Using Ship Noise For Ocean Bottom Geoacoustic Parameters Inversion

The ocean bottom geoacoustic parameters have significant effects on the soundpropagation and sonar performance predictions in shallow waters; therefore geoacousticinversion has been a very hot topic in underwater acoustics. Due to the limitations of activesources, e.g., environmental protection and security reason in particular situations, the usingsound sources already exist in the ocean for geoacoustic inversion is drawing more andmore attentions. In shallow waters, there exist plenty of ship activites (fishing and shipmen),whose noise is a good resource for passive acoustic applications. Based on theseconsiderations, this thesis studies the using ship noise for ocean bottom geoacousticinversion, which includes the following main aspects:(1) The first part reviews the past researches on geoacoustic inversion, which brieflyintroduces the geoacoustic history and the main advantages and disadvantages of traditionalgeoacoustic inversion techniques. This section emphasizes these researches on the oceansound noise characteristics and these applications based on the noise sources. Due to theuncertainty and complicity of oceanic noise, it is hard to directly apply active acoustictechniques in passive sources, which needs newly developed equipments or new methods.Since a vector sensor can simultaneously measure the pressure and vector field of the soundfield at the same point, the using vector sensor in acoustic applications are more and morepopular in underwater acoustics. The vector sensor research history and applications arealso reviewed in this section.(2) The second part introduces the linear acoustic propagation theory in shallow watersand theoretical basises for our applications. The broadband vector field of a ship source ofopportunity is found also exhibits interference structure but has different structure with thatof pressure field. Their structure differences are determined by environmental properties andcan therefore be used for environmental inverse problems. Through investigation, thecombination of pressure filed and vector field can achieve physical observables that betterinterpret the waveguide interference characteristics, which is favorable for thoseapplications exploiting the waveguide interference characteristics. The waveguidecharacteristics impedance is obtained also through the combination of pressure field andvector field, which is source spectal level independent but very sensitive to geoacousticparameters. These features make it favourable to be observed for passive inversion purpose.In this section, some numerical researchs on the time-domain broadband vector signal predictions are also given.(3) The third section discusses the using interference structure of a broadband shipnoise for sediment geoacoustic inversion. The low-frequency striations structure of a shipnoise of opportunity is found weakly affected by the source spectrum shape, but theirlocations are slightly changed according to environmental perturbations (here refer tosediment sound speed and thickness). Their relationship is theoretically analysed andnumerical interpreted under a typical shallow water environment. Based on the relationship,an interferometer technique is proposed to characterize the sediment property variations in asofted-sediment shallow water environment. The ship noise data collected in MediterraneanSea is processed and the results agree well with those of active measurements in the sameexperiment site. The robustness of the method to uncertainties of source depth and range isalso discussed.(4) The fourth part is about the research on using the waveguide characteristicimpedance measured by a vector sensor due a ship noise for ocean bottom geoacousticinversion. A global search optimize algorithm based inversion scheme is used to estimatethe geoacoustic parameters and their uncertainties. In this section, we process the ship noisedata measured at Dalian sea, the inversion scheme is respectively applied to the pressuredata, vector data and the waveguide characteristic impedance due to a passing-by ship. Theinversion results of the waveguide characteristic impedance show better agreement withprior information of the geoacoustic properties of this sea area, demonstrating the accuracyof using waveguide impedance for passive geoacoustic inversion.