Study On Acoustic Multi-frequency Inversion Of Suspended Sediment Concentration In Water

Acoustic inversion of suspended sediment concentration is considered to be one of the main methods to obtain real-time data of suspended sediment concentration in water.Compared with the traditional fixed-point water sampling measurement technology,the acoustic measurement method has technical characteristics such as non-invasiveness to the measured environment,higher space-time resolution and the ability to achieve profil ing measurement,and has thus attracted much attention in recent years.The thesis focuses on the application of acoustic methods to the scientific problems involved in the measurement of suspended sediment concentration in water bodies,with the purpose of exploring effective acoustic inversion schemes for suspended sediment concentration.The core of the acoustic method to measure the concentration of suspended sediment lies in the principle of acoustic backscatter ing.After the emitted sound wave is scattered by suspended sediment particles in the target water area,the scattered echo signal is cl osely related to the physical characteristic information of the scatterer particles.By exploring the characteristics of the scattering echo and the scattering process,the physical parameters of the suspended sediment such as the concentration and particl e size information can be obtained.Therefore,this thesis constructs a top-down acoustic backscatter model based on the Acoustic Doppler Current Profiler(ADCP).The simulated emission signal and actual measurement environment are substituted into the model to simulate the energy change and phase change caused by the propagation of sound waves in the water,and the reconstruction of the scattered echo signal is completed from the frequency domain.The energy change is described according to the active sona r equation,which mainly discusses the acoustic attenuation characteristics(the absorption attenuation of sea water and scatterers and the expansion attenuation of sound waves)and the acoustic scattering characteristics(scattering effects of the scatterers);the phase change adopts uniform distribution according to statistical analysis.In the end the correctness of the acoustic backscatter model is verified by analyzing and comparing the statistical characteristics of the simulated echo signal and the measured echo signal.The thesis derives in detail the theoretical basis for inverting the concentration and particle size of suspended sediments using acoustic backscattering technology,and combs the algorithm flow of single-frequency concentration invers ion.Based on those,dual-frequency concentration inversion and multi-frequency concentration inversion algorithms(energy ratio algorithm,difference algorithm and K-nearest neighbor algorithm)are separately developed,the influence of the scattering att enuation term generated by suspended sediment concentration on the performance of various inversion algorithms is explored when the concentration of suspended sediment is at a high level.The 200,300,and 500 k Hz scattered echo signals reconstructed by the acoustic backscatter model are used to simulate the inversion algorithm.The results show that dual-frequency inversion can effectively solve the problem of iterative instability in single-frequency inversion,and multi-frequency inversion can achieve simultaneous inversion of particle size and concentration information.Multiple ADCPs with different frequencies were used in the Zhoushan waters to carry out multiple related on-site experiments.The data processing results show that the suspended sediment concentration profiling information estimated by the single-frequency,dual-frequency and multi-frequency inversion algorithms is roughly consistent with the actual measurement environment,confirming the feasibility of the acoustic inversion algorithm for suspended sediment concentration proposed in this thesis.