Researches On The Method Of Geoacoustic Inversion With Bottom Parameters In Deep Ocean

Geoacoustic parameters such as sound velocity,density,attenuation coefficient and layering characteristics of the seabed are important parameters of the sound field.In the deep ocean environment,since direct measurement is very difficult,geoacoustic inversion is one of the effective methods to obtain the bottom parameters.With the acoustic survey experiments in deep ocean participated by our research group,the inversion study of bottom parameters in deep ocean is carried out in the paper.Based on the bottom reflection characteristics,reverberation and ship noise of underwater acoustic signals,the corresponding geoacoustic inversion methods are studied.The theoretical research in this paper are closely combined with the actual experimental data.The proposed geoacoustic inversion method in the deep ocean have been verified by simulation and experimental data,which have the important application value.The specific research work includes the following aspects: 1.Based on the bottom reflection loss,a geoacoustic inversion method is developed for the reliable acoustic path and sound shadow zone in the deep ocean.Combining the sound propagation characteristics with the bottom reflection characteristics,the geoacoustic inversion methods which are suitable for the reliable acoustic path and sound shadow zone in the deep ocean are studied.For the environment of reliable acoustic path,the sensitivity analysis of bottom reflection loss to different bottom parameters of the uniform seabed is studied in simulation with high and low sound velocity seabed.The bottom reflection loss with grazing angle for the uniform and stratified seabed is obtained.The bottom reflection loss of the uniform seabed is independent of the frequency,and the bottom reflection loss of the stratified seabed shows complex oscillating phenomenon with the frequency and grazing angle.The multipath of acoustic signal obtained with the reliable acoustic path in the experiment of deep ocean is analyzed,and the bottom reflection loss is extracted by using the energy difference between the direct wave and the bottom reflected wave.For the environment of sound shadow zone,the bottom reflected wave is the most common and important multipath signal.The multipath of acoustic signal obtained with the sound shadow zone is analyzed in simulation.We choose four pairs of acoustic ray,which are direct wave and bottom reflected wave,surface reflected wave and surface-bottom reflected wave,bottom-surface reflected wave and bottom-surface-bottom reflected wave,surface-bottom-surface reflected wave and surfacebottom-surface-bottom reflected wave.The latter two pairs are selected for extracting the bottom reflection loss.By matching the simulation and experimental data,the bottom parameters of the reliable acoustic path and sound shadow zone in the deep ocean are obtained by inversion using the relationship between the bottom reflection loss with the grazing angle.2.The model of bottom reverberation level for decline tendency with travel time and a physical-statistical bottom reverberation model are eatablished and studied.The two models have a clear physical meaning and can be used for geoacoustic inversion in the deep ocean.Based on the formation process of bottom reverberation,the relationship between scattering intensity and bottom parameters is considered and a bottom reverberation model in the deep ocean with a clear physical meaning is established.With the three-dimensional coordinate system of ocean space,the bottom reverberation modeling in the deep ocean is divided into propagation and scattering processes.The ray model is used for the propagation process,and the Jackson model is used for the scattering process.The discrete time series of the bottom reverberation level are calculated using the two-way propagation time from the source and the receiver to the scatterer.The scattering intensity is assumed to be a sum of the roughnessscattering and volume-scattering cross sections.The experimental data of bottom reverberation in the deep ocean is analyzied.The method of using the decline tendency of bottom reverberation with travel time is proposed to achieve the purpose of effectively obtaining bottom parameters in the inversion.Based on the experimental data in the South China Sea,the envelope of bottom reverberation is more consistent with the K distribution.A double exponential distribution is used to generate the amplitude of reverberation,of which envelope obeys the K distribution.At the same time,the propagation and scattering model are introduced as the parameters of the density function of double exponential function,so that the model has a certain physical mechanism.Therefore,a bottom reverberation model with a clear physical meaning can be used for geoacoustic inversion.The cost function is to minimize the difference between the envelope of the bottom reverberation of experimental data and simulated data.3.A method of striation-based depth estimation with a vertical line array in the deep ocean is proposed,which can be used for estimation of the depth of targets and the geoacoustic inversion in the deep ocean.An acoustic tomography method for seabed based on a vertical line array is proposed,which can be used to obtain bottom structure as priori information for geoacoustic inversion.The inversion using ship noise is ecological and economical compared with a controlled source,which can be applied to bypassing ships.The simulation of sound field with a vertical line array in the deep ocean shows that the position and the number of striations are closely related to the source depth.The intensity plot with the different depth of source versus the frequency and D-DOA exhibits striation structures.And the number of striations increases with increasing source depth.A cost function is presented to quantify the difference between the measured striations and model-based striations and the depth of source is obtained as the source for inversion.The bottom impulse response is obtained by an acoustic tomography method for seabed based on a vertical line array.The bottom structure and layer thickness are extracted as the prior interval of the inversion.The noise of surface ship can be an excellent source for inversion compared with controlled sources.The spectrograms of upward propagation shows obvious striation structure and are used for the inversion as the interaction between the bottom reflected wave with the bottom.The strong tones of the upward propagation spectrogram are identified for the inversion to provide the detailed geoacoustic properties of the seabed.