 - Departure from the Port of Majuro
Sept. 3 - Return to the Port of Majuro with all five systems
Sept. 16 - Unloading of the systems at Yokosuka

For further information, contact the Japan Marine Science and Technology Center.
If you wish to talk with someone in the Ocean Research Department, ask for Mr. Nakano. Tel: +81-468-67-3885
If you wish to talk with someone in the Public Relations, Training and Education Division, ask for Mr. Taya or Mr. Kimura. Tel: +81-468-67-3806

1. Ocean acoustic tomography system
This system works on the same principle as a medical X-ray, CT but in place of X-rays it uses acoustic waves. By emitting and receiving acoustic signals in the sea from various angles, the system can be used to reconstruct a tomographic image of water temperatures and currents.

2. Surface buoys
These buoys are equipped with an INMARSAT transmitter for data telemetry, and with a GPS receiver for adjustment of the internal clock. Each buoy is connected to a submersed transceiver by a transmission cable measuring some 3000 meters in length.
Surface buoy specifications:

(1) Data transmission - Whenever 8 KB of data is stored. (at irregular intervals)

(2) Regularly scheduled transmissions - Once per day, at 00:00 Japan time

(3) Buoy position signal-When a buoy breaks away and begins drifting, it emits a signal once every three hours to indicate its position.

(4) Signal reception window-In order to allow for commands to be sent from land, the receiver turns on every Wednesday.

3. Transceivers
Each transceiver consists of an underwater speaker (for emitting acoustic signals), an underwater microphone (for receiving acoustic signals), and various electronic circuits that make up the heart of its tomography system. The electronic circuits include a high-precision clock, a device for the processing and recording of data, and an acoustic amplifier. The transceivers accurately measure the time it takes for acoustic signals to travel from one transceiver to another. Each transceiver is also equipped with a rubidium atomic oscillator that is used for clock adjustment if the surface buoy breaks away from its moorings and becomes lost.

4. Lithium batteries (coated)
These lithium batteries make use of thionyle chloride, and are an especially powerful type of lithium battery. The lithium on the negative terminals on these batteries is made so that it can be easily coated with lithium chloride.

The use of this coating prevents direct contact between the lithium and the thionyle chloride, thus preventing power leakage and consumption of the batteries. The drawback with this type of battery is that a thick coating can easily result in a drop of voltage.