Shallow Survey 08 Portsmouth NH Interferometry for bathymetry

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Interferometry for Bathymetry Sonars: Accuracy, Resolution and Quality Factor Xavier LURTON Ifremer Acoustics & Seismics Department BP 70, 29280 Plouzané, France lurton@ifremer. fr 1

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Summary 1. 2. 3. 4. Interferometry: the Accuracy-Resolution trade-off Multibeam echosounders : the phase-ramp A new quality factor for bathymetry sonars Conclusions 2

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Interferometry Fundamentals a A B Y Spacing x DFAB = 2 pa/l. sin(q - y) q-y z Wavelength R Angle q M q = y + asin(DFAB/ka) (ambiguous since DF modulo 2 p) Phase difference DFAB Angle q x = R. sinq Time t 2 R/c Range R z = R. cosq 3

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Bathymetry Accuracy Neglecting errors / time sampling, motion, SSP… Degradation of measurement quality w/ NOISE • Signal intrinsic fluctuations & decorrelation • Additive noise d. DF(°) Phase-diff. error d. DF Angle error dq SNR = 10 d. B d. DF = 40° Time error dt Bathymetry error dz SNR (d. B) 4

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Bathymetry Accuracy (2) • Improvement of d. DF = averaging over N signal realisations – In radar : multi-look (several snapshots of a scene) – In sonar : averaging of N neighbouring samples • For a fluctating signal in noise: d. DF(°) (Lurton, IHR 2003 ; Llort et al, subm. JOE) • Improvement from N-averaging (large N) SNR = 10 d. B N = 100 d. DF < 2° N=100 20 5 2 1 SNR (d. B) The Trade-Off : Gain of N in accuracy resolution degradation by N 5

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » MBES Interferometry s 1(t) Sub-arrays Phase difference s 2(t) q 0 Dj t Zero phase difference time • SNR improved by narrow Beamforming • No phase ambiguity • Good time accuracy @ zero-crossing • Resolution Phase-ramp fitting length NOT REALLY a local measurement Degradation with incident angle (1/cos²q) 6

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Resolution Loss by Phase-Ramp Fitting Numerical simulation : • Vertical plane, seafloor = discrete scatterers (+noise) • Inside formed beams : – Backscattered time signals phase ramp – Fitting, and zero-phase instant detection • Retrieving of the seafloor profile Phase difference at q DF t t. D (R, q) computation 7

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Interfero. bathy resolution – Phase ramp = 100% beamwidth Shallow water 50 m ; 200 beams ; V = 8 kts Shallow water 50 m ; 4 x 800 beams ; V = 8 kts Performance evolution : resolution Acrosstrackrange(m) Acrosstrack Depth(m) Depth 8

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Phase-Ramp Reduction Results • 100 k. Hz MBES • 100 beams @ 2° • D = 50 m • 2 features 10 x 2 m 9

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Resolution Loss by Phase-Ramp Fitting • Spatial resolution = fitting-interval physical length • The phase-ramp fitting plays the role of a smoothing filter (sliding average) of the same length directly applied to seafloor topography Simulations are run for various : beamwidth, beam density, sampling frequency, phase ramp fit length, water depths… Ø Beamwidth controls SNR (array gain) & indirectly interferometric resolution (via phase ramp length) Ø Sampling frequency controls SNR (averaging gain), not resolution Ø Beam density controls DTM spatial sampling, not resolution 10

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Improvement Strategies • Decrease beam width, with Dx = 100% –Straightforward & expensive Dj Dj t t • Decrease Dx inside given beamwidth Dj –EM 300 optional setting (J. P. Allenou, X. Lurton, Kj. Nilsen, 1999 ) Dj t t • Split Dx several soundings / beam –EM 3002 « Hi-Resolution mode » (Kongsberg, 2006) Dj Dj t t 11

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Towards Hybrid Bathymetry Sonars MBES beam number evolution : • 16 (3°, Seabeam, 1976) 880 (0. 5°, Seabat 7150, 2008) > 50 x ! • Need for so many / so narrow beams? The best of the two worlds: High-resolution interferometry inside formed beams • Several soundings / beam @ limited beam number • Moderately narrow beams (2°? ) good SNR & no phase ambiguity • Sonar image ideally co-registered with bathymetry • Coexistence seafloor & in-water targets 12

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Classical Resultats These GG Hybrid processing of MBES data Ph. D. Gerard Llort-Pujol, Telecom-Bretagne (2006) 13

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » A Sounding Quality Estimator Context : development (2008) by Ifremer of a customized Bottom Detection Algorithms for Seabat 7150 & 7111 on R/V Pourquoi pas? Definition of a Bathymetry Quality Estimator dz/z for: • Objective assesment of soundings • Data flagging Based upon : • Elementary modelling dt/t • Measured signal characteristics • Local configuration parameters Concept = independent of the sonar type / model 14

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Quality Factor / Phase • Time (depth) accuracy Phase fluctuation Dj dt D t Measured: • Phase std. dev. • Phase slope A t. D QF = Log 10(q. F) QF dz/z < 2. 0 > 1% 2. 5 0. 3% > 3. 0 < 0. 1% 15

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Quality Factor / Amplitude • Arrival time = center of gravity (intensity, or amplitude) Dt. D QF = Log 10(q. F) • Dt. D envelope width • f. S = sampling frequency • B 5 (for intensity c. o. g. ) 16

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Application Example of the QF D(m) QF Beam Number • Reson Seabat 7111 – RV Pourquoi pas? - July 2008 17

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Application Example of the QF Bathymetry Reson Seabat 7111 RV Pourquoi pas? July 2008 Quality Factor Detection type : • Amplitude • Phase 18

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Properties of the Quality Factor • A direct estimator of the bathymetry quality (what e. g. the SNR is NOT) • Based upon actual signal measured statistics the signal characteristics (SNR…) are implicit • QF = independent of the sonar type/model (e. g. adaptable to interferometric SSS) • Algorithm readily implemented / computed for Ø Flagging of soundings recorded in datagrams Ø Local bathymetry accuracy-resolution assesment Ø Direct validation of soundings in DTMs Ø Bathymetry processing using quality criteria (CUBE) 19

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Conclusions • Averaging interferometry samples = indispensable for accuracy, but detrimental to resolution • Zero-Phase-Difference = not a local measurement in itself; the MBES phase ramp acts like a low-pass filter of same length • Future hybrid systems should mix advantages of beamforming and time-sample interferometry, for hi-resolution 3 -D measurements • A local bathymetry quality estimator can be defined from the phase/amplitude signal features 20

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Sonar. Scope® Presentation • Ifremer processing software toolbox • Applied to (part of) Common Dataset • Results posters / back of the conference room • SW demonstrations by Jean-Marie Augustin 21

Shallow Survey’ 08 – Portsmouth, NH – « Interferometry for bathymetry… » Averaging : Signal or Depth Signal Averaging Depth Averaging 22