Real-time Monitoring of

Transcription

1 Real-time Monitoring of Uncertainty due to Refraction in Multibeam Echosounding J. Beaudoin Ocean Mapping Group University of New Brunswick Oct. 21 st 2008 Shallow Survey

2 Introduction CSL Heron CCGS Creed CCGS Matthew CCGS Amundsen 1450 m/s 1500 m/s Oct. 21 st 2008 Shallow Survey

3 Assessing Refraction Artifacts in Real-Time Highly subjective Requires constant vigil Can overreact over flat seafloors Can underreact over complicated topography Impossible for iso-velocity displays (e.g. Reson 81XX display) Oct. 21 st 2008 Shallow Survey

4 Proposed Approach: Raytracing Simulation Isolates raytracing portion of depth reduction procedure: no sounding data required! Requires accurate model of raytracing procedure: Draft Angular sector Survey depth Surface sound speed probe Depression angle Sound speed TWTT Investigation depth Depth TWTT 1. Compute common TWTT 2. Raytrace in test watercolumn Difference the solutions Depth error Oct. 21 st 2008 Shallow Survey Horizontal error

5 Application to Real-Time Monitoring Sound speed Smile artifacts near surface Bias cancels out at mid-depth Frown artifacts at depth It s important to see the whole picture Oct. 21 st 2008 Shallow Survey Depth

6 Sound speed Uncertainty Wedge Depth Error (%w.d.) Oct. 21 st 2008 Shallow Survey

7 Simplification for Real-Time Decision Making Sound speed Depth % w.d % w.d % w.d % w.d. > 1.0% w.d. IHO Order Allowable depth dependant portion of TVU Special 0.75% w.d. 1a 1.3% w.d. 1b 1.3% w.d. Oct. 21 st 2008 Shallow Survey % w.d.

8 Snapshots of Refraction Bias Through an Evolving Watercolumn 3 vs. 4 4 vs. 5 5 vs sound speed depth depth 3 2 vs. 3 1 vs Oct. 21 st 2008 Shallow Survey

9 Time Evolution of Bias Between Casts sound speed 5 4 HUGE ASSUMPTION: Linear growth of bias with time Not unreasonable if you re sampling at a high rate but DEFINITELY not applicable if you re undersampling Oct. 21 st 2008 Shallow Survey depth

10 Look direction Real-Time Uncertainty Visualization Comparison of cast 1 & 2 Comparison of cast 2 & 3 sound speed depth It s important to be able to visualize the time evolution and history of error Oct. 21 st 2008 Shallow Survey

11 Uncertainty Visualization Sound Speed Field Depth Bottom Uncertainty Field Error Analysis Time Depth Oct. 21 st 2008 Shallow Survey

12 Real-Time Monitoring Time Oct. 21 st 2008 Shallow Survey Depth

13 Other Applications Error Analysis with Raytrace Simulation Pre-survey Analysis CSL Heron, Port of Saint John (2008) Quality Assurance CCGS Matthew, Advocate Bay (2008) Pre-survey Analysis CCGS Matthew, EM710 acceptance trials (2005) Oct. 21 st 2008 Shallow Survey

14 Example 1: CSL Heron, Port of Saint John MVP30 Sound speed Temperature Salinity Oct. 21 st 2008 Shallow Survey

15 Sound speed field Problem area Error Field Casts within problem area highlighted How does this affect survey planning? - Less of our angular sector is within tolerable uncertainty, so can reduce line spacing in these areas to maintain accuracy HIGH TIDE Geographic plot of depth error highlights areas where the watercolumn s rate of change exceeds our ability to sample it - Could reduce vessel speed to increase spatial sampling of the rapidly changing watermass Casts within problem area highlighted - Could survey at low tide Error Field LOW TIDE Oct. 21 st 2008 Shallow Survey

16 Example 2: Post-Survey Quality Assurance Bay of Fundy CCGS Matthew - EM710 (140 sector) - MVP200 Sound speed Temperature Salinity Oct. 21 st 2008 Shallow Survey

17 Post-Survey Quality Assurance MVP casts over 9.5 hr survey, 2 min. sampling interval Error analysis - Uncertainty due to refraction maintained within +/- 0.02% w.d.!! Oct. 21 st 2008 Shallow Survey

18 Post-Survey Quality Assurance Cross sectional view of soundings - Refraction uncertainty is noise in the error budge - Largest source of uncertainty is water level - Soundings shown here are tidally reduced with WebTide (2D barotropic hydrodynamic model) CHS uses GPS/RTK tide Oct. 21 st 2008 Shallow Survey

