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Backscatter Patch Test inter-comparison of systems using shared reference areas for testing, calibration, and quality assessment M. Roche, T. P. Le Bas, X. Lurton, K. Degrendele, L. De Mol, V. Van Lancker, M. Baeye, J. De Bisschop, C.

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SLIDE 1

Backscatter Patch Test – inter-comparison of systems using shared reference areas for testing, calibration, and quality assessment

  • M. Roche, T. P. Le Bas, X. Lurton, K. Degrendele, L. De Mol, V. Van Lancker,
  • M. Baeye, J. De Bisschop, C. Vrignaud, S. Papili, O. Lopera, J.M. Augustin,
  • N. Le Bouffant, L. Berger & D. Eleftherakis
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SLIDE 2

Monitoring the seabed = EU legal obligation The European Union Marine Strategy Framework Directive (“MSFD”)

“Coastal waters, including their seabed and subsoil, are an integral part of the marine environment, and as such should also be covered by this Directive…”

GES 2020

Six-year review

  • f the different

elements

  • f the strategy

2018-2021 Implementation

  • f the

Marine Strategy 2016 Monitoring programs 2014 Programs

  • f measures

2015 Initial assessment,

  • bjectives, targets

and indicators 2012 (+ 6 years)

dura lex, sed lex…

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SLIDE 3

Combined MBES Bathy + BS = the standard technology for mapping the seabed Bathymetry Backscatter Strength

NE Malta – EM3002D – acquired April 2010 on RV Hercules Ref: Aaron Micallef, Tim Le Bas, Veerle Huvenne, Philippe Blondel, Veit Hühnerbach, Alan Deidun, (2012) A multi- method approach for benthic habitat mapping of shallow coastal areas with high-resolution multibeam data, Continental Shelf Research, 10.1016/j.csr.2012.03.008.

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SLIDE 4

Benthic Terrain Mapping and supervised backscatter texture analysis

Combined MBES Bathy + BS = the standard technology for mapping the seabed

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SLIDE 5

MBES BS for monitoring the seabed nature variation? Yes, if…

2005 2012 2011 2009

TIME

  • 15
  • 35

BS

dB

500 m 500 m

Kwintebank West channel KWGS area, Belgium EM3002D

  • Sediment / habitat limits spatial change ?
  • Mean dB level trend

Sediment / Habitat change ?

Yes, if BS measurements are enough accurate and stable…

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SLIDE 6

BS required accuracy for detecting sediment change trends

  • average difference between the mean BS levels of the different sediment types ± 2 dB

1 dB (1/2 average difference) = accuracy required

  • Plateau = most discriminant part = MBES BS PARADISE
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SLIDE 7

The plateau approach to quantify the mean BS level

IFREMER dataset (RV Thalia EM2040 dual receiver @ 200 kHz)

BS versus Incidence angle for different seafloor interfaces from various sites

  • Restriction to sector 35°-45° Loss of information (specular + fall-off)
  • Remains the most relevant BS data derived from only oblique insonification
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SLIDE 8
  • 16
  • 15,9
  • 15,8
  • 15,7
  • 15,6
  • 15,5
  • 6
  • 5
  • 4
  • 3
  • 2
  • 1

BS mean level stability ?

BS mean level -15.7 std 0.1

BS mean level variance evaluation by repetitive measurements during a tide cycle: BS mean level Tide level Average level of BS is fully stable over a short period ~ 13 h

RV Belgica EM3002D

  • same one line

area surveyed several times (heading = cst)

  • Beam averaged

1x1 m mosaic

  • Stats on BS

mosaic values from the same area and for incidence angle interval 35°-45°

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SLIDE 9

Sources of instability (out of the sonar equation)

The Very Big Question: To what extent mean BS level variation from one cruise to another really represents significant change in seabed properties and not change due to the followings? Potential sources of instability (non exhaustive list, see BSWG C3 and C5) Scientific literature Manufacturer references Changes of acquisition parameters on board

±

Antenna state - biofouling

?

Aging of antenna (electronic and coating)

?

Water column influence? Influence of dense near-bed and water column suspensions on the MBES BS?

  • MBES + acoustic doppler current profiler (ADP) measurements
  • Benthic landers + ADPs + acoustic backscatter sensor (ABS) near-bed sand transport
  • Water samples to calibrate the optical and acoustical sensors
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SLIDE 10

Sea state?

Beaufort 6-7 The Scientists… The Captain and a world-famous Belgian surveyor…

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SLIDE 11

BS time series on a sandbank area covered by very large dunes: Negative offset of up to 3 dB of the mean dB levels

Sea state?

EM1002 RV Belgica

calm sea rough sea

If target is to monitor the seabed, stop the survey if the weather is too rough!

