Page 47 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition
P. 47
Echo trace
Smooth simple bottom
FIGURE 2.7
Acoustic seabed classification.
discrimination (e.g., mud, sand, rock, sea grass, and corals). Newer acoustic techniques for collect- ing hyperspectral imagery are now available through processing of acoustic backscatter.
Acoustic seabed classification analyzes the amplitude and shape of acoustic backscatter echoed from the sea bottom for the determination of bottom texture and makeup (Figure 2.7). Seafloor roughness causes impedance mismatch between the water and the sediment. Further, reverberation within the substrate can be analyzed in determining the overall composition of the bottom being insonified. Acoustic data acquisition systems and a set of algorithms that analyze the data allow for determining the seabed acoustic class.
2.2.8 Sonic velocity and sound channels
Sound propagation (vector and intensity) in water is a function of its velocity. And velocity is a function of water density and compressibility. As such, sound velocity is dependent upon tempera- ture, salinity, and pressure and is normally derived expressing these three variables (Figure 2.8). The speed of sound in water changes by 35m/s/C, by approximately 1.3m/s/PSU salinity change, and by about 1.7 m/s/100 m change in depth (compression). The speed of sound in seawa- ter increases with increasing pressure, temperature, and salinity (and vice versa).
The generally accepted underwater sonic velocity model was derived by W. D. Wilson in 1960. A simplified version of Wilson’s (1960) formula on the speed of sound in water follows:
c 5 1449 1 4:6T 2 0:055T2 1 0:0003T3 1 1:39ðS 35Þ 1 0:017D
where c is the speed of sound in meters per second, T is the temperature in degrees Celsius, S is the salinity in PSU, and D is the depth in meters.
Temperature/salinity/density profiles are important measurements for sensor operations in many underwater environments, and they have a dramatic effect on the transmission of sound in the
2.2 Chemical oceanography 35
Echo trace
Rough complicated bottom
