Research On Acoustic Correlation Log For Velocity Measurement

The principle of"waveform invariance principle"is used for the velocity measurement among acoustic correlation log (ACL), which is suitable to be equipped on submarines and autonomous underwater vehicles (AUV). The ACL has striking advantages compared with the acoustic Doppler log (ADL). It can work with a single beam pointing directly down at the sea bed and the return signal is much larger than from the ADL because of the normal incidence. Furthermore the beam can be fairly broad and, for the same size of transmit transducer, this enables the use of lower acoustic frequencies for which the attenuation is less. The ACL has the potential therefore for operating on the sea bed echoes at much greater depths than is possible with the ADL. The design and utilization of ACL become more and more important both for the defense of the national ocean equity and for the scientific research and exploitation of islands and ocean resource.In this thesis, the basic principle of ACL and its applying area, is presented in detail. The correlation properties of seafloor reverberation signals between different hydrophones on receiving arrays are discussed which is determined by the oceanic depth and launching ankle. Some problems, such as the choice of frequency and distance between transducers, are discussed according to sonar equation, which should be considered in the design of ACL.The design method of transducer arrays assembly plays a key role in the velocity measurement for the ACL. A design method of one dimension and two dimension arrays for the ACL is studied in this dissertation. Based on the method of restricted minimum-redundancy linear array (RMRLA), the redundancy factor is got, an ideal virtual array map is built, the conception of virtual array coverage rate is proposed and a fast two-dimensional arrays design method is obtained.With sufficient advisement based on sonar equation, the seabed echo simulation model for acoustic correlation log is founded by gridding method, in which the result is basically consistent with the simulation. Then, the theoretical analysis and emulation study is taken on the spatial correlation according to the model, discussing the Hermite interpolation fit and cubic spline interpolation fit to the spatial correlation function or interpolation effect, the statistical velocity measurement result of beeline array and plane array have come out, the precision of velocity measurement is preferable.The theoretical analysis and emulation study is taken on the temporal and the spatial correlation according to the model, the theory of the temporal-spatial correlation is deduced in details, and data the correlation matrix is set up. Using the maximum likelihood estimation method to estimate the unknown velocity parameters. Nelder-Mead Method is introduced in the paper to find the likelihood function minimization. Due to the possibility to bet into local optimum with Nelder-Mead Method, the improved method is put forward with the Simulated Annealing algorithm. Simulation proves the effectivity of the improved method.