Sound Propagation Fluctuation Caused By Internal Waves And Detection Of Internal Soliton Properties

The property of inhomogeneity in ocean is one of the most important factor which affects the sound propagation. Internal waves are the prominent reason that gives rise to the fluctuation of sound propagation in the region where the motion of internal wave is frequent. The oceanic internal waves are composed of linear and nonlinear internal waves, and often behave their superposition. The study of influence of internal waves on sound propagation includes two aspects, one is the parametric establishment of internal wave model, and the other is the selection of proper sound propagation model.In this paper, associated with the GM spectrum, the vertical displacements of linear internal waves were obtained by using CTD data and linear internal wave modeling. The spatio-temporal variation of sound speed was studied employing the relation between the vertical displacements of linear internal wave and perturbation of the sound speed. Based upon the results of linear internal waves, the solitary internal wave modeling (KdV equation) was established and its coefficients were determined using the mode function of linear internal waves. Then the spatio-temporal variation of sound speed profiles induced by internal solitons is studied numerically. It can be seen that both linear internal waves and internal solitons have important effect on variations of the sound speed. As to the whole sound speed fields, the linear internal waves play a prominent role on the disturbances of sound speed due to the small effective scope of the internal solitons.Combined with range-dependent PE modeling of the sound propagation, the acoustical fields were computed with the effect of linear internal waves being taken into account. The transmission loss via range curves were illustrated at differentsound source depths, send frequencies, received depths. These results were compared with those of no internal waves, and the decibels of sound intensity fluctuation, which caused by linear internal waves were obtained. Moreover, the variation of transmission loss with range were illustrated at different amplitudess of internal solitons, sound source depths, send frequencies, received depths when the internal solitons appeared. It was shown from numerical results that linear internal waves play a prominent role on the transmission of sound propagation due to its larger effective scope than that of internal soliton. In addition, with the increase of amplitudes of internal soliton, the transmission loss caused by internal soliton becomes larger.No mater internal or external pycnocline region the sound source locates, the sound intensity under the pycnocline is larger than that within pycnocline at the same received range due to negative pycnocline gradient. For a fixed sound source depth, the acoustical fields caused by high send frequency has strong sound intensity fluctuation, and their variation is sharp enough in an adjacent range. When the sound source locates pycnocline, we found that a shadow area within some plane range can be formed in the bottom of sea floor if the send frequency is large. However, the shadow area cannot be formed when the sound source is located in the bottom of sea floor.A characteristic parameter inverse modeling of internal soliton was established based on the relation between the ocean acoustic fields and internal waves. Combined with some parameters of internal waves obtained from SAR image, the amplitudess of internal solitons was inversed by the information of sound transmission loss. Finally, the inverse modeling was confirmed by the comparison inversed results and experimental ones. The results shows that this inverse method is reasonable.