Proceedings of Meetings on Acoustics

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1 Proceedings of Meetings on Acoustics Voume 19, ICA 213 Montrea Montrea, Canada 2-7 June 213 Underwater Acoustics Session 1pUW: Seabed Scattering: Measurements and Mechanisms II 1pUW5. Non-invasive characterization of fuid mud from scaar and vector noise fieds due to a sma boat Qun-yan Ren and Jean-Pierre Hermand* *Corresponding author's address: Acoustics and Environmenta Hydroacoustics Lab, av. F.D. Roosevet 5, Brusses, 15, Brusses, Begium, jhermand@ub.ac.be The geoacoustic characterization of sediment using the ratio between passive pressure and vector fieds that are measured by a singe pressuregradient receiver system is discussed. The ratio is highy sensitive to environmenta properties but independent of unknown source spectrogram, e.g., boat noise that exhibits compex time-varying spectra shape. Noise data sets due to different runs with a sma boat were recorded on two cosey-spaced hydrophones offshore the Amazon Rio mouth in June 212 and processed by a noninear inversion scheme. Goba optimization based on genetic agorithms provides 12 parameters describing the run geometry and physica properties of a fuid mud and underying mud sediment, and the associated margina posterior probabiities. Good consistency among the inversion resuts of the different datasets demonstrates the feasibiity of the method for environmenta characterization under far from idea conditions, in term of ack of precise navigation data, unknown source spectra, uncertain receiver depth and tit, etc. Pubished by the Acoustica Society of America through the American Institute of Physics 213 Acoustica Society of America [DOI: /1.4849] Received 22 Jan 213; pubished 2 Jun 213 Proceedings of Meetings on Acoustics, Vo. 19, 714 (213) Page 1

2 INTRODUCTION The vector sensor can simutaneousy measure the pressure and vector fieds at the same point of a waveguide [Lesie(1956)], making it is possibe to use their reationship that may be independent of source characteristics for passive acoustics [Peng(28)]. This paper discusses a cost-effective method to obtain bottom acoustic properties from passing ship noise of opportunity from pressure and vector data measured at a signa point by an easiy depoyabe system. The use of the ratio between pressure p and vertica partice veocity V z, or simiary the pressure gradient p g, for geoacoustic inversion is emphasized here. The range variation of such ratio is independent of source spectrogram [Ren(212)] but very sensitive to geoacoustica properties, incuding to parameters that are typicay insensitive in matched fied processing techniques, e.g., bottom ayer densities [Bonne(211), Gervaise(212), Hermand(1999)]. An experiment for soft sediment geoacoustic characterization using ship noise of opportunity was conducted offshore the Amazon Rio mouth in June 212. Noise data sets due to different runs with a sma boat were recorded on two cosey-spaced hydrophones depoyed from another sma boat. The measurement conditions were far from idea in the term of unknown source spectra, ack of precise navigation data, uncertain receiver depth and tit, etc. A goba optimization method based on genetic agorithms [Godberg(1988)] is appied to the recorded data, which provides an estimation of the geoacoustic parameters of a fuid mud and underying mud sediment, and of their margina posterior probabiities. Good consistency among the resuts obtained with different data sets demonstrates the feasibiity of the method for environmenta characterization even under difficut measurement conditions. METHOD DESCRIPTION Vertica waveguide characteristic impedance At ranges greater than severa water depths, the acoustic compex pressure fied p and pressure gradient p g generated by an omnidirectiona point source at depth z emitting a singe tone at anguar frequency of ω, received at range r and depth z, can be respectivey expressed as and L p (ω, r, z) = πis(ω) φ (z )φ (z) H (1) (ξ r), (1) p g (ω, r, z) = L z p (ω, r, z) = πis(ω) κ,z φ (z )φ (z) H(1) (ξ r), (2) where L is the propagation mode number, H (1) is the zero order Hanke function of the first kind, φ and ξ are the moda function and eigenvaue for mode, φ is the derivative of φ with respect to depth, and κ,z is the vertica wavenumber for mode. S (ω) is the source spectrogram, which when being unknown, can significanty affect the accuracy of acoustic inversion resuts. It woud therefore be favourabe for passive acoustics if one can effectivey use observabes that are independent of S (ω). The ratio between p and p g can be written as Z g = p = iωρ L φ (z )φ (z) H (1) (ξ r) p g Mm κ m,z φ m (z )φ m (z) H (1) (ξ mr). (3) Proceedings of Meetings on Acoustics, Vo. 19, 714 (213) Page 2

