Side-Scan Sonar Presentation STS

Transcription

1 Training Module Side-Scan Sonar Presentation STS

2 SIDE-SCAN SONAR SAFETY Training Module Content: This module includes information on: Types of Side-Scan Benefits and Disadvantages System Configuration and Options Effects of Pitch and Roll Winches and Options Tow Arrangements Search Planning Search Patterns Calculating Target Position Beam Depression

3 SIDE-SCAN SONAR SAFETY Where is his PFD?

4 SIDE-SCAN SONAR The sonar transducers and electronics package is positioned close to the seabed and moved in a straight line and at a fixed speed; fan-beams of sound are emitted perpendicular to the tow direction on either side of the instrument. The image is built up line after line. The quality of the record is heavily influenced by consistency in tow speed, transducer stability, and geometry of the transducers relative to bottom.

5 SIDE-SCAN SONAR BEAM COVERAGE

6 Types of Side-Scan Include: Single Beam (hull-mounted, towfish deployed) Dual Frequency (either/or) Simultaneous Dual Frequency Multibeam Multi-pulse Interferometric Synthetic Aperture Side-Scan

7 SIDE-SCAN System Benefits: Area coverage versus time Image clarity (high frequency systems) Availability of post-processing software to create mosaics Side Scan is an excellent search and mapping tool for large areas where the seabed is relatively flat.

8 SIDE-SCAN System Disadvantages: The length of tow cable length needed to get sonar to depth (typically 3/1 or 4/1 ratio when towfish deployed) Requirement to keep the sonar head towed in a straight line and at a fixed speed The ability to position a target when a long length of tow cable is used The danger of losing the sonar because of the need to tow it close to the seabed The difficulty of obtaining data coverage in areas of rapidly changing seabed elevations and man-made structures

9 TRANSDUCER BEAM ANGLE

10 TRANSDUCER NEARFIELD Transducer Mathematical Model of Near and Farfield

11 BASIC SIDE-SCAN COMPONENTS Sonar Display Responder trigger Side-scan Processor Peripheral Sensors Target Output Data Size, Weight and Towfish Design Responder or Transponder, and Location Pinger Software or Hardware Based Hand-deployed Cable Graphic Recorder Data Recorder Cable length Connects to Towfish Single or Dual Frequency, Chirp, Multipulse, Multibeam, Interferometric, 3D, SAS Compass, Pitch & Roll, and Temperature Sensors Option considerations

12 WATERFALL DISPLAY Record courtesy Steve Wright, EDGETECH Nadir Side Scan Waterfall Display (barge and wreckage)

13 WATERCOLUMN REMOVED Dual Channel Single Beam (330 khz) record with water column removed.

14 SINGLE CHANNEL DISPLAY (rotated 90 degrees) Single channel record 600 khz courtesy Marine Sonics Technology Ltd.

15 SIMULTANEOUS DUAL FREQUENCY SIDE-SCAN 120 khz 330 khz The center image illustrates the advantages of using simultaneous dual frequencies; the wreck of the Atlantic, has absorbed the 120 khz but highlighted the circled target. Had just the lower frequency been used, the wreck may have been missed. Image below courtesy Kongsberg Mesotech, Ltd. In most cases, the HF channels will provide superior target detection than the LF channels but will not support the LF range capability.

16 SIMULTANEOUS DUAL FREQUENCY SIDE-SCAN Record courtesy Steve Wright, Edgetech, and DSTO Australia Why do we observe the eel grass on the LF and not the HF channels?

17 MULTIBEAM SIDE-SCAN Record and image courtesy, Garry Kozak, L3 Communications, Klein Associates, Inc. Multibeam Side-Scan Towfish and Depressor Using a combination of receive elements a dynamically focused multibeam provides consistent along track and cross track resolution.

18 SYNTHETIC APERTURE SIDE- SCAN (SAS) Drawing courtesy Kongsberg Maritime The principle of SAS is that a long transducer array (much longer than the physical antenna) can be synthesized in the horizontal plane. This is achieved by coherent combination of pings recorded at intervals along the line of platform motion.

19 SAS The more widebeam the real single element aperture is, the longer the aperture that can be synthesised, and the better the alongtrack resolution that can be achieved.

20 SAS SAS resolution does not degrade with range!

21 SAS The issue with SAS is the transducer position needs to be known to ¼ wave length.

22 TRANSDUCER YAW, PITCH and ROLL Top view Yaw Direction of travel Roll Pitch Facing View Side view

23 TRANSDUCER PITCH When operating from a small vessel, pitching of the towfish will occur when short lengths of umbilical are deployed - in even relatively light to moderate sea states. The result of this motion is viewed as acoustic banding on the side-scan record. Higher frequency systems are typically more prone to the effects of banding under these conditions due to their inherent narrow transverse beam patterns. As the stern of the vessel rises with the crest of the wave, the tow cable is jerked upward. The cable strain quickly relaxes as the boat falls into the wave s trough which causes the towfish to nosedive. The illustrated pitch motion effects the receive directivity index of the transducer; the energy of the echoed return is not optimally aligned to the geometry of the array. Check to see if the tow cable can be safely deployed at the fore/aft pivot point of the vessel. Securing the tow cable at this position will reduce some of the pitching motion. The motion is exacerbated in a following sea as there is a tendency for the stern to yaw in the approaching waves. Try running the survey line into the waves (or at an offset angle) to minimize this motion effect.

24 TRANSDUCER PITCH Image courtesy Steve Wright, Edgetech Why does the pitch/roll effect the HF more than the LF record?

25 SIDE-SCAN and KITING

26 WHERE IS THE TARGET?

27 WHERE IS THE TARGET 2?

28 HERE IT IS!

29 WHAT CAN MESS UP TARGET POSITIONING

30 SIDE-SCAN MOSAICS

31 TRANSVERS FOOTPRINT COVERAGE Why is understanding the sonar s transverse footprint so important in target detection?

32 I M DEPRESSED!

33 WHAT S SLANT RANGE CORRECTION?

34 SLANT RANGE CORRECTION looks good!

35 RANGE CORRECTION not so good!

36 WHAT S SPEED CORRECTION?

37 HOW DO I COVER MY BOTTOM?

38 LET S GET TURNED AROUND

39 DIFFERENT TOW ARRANGEMENTS

40 DIFFERENT TOW ARRANGEMENTS 2

41 DIFFERENT TOW ARRANGEMENT 3

42 . Whatsa knot?. Whatsa nautical mile?. Whatsa metre? MPH metre/feet feet per sec Door #1 Door #2 Door #3

43 HOUSTON, WE HAVE A PROBLEM: You have a 1 square nautical mile to search It is a body size target (assume a flat bottom) You have to use a sonar range of 25 metres (50m swath) to resolve the smallest body dimension You have to have 50% overlapping coverage The boat speed is 4 knots for this search It takes 5 minutes to get the boat turned around and start the next line HOW LONG WIILL THIS SEARCH TAKE?

44 ANOTHER WAY TO CALCULATE COVERAGE

45 LET S GO SIDE-SCAN! The END!