Acoustic methods form our principal means of mapping the seabed. The basics of marine acoustics are introduced and each technique is introduced seperately, the theory of operation discussed and examples of data are presented. Introduction Sound provides the only convenient means for transmitting information over great distances in the ocean, and it is the only signal that can be used for the remotely sensing of the ocean below a depth of a few tens of metres. The sound speed in water is dependent on the temperature, salinity, and pressure. A typical value of V p is 1480 ms -1 .
Velocity depends primarily on temperature, less on pressure, and very little on salinity.
In practice the last 2 terms in the above equation are often ignored. V p is highly variable. As noted above, V p for salt water is approximately 1480 ms -1 . V p varies between different sediment types and rocks. Furthermore, the general empirical rule applies that the compressional wave velocity increases in step with density in similar material types.