3 Time (s) Frequency (Hz) FIGURE 1: Estimated spectrogram for the ship noise of opportunity with a CPA range of about 15 m. The effect of source characteristics term S (ω) is eiminated in this ratio, making the Z g independent on source spectra. However, the range variation of Z g is highy correated with the eigenvaues (ξ ) determined phase terms H (1) (ξ r), moda functions φ (z) and their derivatives, especiay for those in the denominator. ξ and φ (z) are determined by the environmenta properties. Z g is very sensitive to environmenta properties and is therefore a good physica variabe to be observed for environmenta inversion based on passive acoustic data. Goba optimization method A goba optimization agorithm (genetic agorithms) is used here to estimate the searched parameters by minimizing the foowing objective function that is based on the cross correation coefficient between data and prediction at different frequencies, φ(m) = 1 ˆφ(m), (4) and ˆφ = 1 N f r i Z d g (f, r i ) Z p g (f, r i ) N f r i Zd g (, f, r i ) r i Z p g (f, r i ), where N f is the tota number of seected frequency bins, Z d g (f, r i ) is the rea data for frequency f at range r i, and Z p g (f, r i ) is the corresponding repica predicted by RAMGEO propagation code [Coins(1991)]. The observations generated by the optimization agorithm are then used to estimate the margina posterior probabiities of the searched geoacoustic parameters of the fuid mud and underying mud sediment. AT-SEA EXPERIMENT AND DATA PROCESSING An experiment for ocean bottom geoacoustic characterization with the vector fied due to a ship noise of opportunity has been conducted in an area of the Amazon River mouth in June 212. A receiver array composed of two cosey and verticay spaced acoustic pressure sensors was used to record the ship noise data. The measurement conditions of this experiment were far from idea, due to the strong current, surface waves, ack of positioning system, etc. Figure 1 is the measured noise spectrogram for one passive run. Denote the received pressure spectrogram for the two receivers Proceedings of Meetings on Acoustics, Vo. 19, 714 (213) Page 3

4 WD (m) α sed (db/λ) SD (m) H (m) C (m/s) bot RD (m) C (m/s) sed ρ (g/cm 3 ) bot CPA (m) ρ sed (g/cm 3 ) α bot (db/λ) Tit (dec) FIGURE 2: Estimated posteriori distributions for a search of the tweve parameters of water depth (WD), sediment thickness (H), sediment sound speed (C sed ), sediment density (ρ sed ), sediment attenuation (α sed ), bottom sound speed (C bot ), bottom density (ρ bot ), bottom attenuation (α bot ), source depth (SD), receiver depth (RD), CPA range and tit. as P 1 (ω) and P 2 (ω). Their average is P avg (ω) = (P 1 (ω) + P 2 (ω))/2, the pressure gradient is P g (ω) = (P 1 (ω) P 2 (ω))/2 spacing, and Z g can be derived as P avg /P g. Noise spectra estimates at seven frequencies (115 Hz, 125 Hz, 15 Hz, 175 Hz, 2 Hz, 225 Hz and 25 Hz) as a function of range are used for the inversion. For the genetic agorithms used in this paper, the tota number of generation is 6 and each generation has a popuation size of 1. The estimated 1D posterior for the searched 12 parameters is shown in Fig. 2 for one of the data sets. It is seen that the parameters are we resoved with reativey sma variabiity. The resut for source depth exhibits two comparabe peaks, probaby due to the different depths of the noise sources (engine and propeer) of the ship of opportunity. For the other geometrica and sediment parameters, the resuts are cose to the prior information. It shoud be noted that, the inverted densities cose to that of direct measurements. For the other data sets, the estimated geoacoustic parameters are simiar to the resuts in Fig. 2, demonstrating the robustness of vertica impedance method for passive geoacoustic inversion. CONCLUSION This paper discussed a passive geoacoustic characterization technique using vector fied measured by two verticay spaced hydrophones. The ratio between pressure and pressure gradient measured from a noise source of opportunity is used in a goba optimization method to estimate the geoacoustic parameters of fuid mud and underying mud sediment, and corresponding posteriori distributions. The inverted resuts are in accordance with prior information and consistent with concomitant and independent direct measurements. Furthermore, consistency among the different data sets demonstrates the robustness of the method for passive geoacoustic inversion. Proceedings of Meetings on Acoustics, Vo. 19, 714 (213) Page 4

5 ACKNOWLEDGMENTS Work supported by Begian Nationa Fund for Scientific Research (F.R.S.-F.N.R.S.), Nationa Counci for Scientific and Technoogica Deveopment (CNPq), Waonie-Bruxees Internationa (WBI), and Coordenação de Aperfeiçoamento de Pessoa de Níve Superior (CAPES). REFERENCES [Lesie(1956)] Lesie, C. B. Hydrophone for measuring partice veocity, J. Acoust. Soc. Am. 28(4), (1956). [Peng(28)] Peng, H. S. and Li, F. H. Geoacoustic inversion from vector hydrophone array in shaow water, Technica acoustics. 27(2), (28). [Ren(212)] Ren, Q. Y. and Hermand, J.-P. A sequentia fitering technique for geoacoustic inversion with ship of opportunity and a vector sensor, In Proceedings of OCEANS 12 MTS/IEEE HAMPTON ROADS Conference (Harnessing the Power of the Ocean). (212). [Bonne(211)] Bonne, J. and Chapman, N. R. Geoacoustic inversion in a dispersive waveguide using warping operators, J. Acoust. Soc. Am. 13, EL11 EL17 (211). [Gervaise(212)] Gervaise, C., Kinda, B. G., Bonne, J., Stéphan, Y., and Vaez, S. Passive geoacoustic inversion with a singe hydrophone using broadband ship noise, J. Acoust. Soc. Am. 131(3), (212). [Hermand(1999)] Hermand, J.-P. Broad-band geoacoustic inversion in shaow water from waveguide impuse response measurements on a singe hydrophone: Theory and experimenta resuts, IEEE J. Ocean. Eng. 24, (1999). [Godberg(1988)] Godberg, D. E. and Hoand, J. H. Genetic agorithms and machine earning, Machine Learning. 3, (1988). [Coins(1991)] Coins, M. D. Higher-order Padé approximations for accurate and stabe eastic paraboic equations with appication to interface wave propagation, J. Acoust. Soc. Am. 89, (1991). Proceedings of Meetings on Acoustics, Vo. 19, 714 (213) Page 5