Research On Acoustic Detection Method Of Deep-sea Cold Seep Based On Resonant Frequency

The leakage of fluids rich in natural gas hydrate from the seabed,which has unique physical characteristics compared to the surrounding seawater and is called the cold seep.The cold seep can prove the existence of natural gas hydrate from the seabed,it is closely related to the marine atmospheric environment and the deep-sea benthic biological community.However,the target intensity of the deep-sea cold seep is weak,and its morphology is complex.There is a great challenge to achieve efficient detection and recognition.The main research objective of the thesis is to improve the detection ability of deep-sea cold seep based on the resonant acoustic detection method.Furthermore,the scattering intensity and the three-dimensional(3D)acoustic image can be obtained.The work is of great significance in the scientific research of deep-sea exploration.Firstly,the acoustic scattering characteristics of the cold seep are studied.The theoretical model is established based on the wave equation to acquire the acoustic scattering principle of bubble plumes.The resonance frequency of the cold seep bubble and its correction formula is derived,the relationship between the resonance frequency,depth,and bubble radius is provided,which verified the accuracy of resonance frequency calculation has been improved by 10% after correction.Simulation results under ideal conditions can prove that the cold seep bubble has strong acoustic scattering characteristics near resonance frequency.Secondly,deep-sea resonance cold seep detection and scattering intensity inversion methods are studied.The shipborne detection based on the acoustic resonance characteristics of the bubble is proposed,compared with conventional high-frequency detection methods,the scattering intensity can be increased by more than 10 d B.The resonant cold seep detection system is designed by using broadband detection,and it can cover the resonance frequency of the cold-seep bubble,which is effectively balancing detection efficiency and accuracy.A deepsea cold seep was discovered in the Dongsha Sea using this system.The scattering intensity inversion method is proposed to compensate for the deviation between the bubble scattering intensity under resonance and its theoretical value.In addition,the deep-sea cold seep recognition method is studied,which can recognize the bubble plume from the acoustic image.By processing the bubble scattering intensity and distribution characteristics within the frequency range of 14 k Hz to 19 k Hz in Dongsha sea trail,the maximum error between the measured results and the correction scattering intensity is 3 d B.Finally,the 3D reconstruction method of the cold seep plume image is studied.Based on the analysis of the plume spatial distribution and variation characteristics,the 3D coordinate system is established.The bi-sliding window correction(BSWC)method is proposed to inverse the actual contour of the cold seep.The geometric-constrained convergence method(GCCM)based on the steepest descent is proposed to fit the contour of the cold seep on each depth profile.Using the discrete profile acoustic image information collected from the sea trial,the contour correction,contour fitting,and 3D image reconstruction results of the measured deep-sea cold seep are formed.