Page 46 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition
P. 46
34 CHAPTER 2 The Ocean Environment
Transducer depth
Sounded depth
Vehicle depth
Vehicle altitude
FIGURE 2.6
Vessel-mounted and sub-mounted sounders.
According to Bowditch (2002), “the major difference between various types of echo sounders is in the frequency they use. Transducers can be classified according to their beam width, frequency, and power rating. The sound radiates from the transducer in a cone, with about 50 percent actually reaching the sea bottom. Beam width is determined by the frequency of the pulse and the size of the transducer. In general, lower frequencies produce a wider beam, and at a given frequency, a smaller transducer will produce a wider beam. Lower frequencies penetrate deeper into the water, but have less resolution in depth. Higher frequencies have a greater resolution in depth, but less range, so the choice is a trade-off. Higher frequencies also require a smaller transducer. A typical low-frequency transducer operates at 12 kHz and a high-frequency one at 200 kHz.” Many smaller ROV systems have altimeters, such as the Imagenex 852, on the same frequency as their imaging system for easier software integration (the same software can be used for processing both signals) and reduced cost (Figure 2.6).
Computation of depth as determined by an echo sounder is determined via the following formula:
D5ðV 3T=2Þ1K 1Dr
where D is depth below the surface (or from the measuring platform), V is the mean velocity of sound in the water column, T is time for the round trip pulse, K is the system index constant, and Dr is the depth of the transducer below the surface.
2.2.7.2 Opticacoustic seabed classification
Traditional seafloor classification methods have, until recently, relied upon the use of mechanical sampling, aerial photography, or multiband sensors (such as Landsatt) for major bottom-type
