Scanning Sonar and ROV Operations For Underwater SAR Imaging Applications
This training module provides information specific to integrating scanning sonar onto an ROV platform and fundamentals in using the sonar to detect and prosecute targets It includes: Sonar mounting and alignment ROV trim Transducer guards Transducer height and vertical beam geometry Mid-water target detection Orthogonally mounted scanning sonar Geo-referencing targets Contiguous sonar coverage to create mosaics Search Patterns
There are four classifications of ROVs: 1. Work Class 2. Light Work Class 3. Observation Class 4. Micro Class
Note the location of the scanning sonar and consider its effect on on target detection.. Consider the location with respect to possible damage from physical impact.
There is a reason why you cannot see a target when the sonar is positioned too high on the vehicle!
CREATIVE SONAR MOUNTING
The main limitation with Micro Class ROVs is their size, power, and payload capability.
Micro class ROV are notoriously unstable unless they have auto heading and depth capability. To obtain a readable scanning sonar image requires that the operator sit the vehicle on bottom to provide the required stability. Because the sonar is positioned so close to the seabed the geometry limits the sonar image.
Check the transducer beam pattern and determine whether mounting the head with the transducer up or down has any geometric implications with respect to acoustic coverage.
Head Up/Down and Display:
SONAR HEAD ALIGNMENT
1. Ensure that the 0º alignment of the sonar head matches the 0º heading of the ROV. 2. Measure and record the height of the sonar head above the skid of the ROV. 3. Measure and record the X,Y (horizontal offsets of the sonar transducer to the acoustic tracking beacon). 4. Install a protective guard over the sonar transducer if it is positioned where it may become entangled in a line or be damaged when docking or during recovery of the ROV. 5. Position the sonar so it has a clear field of acoustic view while balancing this requirement with the need to protect the head. 6. Measure and record the X,Y and horizontal offsets of the sonar transducer to the ROV compass. 7. Check the sonar record and see if the ROV flotation causes acoustic multipath (the sonar will need to be repositioned on the ROV if this is the case). 8. Ensure that the sonar data is transmitted at the highest possible telemetry speed to the surface so as not to restrict the sonar s ping rate.
When the vehicle has a slight positive bow trim, it can be rocked to acoustically view most inclined (upslope) gradients. On a flat seabed, it takes very little power to thrust down and forward to see ahead. Since thruster wash is aft, the visibility for the camera is maintained even if the ROV thrusters stir up the bottom sediment. When the vehicle is trimmed level, the operator must thrust down and reverse to look up-slope. In doing so, the sediment in the backwash of the thrusters obscures camera visibility, and will be detected on high frequency imaging sonars (and potentially limit the sonar s range).
Protect your sonar investment by using a transducer guard. Be aware that by mounting the sonar near a very reflective surface may change its beam pattern!.
BEAM PATTERN CHANGES
Mounting the Sonar Close to Bottom: When the sonar is positioned too close to the seabed, its range is limited.
HORIZONTALLY MOUNTED HEAD Certain applications are better addressed using a horizontally mounted scanning and/or two sonars with one orthogonal to the other.
MID-WATER TARGETS Mid-water target detection using a horizontally mounted scanning sonar.
SCAN PATTERN for HORIZONTALLY ALIGNED HEAD Remember that the wide component of the beam is now in the horizontal plane and that the heading of the vehicle should not be altered more than the beam width to get 100% coverage.
VERTICAL STRUCTURE VISUALIZATION Oil rig damage assessment survey after Katrina using a horizontally mounted scanning sonar.
Two heads, one orthogonally mounted to the other.
Geo-referencing Sonar Targets: What needs to be considered?
Geo-referencing Sonar Targets: 1 1 Ship s Heading (gyro compass) 2 4 2 Angle between ship s heading and True North 3 Offset between navigation receiver and acoustic tracking transducer 3 5 11 6 target Satellite Navigation Receiver Acoustic Tracking System Transducer Acoustic Tracking System Transponder (responder) 12 Range and angles that must be known to geo-reference a sonar target.
Integrated Navigation and Sonar Display:
CREATING A CORIDOR When 100% coverage is required, use an automated track plotting system that shows the area ensonified by each scan.
Practical Horizontal Excursions with an Observation Class ROV
Clump Weighting the ROV Umbilical: Using a clump weight on the umbilical reduces the chance it is eaten by the vessel s propeller.
Umbilical Management on a Sloped Seabed: Use this method with or without an acoustic tracking system.
End ROV-deployed SONAR