32 



2.1.7.f Directional Hydrophone 



A directional hydrophone consists of a housing containing a row of transducers 

 or hydrophones which can receive an acoustic pulse and provide a bearing 

 to the pulse source. This capability permits the ROV to home-in on an active - 

 and frequency compatible - acoustic source (i.e. , pinger) . Frequency range 

 of the hydrophone is generally from 25 to 50 kHz (Helle Model 6550) , but 

 wider ranges (0-200 kHz) are also employed. 



2.1.7.g Scanning Sonar 



Scanning sonars serve multifunctions on ROVs : obstacle avoidance, object 

 location, pinger location, transponder navigation and relative bearing to 

 surface vessel. All scanning sonars listed inTable 2.3 (except for PINGUIN B6) 

 are of the CTFM (Continuous Transmission Frequency Modulated) type produced 

 by Ametek. The ranges are from 2m to 914m (6 to 3,000 ft), the frequencies 

 vary depending upon the operational mode. The transducer scans 360 degrees 

 (90 degrees sector scan) and can be tilted il5 degrees. Commercial vehicles 

 are tending towards a less sophisticated scanning sonar, such as offered 

 by the Wesmar Corp. , to provide obstacle avoidance and object location capa- 

 bilities where visibility is limited to less than a few meters. 



2.1.7.h Side Scan Sonar 



There are a variety of commercial side scan sonar manufacturers (e.g., EG&G, 



International, Klein Associates, Wesmar, Edo Western, UDI Ltd.), the characteristics 



of each device varies so that one description does not necessarily represent 



all models available. The general purpose of a side scan sonar is to distinquish 



topographic features of the sea floor and objects on or above it. Side scan 



sonar employs an acoustic beam whose main axis is slightly below horizontal. J 



The beam is narrow (0.75 to 1.5 degrees) in the horizontal plane, yet adequately ' 



broad in the vertical (35 to 60 degrees) to obtain echos from a point on the 



bottom directly below the transducer to points some 500m (1,600 ft) abeam of 



the transducer. The combination of the beam shape and short length of the 



acoustic pulse (about 0.1 millisecond) provides the capability to resolve 



small topographic irregularities and small objects (man-made and natural) 



on or above the sea bottom. The transducer is carried by the ROV at an 



appropriate altitude above the bottom, the reflected echoes are graphically 



recorded to produce a 3-dimensional facsimile or plan view of the bottom. 



Only two vehicles list side scan sonar as a part of their standard equipment 

 suite, however, virtually all of the larger vehicles have the pay load and 

 wiring capability to carry side scan sonar. Ocean Systems TROV S-4 deployed 

 a side scan sonar during a recent bottom route survey in the Mediterranean, 

 although it is not listed in Table 2.3. 



2.1.7.i Sub-bottom Profiler 



Sub-bottom profiling devices emit an acoustic pulse of suitable characteristics 

 to provide a record of the ocean bottom topography and its sub-structure 

 along the track of the ROV. The sub-bottom profilers are used to detect 

 and measure depth of pipeline burial and to discern the nature of the sub- 

 bottom preliminary to excavation or plowing. 



