MPL Short range geoacoustic inversion Short range geoacoustic inversion with a vertical line array with a vertical line array Yong-Min Jiang 1 , N. Ross Chapman 1 and Peter Gerstoft 2 1 University of Victoria, Victoria, BC, Canada 2 Marine Physical Laboratory, Scripps Institution of Oceanography, La Jolla, CA, USA Work supported by ONR Work supported by ONR Work supported by ONR Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
MPL Objective: Objective: Travel time geoacoustic inversion in a time varying ocean environment using broadband signal and vertical line array Outline: Outline: • Experimental site & geometric setting up • The time varying environment • The inversion procedure • Energy function, forward model and inversion algorithm • Comparison of the inversion results with geophysical survey • Conclusions Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
Experimental site & set up Experimental site & set up MPL 15 m 25 m 35 m 45 m 55 m 65 m Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
The time varying oceanic SSP The time varying oceanic SSP MPL end start Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
MPL The effect of SSP on signal amplitude The effect of SSP on signal amplitude Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
MPL The effect of SSP on arrival time The effect of SSP on arrival time Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
Travel time inversion Travel time inversion MPL � The difficulties of reflection coefficient inversion • Signal amplitude fluctuation, micro-path interference in the direct arrivals 15 m 25 m • We need to treat each source depth data 35 m separately because of time varying SSP 45 m 55 m 65 m � Travel time inversion procedure • experimental geometry at SD = 65 m: WD, R, SD and VLA tilt • at other source depths, invert for water column SSP • sediment sound speed and layer thickness at all source depths Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
Environmental parameterization Environmental parameterization MPL • Water column SSP is included in the inversion in terms of EOF • SSP casts measured on JD239 + 2 SSPs measured before and after the experiment • Fit only the thermocline region in the inversion • Only the first 4 EOF coefficients are included in the inversion Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
Geoacoustic parameterization Geoacoustic parameterization MPL Source depth: 25 m Source depth: 35 m (a) R R (b) 15 15 S B B S Geoacoustic R R . 13 . 13 o o e n e n parameterization l ct arriva al 11 11 rriv n n o o 09 09 is based on the h h t a p p ro 07 ro direc 07 dire resolvable layers yd yd 05 BR 05 BR H H S S in the data R R 03 R 03 R SRB SRB 01 01 0 20 40 60 0 20 40 60 Time (ms) Time (ms) Source depth: 45 m Source depth: 55 m Source depth: 65 m (c) (d) (e) 15 15 15 BR BR S BR R R R SRB SRB S SR SR . . . 13 13 o o o 13 R e n e n e n l l ct arriva ct arriva rrival 11 11 11 n n n o o o 09 09 09 h h h t a p p p ro 07 ro 07 ro 07 direc dire dire yd yd yd 05 05 05 H H H R R 03 03 R B SRB 03 B S R S R B R 01 01 01 0 20 40 60 0 20 40 60 0 20 40 60 Time (ms) Time (ms) Time (ms) Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
MPL Geoacoustic parameterization continued… Geoacoustic parameterization continued… Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
Energy function, forward model and Energy function, forward model and MPL inversion algorithm inversion algorithm N N ∑∑ L H Energy function: = − 2 E ( m ) ( t T ( m )) i , j i , j = = i 1 j 1 Forward model: Based on ray theory. Inversion algorithm: Optimization algorithm - ASDE (Adaptive Simplex Differential Evolution) Embed downhill simplex (local search) into differential evolution (global search) Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
Example of SSP inversion result Example of SSP inversion result MPL Inverted SSP for SD = 25 m data Travel time fit for SSP inversion Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
Example of geoacoustic inversion result Example of geoacoustic inversion result MPL Travel time fit for 2-layer Travel time fit for I-layer geoacoustic inversion geoacoustic inversion Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
Comparison with geophysical survey data Comparison with geophysical survey data MPL From John Goff WP19, ~ 230 m Source depth Parameter 25 m 35 m 45 m 55 m 65 m TABLE I. Travel time inversion results for one layer layer thickness (m) 21.5 20.6 22.3 22.3 20.4 over half space model sound speed (m/s) 1609 1597 1600 1600 1584 TWT (ms) 26.7 25.8 27.9 27.9 25.8 layer I layer II Source depth TABLE II. Travel time inversion sound speed layer thickness sound speed layer thickness results for two-layer 55 m 1581 m/s 11.9 m 1602 m/s 9.1 m over half space model 65 m 1584 m/s 14.5 m 1611 m/s 8.4 m Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
Summary Summary MPL • Water column SSP has great impact on geoacoustic inversion. SSP in terms of EOFs included in the inversion ( geometric, geoacoustic parameters) • The limitation • Future work Quantify the uncertainty of the estimates Acknowledgements Acknowledgements • Office of Naval Research: for sponsoring the research • Dr. William Hodgkiss from MPL (acoustic data) • Dr. John Goff from Institute of Geophysics, University of Texas at Austin ( geophysical chirp seismic reflection data ) Jiang, Chapman and Gerstoft ASA155, Paris, France, 29 June – 4 July 2007
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