Page 417 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

410 CHAPTER 15 Sonar
(100 kHz/500 kHz) towed Klein 5000 sonar was used. The low-frequency band penetrated through the relatively light viscosity oil while the higher frequency bounced off the top of the oil/water interface, allowing for differentiation of the relatively rough mud bottom from the smooth oil as it migrated on the bottom (Figure 15.21). Once the search area was covered and the sonar data analyzed, an observation-class ROV visually characterized the bottom based upon sonar returns. Excellent tracking results were obtained of the oil plume, allowing for a higher incidence of recovery.
3. CHIRP sonar: This frequency-shifting sonar technology (along with its numerous technological advantages) is described more fully in the next section.
4. Tunable frequency: This type of transmitter/receiver combination uses frequency tuning to glean differing characteristics of the target area based upon the backscatter characteristics of the frequency transmitted. The strength of this technique is the ability to insonify the target and analyze backscatter at the various frequencies to gain better target characterization. The weakness is the rapid degradation in efficiency of the transducer as the frequency departs the nominal transducer design range.
15.2.5 CHIRP technology and acoustic lens systems
15.2.5.1 CHIRP sonar
CHIRP (compressed high intensity radar pulse) techniques have been used for a number of years above the water in many commercial and military radar systems. The techniques used to create an electromagnetic CHIRP pulse have now been modified and adapted to commercial acoustic imaging sonar systems.
To understand the benefits of using CHIRP acoustic techniques, one needs to analyze the limita- tions using conventional monotonic techniques. An acoustic pulse consists of an on/off switch mod- ulating the amplitude of a single carrier frequency (Figure 15.22).
FIGURE 15.21
Dual-frequency sonar used during oil spill plume tracking with (a) tow fish hand launch and (b) sonar display.