Inversion Of Sound Speed Profile In Three-dimensional Shallow Water

Sound speed profile (SSP) inversion is an effective method of obtaining the equivalentSSP rapidly in a large area by distributed acoustic detectors, which can hardly be realized bymanual mearsurment. Because of many advantages, such as fast calculation speed, being notinfluenced greatly by the acoustic parameters of seabed, and being suitable for the calculationof sound field in three-dimensional (3D) environments, SSP inversion based on ray traveltime was applied widely in recent years. The key procedure for this model is the accuratedetermination of eigenray and precise measurement of corresponding transmission time.Horizontal deflection often occurs when acoustic ray reflects from irregular seabed in shallowwater, and leads to3D ray tracing. This phenomenon was not usually considered in givenmodels, thus additonal error will be introduced. In this paper a new method for eigenrayseeking in3D environment had been presented firstly, and the shortest travel time ofeigenrays are used to construct a cost function to invert SSP for some area of South China Sea.In addition, SSP inversion is a problem of multi-dimensional optimization and the calculationspeed of transmission time is the main obstacle to restrict the inversion efficience. To improvethe algorithm speed of inversion, ray travel time is expressed as the sum given by referencedSSP and perturbation terms, and inversion procedure is transfered from non-linearoptimization to the solution of system of linear equations of empirical orthogonal functions(EOFs) by perturbation approximation. The primary innovations are as follows:1. The model for eigenray seeking in3D environments and travel time calculation hasbeen developed.The rule of horizontal deflection when a ray reflects from irregular seabed has beenrevealed; the model of ray tracing in shallow water with irregular seabed has been built andsound field calculation software for eigenray seeking in3D environments and travel timecalculation has been developed. Accuracy of the results is compared with other models andtheoretical values. It indicates that the precision of sound travel time given by this modelreaches to the quantity of10-6seconds when the horizontal travel range is up to100km.2. The precision and speed of SSP inversion have been improved dramatically bydifferent methods.The error of sound travel time calculation had been reduced, and the precision of SSPinversion has been improved effectively by considering the horizontal deflection induced byray reflection from irregular seabed. The calculation time has been reduced from10hours to several tens of seconds and satisfies real-time monitoring of SSP with only a few precisionloss by perturtbation approximation.3. The algorithmic performance of SSP inversion based on the shortest travel time of theeigenray had been researched deeply.Relationship between the precision of inversion and the number of EOFs used fornumerical simulation had been studied. It shows that the first3orders of EOFs are enough touse for inversion of SSP, the precision of inversion will not be improved obviously whennumber of used EOFs increases.The problems of parameter mismatching including the depth of seabed, the horizontallocation of source and the inclination of array have been researched by theoretical calculationand numerical simulation. The results indicate that the additional errors introduced bymismatched parameters are inversely proportional to transmission time and the influence onSSP inversion can be neglected when the mismatched parameters are relatively small.Moreover, the following contents are also included in the thesis：1. When calculate the values of SSP and EOFs, in order to overcome the difficulty thatno sufficient measuring data about sound speed in larger depth, instead of the method ofconventional ocean data assimilation, the sound speed profiles exceeding the measurementdepth have been extended by empirical formula, using reasonable estimated values oftemperature and salinity.2. Joint inversion of unknown parameters of source and SSP has been developed.As to the problem of incomplete parameters of source location, explosive moment, theyare inverted at the same time with SSP, using more equations of sound transmission time fordifferent recievers.