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SLIDE 12
  • Check BS stability = mandatory
  • Seafloor reference area = most practical approach
  • Basic assumption =

Reference area seafloor cst against required accuracy of BS measurement (~ 1 dB)

  • The ideal reference area:
  • Flat
  • Deep and large enough
  • Isotropic homogenous seafloor
  • Not subject to seasonal change
  • Easily accessible
  • Ground truthing data
  • Time series

MBES BS reference area

Belgian BS reference area and RV Belgica II

(futuristic vision inspired from the world's largest swimming pool of San Alfonso del Mar, Algarrobo, Chile)

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SLIDE 13

MBES BS reference area Similar as the approach used to calibrate the SAR scatterometer Reference surface = Amazonian rain forest Used for years by satellite radars Forest canopy:

  • High reflectivity (-10 dB)
  • Isotropic angular response
  • Excellent stability over the year

(one more point for Brazil!)

From C. H. Buck, Alternative large-scale distributed targets for SAR elevation beam pattern characterization, Tech. rep., ENVISAT project, ESA-ESTEC, 2001.

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SLIDE 14

BS patch test Analog to a IHO bathymetric quality test using a reference surface Analog to a IHO bathymetric quality test using a reference surface Reference model based on reference area BS data: Reference model based on reference area BS data: Ultra dense survey of a reference area at Time Zero using calibrated sensor BS reference model with 35°- 45° data Ultra dense survey of a reference area at Time Zero using calibrated sensor BS reference model with 35°- 45° data Time series of the reference area mean of all the 35°- 45° data BS reference model Time series of the reference area mean of all the 35°- 45° data BS reference model Goal = estimate the relative stability of the BS with a reference model Goal = estimate the relative stability of the BS with a reference model Relative stability estimation: New survey 35°- 45° BS data are compared with the reference model

  • Gap statistics (mean and standard deviation)
  • Relative Quality Level / Acceptability Thresholds remain to be defined
  • Under study approach

Relative stability estimation: New survey 35°- 45° BS data are compared with the reference model

  • Gap statistics (mean and standard deviation)
  • Relative Quality Level / Acceptability Thresholds remain to be defined
  • Under study approach
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SLIDE 15

FR - SHOM - IFREMER areas Reference area In/around the Bay of Brest

  • Several reference areas
  • Depth 25 to 60 m – Flat + wrecks
  • Selected by SHOM for bathymetry trials
  • Candidates for becoming BS reference areas

E.g. Carré Renard:

  • Depth ± 25 m
  • Coarse sediment
  • Long time series with HF MBES (EM 3002, EM 2040)
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SLIDE 16

UK - National Oceanography Centre area Western Solent area

Teledyne Reson 7101 – 240 kHz

  • 7 km², depth from 10 to 30 m
  • sand + gravels
  • strong tidal currents
  • non-mooring area (pipelines + cables)
  • MBES Teledyne Reson 8101 or 7101

surveys for 2008 to 2014 (1 per year)

  • Plus some sidescan and ground truth

data. Reference area

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SLIDE 17

Belgian Teams

  • 6.5 km², depth from 22 to 26 m MLLWS
  • sand + gravels
  • strong tidal currents
  • EM1002 (4 surveys from 2008 to 2011)
  • EM3002D (9 surveys from 2009 to 2015)
  • EM2040 (2 surveys from 2013 to 2014)
  • Sidescan sonar (Remus – SS 900 kHz)
  • Ground truth data: SPI, video, grab samples…

Reference area

2 km 2 km

Flemish sandbank KWGS area Bathymetry BS draped m m

22 22 26 26 22 26

dB

  • 15
  • 15
  • 25
  • 25

dB dB

  • 15
  • 25
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SLIDE 18

HV Ter Streep EM3002D 300 kHz RV Belgica RV Simon Stevin EM3002D 300 kHz EM2040 @ 300 kHz

Inter comparison on reference area

18/09/2012 18/09/2012 21/09/2012

  • 15
  • 35

BS

dB

200 m 200 m

  • Statistics on

common area

  • Along corridor

cross section

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SLIDE 19
  • 21
  • 20
  • 19
  • 18
  • 17
  • 16
  • 15
  • 14

100 200 300 400 500 600 m

Belgica EM3002D Ter Streep EM3002D

Inter comparison on reference area: results dB level from Ter Streep and Belgica EM3002D fully comparable

dB dB

  • 26
  • 24
  • 22
  • 20
  • 18
  • 16
  • 14
  • 12
  • 10

Belgica EM3002D TerStreep EM3002D

Statistics Corridor cross section

Simon Stevin EM2040 @ 300 kHz Simon Stevin EM2040 @ 300 kHz

EM2040 @ 300 kHz not comparable with EM3002 300 kHz (∆ 2-3 dB)

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SLIDE 20

Sharing BS reference area: RV Belgica C1516 – 10-16/06/2015

100 km 100 km 100 km

  • Collegial Survey of the 3 BS reference areas
  • MBES EM3002D
  • SAS Shadows
  • Video and Boxcore
  • Shared BS reference data library
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SLIDE 21

Concluding message

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SLIDE 22

Thank you for your attention! Special thanks to the BSWG Chairing Committee and Editors. Os meus sincero agradecimentos aos colegas brasileiros pela vossa hospitalidade em Salvador da Bahia, cidade de todos os Santos e dos grandes Jorge Amado e Dorival Caymmi.