MBES BS calibration on reference patch areas using singlebeam calibrated data

Transcription

1 MBES BS calibration on reference patch areas using singlebeam calibrated data Southampton, U.K. Brussels, Belgium NOC 3 FPS Economy 2, Continental Shelf Service D. Eleftherakis 1, X. Lurton 1, J.M. Augustin 1, K. Degrendele 2, T. Le Bas 3, C. Vrignaud 4, S. Loyer 4 & M. Roche 2 Brest, France 1, Institut Français de Recherche pour l Exploitation de la Mer 4, Service Hydrographique et Oceanographique de la Marine GeoHab 2016, Winchester, U.K. 2-5 May km

2 The Very Big Question: We survey the same area with the same system and settings at different times and we observe variation in the BS level. Is this due to MBES? (e.g. aging) Seabed? (e.g. changes in the sediment composition) Assuming that the water column influence is negligible or accurately compensated BSWG (ch. 5)*: a consistent and meaningful product depends on the combination of comparable data Use calibrated BS *X. Lurton, G. Lamarche, Backscatter measurements by sefloor-mapping sonars, Guidelines and Recommendations, a collective report by members of the GeoHab Backscatter Working Group (May 2015), free download:

3 Concept: BS reference area Advantages of acquisition over a reference seafloor: Straightforward The target and configuration are the same as in normal operational conditions It can be integrated with bathymetry calibration operations Criteria for an ideal reference area: Predictable Backscatter Strength BS(θ, f) Significant extent Low-relief topography Spatial homogeneity Time stability High reflectivity Angular isotropy Slow variation in θ Minimum specular effects No dependence in azimuth (futuristic vision inspired from the world's largest swimming pool of San Alfonso del Mar, Algarrobo, Chile)

4 Carré Renard (Brest - France) Used by SHOM for bathymetry trials Time series with HF MBES (EM 3002S, EM2040C, EM 2040) Flat Isotropic Homogeneous Coarse sediments msg mg mg

5 - Non-mooring area - Strong tidal currents Western Solent (Southampton - U.K.) Flat x Dunes: Height= m Length= 10 m Homogeneous Isotropic ~ Coarse sediments

6 Kwinte (Belgium) Long time series with EM1002, EM3002D + EM2040, sidescan sonar datasets Strong tidal currents Coarse sediments Flat Homogeneous Isotropic SPI images scale: 5 cm

7 BS calibration approaches Monitoring - Consistency between successive surveys Discovery - Modelling & inversion - Comparison of different systems The Surveyor s Dilemma

8 BS relative control Relative control is based on the time series Step 1: plot mean BS curve per survey Step 2: average curve for all surveys KWINTE Step 3: stability accepted if: survey means [overall mean ± 1 db]

9 BS relative control Relative control is based on the time series Step 1: plot mean BS curve per survey Step 2: average curve for all surveys Step 3: stability accepted if: survey means [overall mean ± 1 db] KWINTE Range: ±[30 :50 ] Carre Renard Western Solent: in progress

10 BS absolute calibration SBES EK60 Tank calibrated EK60 MBES * Tilted Angles of EK60: 0, 15, 30, 45, 60 * Frequency: 200 khz & 300 khz * Pulse lengths: (64, 128, and 256 us)

11 Step 4: Correct the complete dataset based on the calibration curve Step 1: 2: plot fit heuristic the EK60 model measurements Carre Renard EK60 Carre Renard = model Kwinte Step 3: calculate calibration curve abs()+ Western Solent abs()+ Carre Renard EM3002D

12 Summary: Criterion/Area Carre Renard Kwinte Western Solent Flat x Significant extent Homogeneous Angular isotropy ~ (NE-SW) Time stability BS Good area for: Absolute and relative Absolute and relative Relative control Survey these areas with your own MBES, you should have: Processing: - Software: SonarScope - 1 m grid - Filter db mean - ± [30 :50 ] BS db Mean = db std = 2.5 db Mean = -10 db std= 2.2 db Mean = -7 db std = 2.2dB Carre Renard Kwinte Western Solent

13 Conclusions & Future Work: - The criteria and the tolerances of acceptance have to be realistic - Both methods (absolute and relative) are reasonably simple - For a reference area the time stability is the critical parameter - The relative control is to be applied in all cases - Expected advantages of absolute calibration: - crosscalibration of different systems - Seafloor type classification - Variance analysis is in prospect A BS standard approach analogous to an IHO bathymetric quality test using a reference surface is under progress.

14 The Round Table of BS patch test reference area Users Rules Thank for the hospitality in this legendary location! Thank you for your Attention! To practise BScontrol most diligently To survey nothing before calibration To grant access to reference areas to all surveyors, each according to his MBES When in honour or disgrace, to calibrate with the greatest accuracy for those who keep the annals To injure no marine mammal

15 Additional slides

16 Generic Seafloor Acoustic Backscatter model Descriptors A: Specular level B: Specular angular extent C: Lambert reference D: Lambert decrement E: transitory level F: Transitory angular extent Specular Lambert Transitory G. Lamarche, X. Lurton, A.L. Verdier, J.M. Augustin, Quantitative characterisation of seafloor substrate and bedforms using advance processing of multibeam backscatter Application to Cook Strait, New Zealand, Continental Shelf Research, (31) (2011).

17 W.S.

18 KW

19 C.R

20 Carre Renard Step 1: plot mean BS curve per survey Step 2: find average curve from all surveys Step 3: stability accepted if: survey means [overall mean ± 1 db] Range: ±[30 :50 ]

21 W.S. Dunes: Height= m Length= 10 m

22 BS azimuth dependence technique Kwinte (3/2016) Carre Renard (EM2040) 1 db db db Western Solent ~

23 Kwinte different systems

24 Carre renard different systems 24

25 Effect of Pulse Length EM2040 Carre Renard 300 khz EM3002