Picture #026 in A Day in the Life of a Tomography Mooring

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".