19 Example 3: What is oceanographically significant? 2005 CCGS Matthew EM710 Acceptance Trials MVP casts collected during transit Sound Only 36 casts speed required to maintain Oct. casts 21 st 2008 Shallow Survey 2008 uncertainty < 0.25% w.d. 19

20 The Goldilocks Watermass 95 casts collected The Oversampled Watermass 36 casts required The Goldilocks Watermass Oct. 21 st 2008 Shallow Survey

21 Conclusion Ability to monitor watercolumn conditions as a source of error gives unprecedented control over refraction Surveyors can have confidence in refraction solution in real-time The ability to tune MVP profile sampling rate can minimize wear on equipment while maintaining a desired accuracy: The Goldilocks Watermass Many other analysis problems are easily solved using the OMG/UNB SVP Toolkit: Pre-analysis, QA Oct. 21 st 2008 Shallow Survey

22 Future Work Incorporation of UNB uncertainty monitoring in ODIM Brooke Ocean MVP controller Automated MVP deployment with error monitoring & error prediction Application of simulator to case of undersampled watercolumn Oct. 21 st 2008 Shallow Survey

23 Acknowledgements NSERC and CFI funding of ArcticNet NCE Sponsors of the UNB Chair in Ocean Mapping U.S. Geological Survey Kongsberg Maritime Royal (U.K.) Navy Fugro Pelagos Route Survey Office of the Canadian Navy Rijkswaterstaat Mike Lamplugh & Jon Griffin, CHS Atlantic ODIM Brooke Ocean Students of UNB HydroCamp 2008 Oct. 21 st 2008 Shallow Survey

24 Oct. 21 st 2008 Shallow Survey

25 Extra Slides Oct. 21 st 2008 Shallow Survey

26 Simulation Subtleties Roll & Pitch Performance envelope Along-track slope Across-track topography Oct. 21 st 2008 Shallow Survey

27 Surface Sound Speed Can mimic use of a surface sound speed probe: Retrieve sound speed at transducer depth from control cast Use this to compute ray parameter for raytrace with test cast Oct. 21 st 2008 Shallow Survey

28 Along-track view Does this actually work?? Refraction Step Artifacts Step artifact at moment of transition Across-track view Depth difference (m) Difference between swaths before and after transition Predicted difference from raytrace simulator Across-Track (m) Oct. 21 st 2008 Shallow Survey

29 What about in between casts? 1 Depends if you are sampling the watercolumn at a high rate Yes? 1. Are you stuck with real-time reduced soundings (e.g. REA)?...or 2. Can you post-process using nearest in time No? 3. Hmmmm Oct. 21 st 2008 Shallow Survey

30 Interpolation of Error Between Casts Case 1: Stuck with real-time reduced soundings sound speed 5 4 depth 0% w.d. 0.81% w.d. Outer beam depth error Cast 4 Cast 5 Time - Cast 4 is used up to the moment that cast 5 is acquired - Error is zero at moment just after acquisition of cast 4 - Error increases with time (linearly?), reaching a maximum just prior to collection of cast #5 Oct. 21 st 2008 Shallow Survey Error returns to zero after acquisition of cast 5, increasing until the next cast

31 Case 1: Stuck with real-time reduced soundings sound speed 5 4 depth 4+ Oct. 21 st 2008 Shallow Survey

32 Interpolation of Error Between Casts Case 2: Will post-process using nearest in time sound speed 5 4 depth 0% w.d. 0.81% w.d. Outer beam depth error t midpoint t 4 t 5 Time 0.81% w.d. 0% w.d. Oct. 21 st 2008 Shallow Survey

33 Case 2: Will post-process using nearest in time sound speed 5 4 depth Oct. 21 st 2008 Shallow Survey

34 5 4+ Case 2: Will post-process using nearest in time Error increases with time (linearly?), reaching a maximum at the midpoint between collection of casts 4 & 5; error then decreases with time, reaching a minimum at the moment cast 5 is collected Oct. 21 st 2008 Shallow Survey

35 Case 1 vs. Case 2 Last observed in time Depth Closest in time Depth Oct. 21 st 2008 Shallow Survey Time

36 Case 3: Undersampled Watercolumn Imprudent to interpolate, BUT Snapshot of uncertainty is still a useful metric that can be used to compile an average uncertainty ESS: uncertainty of averages UNB: average of uncertainties Oct. 21 st 2008 Shallow Survey

37 Interpolation of Error Between Casts Case 3: Undersampled Watercolumn sound speed Cast 15 Cast 16 Time Outer beam depth error depth Interpolate error? Bad idea Time 1510 m/s Depth 1450 m/s Imprudent to interpolate, BUT Snapshot of uncertainty is still a useful metric that can be used to compile an average uncertainty Oct. 21 st 2008 Shallow Survey