Acoustic Measurements Of The Water Column Suspended Particles

Acoustic measurement of suspended sediment is an important method to study the distribution of suspended sediment concentration and migration characteristics. Acoustic measurement offers the advantages of non-intrusive measurements, with high resolution compared to the traditional observation techniques. However, this method comprises a complicated signal processing technique. This thesis focuses on some key problems when acoustic backscatter technology is used in suspended sediment concen-tration profile measurement, and aims to explore the effective suspended sediment concentration inversion method.The basic principle of acoustic measurement is that a sound source produces a pulse signal and acts as a receiver at the same time. The radiated sound reaches the suspended matter and travels back towards the source. Then the scattering echo will be recorded. The backscattering echo carries the physical characteristics of the sus-pended matter, which can be derived from the acoustic inversion method. The scatter-ing and attenuation properties of suspended sediment are essential for acoustic meas-urements, and the co-contribution of suspended particle size and concentration on scat-tering energy deserve special attention.This thesis introduces the attenuation from geometrical spreading and the loss due to absorption and other nongeometrical effects such as scattering. Then the scattering properties are discussed from the perspective of both acoustic cross section and form function. On this basis, a broadband Acoustic Doppler Current Profiler (ADCP) scat-tering echo model is presented. First we analyze the detailed sound propagation under the ADCP measurement scenario, in terms of transmission loss, volume scattering strength etc. Then the broadband scattering is characterized as a filtering process in the frequency domain, and the Fourier synthesis technology is used to reconstruct the scattering echo signal. The model results have been compared with the field measure-ment data, showing a good correspondence between the measured and modeled scat-tering echo signal statistically.This thesis investigates acoustic inversion algorithms to extract suspended sedi-ment information, which can be divided into single and multi-frequency method in terms of the number of frequencies used. We focus on the correction problem for scat-tering attenuation under the single frequency measurement scenario, dual-frequency concentration inversion method and multi-frequency particles size inversion method such as ratio algorithms, differential algorithms and nearest-neighbor algorithm. This thesis explores the application of Particle Swarm Optimization (PSO) algorithm in the study of suspended sediment concentration inversion from the perspective of multi-parameter optimization. The simulation echo data is applied to inversion analysis. And the performance of single frequency inversion method at300kHz and multi-frequency inversion method at200,300, and500kHz are presented.During the thesis research, a prototype Water Column Property Profiler (WCPP) has been designed and built, including sonar system hardware, real-time control and data acquisition software. The prototype systems and broadband ADCP have been used to collect echo data in marine environmental several times. And the processing result and analysis of the experiment data have verified the performance of the prototype systems and relevant signal processing algorithms investigated in this thesis.