The impact of ocean sound speed variability on the uncertainty of - - PowerPoint PPT Presentation

The impact of ocean sound speed variability on the uncertainty of geoacoustic parameter estimates

• N. Ross Chapman and Yong-Min Jiang

University of Victoria Victoria BC Canada University of Victoria, Victoria, BC, Canada Work supported by ONR

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

Objective:

• Describe a simple technique to account for unknown ocean

sound speed profile in matched field inversion sound speed profile in matched-field inversion

Invert for an effective SSP that creates a range independent propagation environment p p g

• Method:

Use EOFs to parameterize the SSP Use EOFs to parameterize the SSP What information is necessary? Large data set of SSP over extended space and time Li it d d t t i i i it f i t i d ti Limited data set in vicinity of experiment in space and time

• Hypothesis is that limited set will be adequate if changes in

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

SSP are not large

Experimental site:

Acoustic array (MPL): Acoustic array (MPL):

• VLA1
• 16 sensors, 3.75 m separation
• the bottom one is 8.2 m from

the bottom one is 8.2 m from the sea floor Source ship stations, distance to VLA1: VLA1:

• WP21, 1 km
• WP22, 3 km
• Wp23 5 km
• Wp23, 5 km

Water depth:

• ~79.0m

Signal frequencies (CW tonals):

• LF: 53, 103, 203, and 253 Hz
• MF: 303, 403 503, 703 and

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

303, 03 503, 03 953 Hz

Geoacoustic model

• Invert for:
• geometric parameters of

the experiment and the experiment, and

• geoacoustic model

parameters

• Approach:

Bayesian Matched field inversion

Geoacoustic model for the SW06 site

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

Recap: Is SSP at the source all we need? p

SSPs measured at source and VLA1

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

Ambiguity surface of MFP (source localization)

SSP data:

SSPs measured at source and VLA1 (derived from CTDs)

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

(derived from CTDs) SSPs measured at SHARK, SW31 and SW32

Approaches: pp

• Limited set from CTDs measured at source ship stations

p

• change only in the thermocline
• requires fewer EOFs (only 4 EOFs)
• Full set from oceanographic moorings and CTDs from

source ship stations source ship stations

• cover whole water column
• need more EOFs

need more EOFs

• how to decide how many EOFs?

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

Comparison of the energy fit versus the number p gy

• f EOFs used:

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

Example: 8EOFs for large SSP sample set:

20 Base line profile 20 1 EOFs 20 2 EOFs 20 3 EOFs 1480 1500 1520 1540 40 60 1480 1500 1520 1540 40 60 1480 1500 1520 1540 40 60 1480 1500 1520 1540 40 60 20 40 4 EOFs 20 40 5 EOFs 20 40 6 EOFs 20 40 7 EOFs 1480 1500 1520 1540 60 1480 1500 1520 1540 60 1480 1500 1520 1540 60 1480 1500 1520 1540 60 8 EOFs 9 EOFs 10 EOFs 11 EOFs 20 40 60 20 40 60 20 40 60 20 40 60

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

1480 1500 1520 1540 60 1480 1500 1520 1540 60 1480 1500 1520 1540 60 1480 1500 1520 1540 60

Comparison of effective SSP for 1 km data Comparison of effective SSP for 1 km data

Small SSP data set Large SSP data set

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

Marginal distributions of SSPs

Inter-parameter correlations for 1 km data – EOF t i d ti t EOFs vs. geometric and geoacoustic parameters

Small SSP data set Large SSP data set

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

2D Marginal distributions of EOFs

Results – comparison of Bayesian geoacoustic Inversion by using different SSP data set at 1 km Inversion by using different SSP data set at 1 km

Small SSP set Large SSP set Large SSP set

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

1D Marginal distributions of geometric parameters

Results – comparison of Bayesian geoacoustic Inversion by using different SSP data set at 1 km Inversion by using different SSP data set at 1 km

Small SSP set Large SSP set

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

1D Marginal distributions of geoacoustic parameters

Results – comparison of Bayesian geoacoustic p y g Inversion using small SSP data set at 1, 3 and 5 km

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

Breakdown – 3 km site, small SSP data set

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

2D Marginal distributions of EOFs with geometric parameters

Breakdown – 3 km site, large SSP data set

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

2D Marginal distributions of EOFs with geometric parameters

Conclusions:

• Water column sound speed profile has significant effect on

geometric parameters and therefore

• affects geoacoustic parameter estimates
• has great impact on matched field processing based

has great impact on matched field processing based source localization

• Geoacoustic parameter estimates using different SSP observations
• Geoacoustic parameter estimates using different SSP observations

are consistent with each other

• for small SSP variations over the propagation path, the

most relevant SSPs are more effective

• for large SSP variations, single effective SSP may not be

adequate

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

q

Acknowledgments:

• Office of Naval Research:

for sponsoring the research

• William Hodgkiss and Peter Gerstoft from MPL

for providing the acoustic data

• David Knobles from ARL

for providing navigation and source depth data

• John Goff from Institute of Geophysics, University of Texas

at Austin f idi h i l hi i i fl i d for providing geophysical chirp seismic reflection data

• Arthur Newhall from WHOI

f idi hi b ti d t

Chapman and Jiang ASA157, Portland, Oregon, 18 - 22 May 2009

for providing oceanographic observation data