水下掩埋目标的散射声场计算与实验

Acoustic scattering from elastic target buried in water-sand sediment

水下掩埋目标声散射问题是识别和探测掩埋目标的理论基础,是声散射研究领域的热点问题.本文基于射线声学推导了掩埋情况下目标声散射计算的格林函数近似式,并在此基础上进一步给出了相应的远场积分公式.在有限元方法的基础上,将推导得到的公式写入有限元仿真软件,对软件功能进行拓展,构建二维轴对称目标的声散射模型,并计算掩埋情况下弹性实心球在不同条件下的目标强度,获得了其散射声场随频率、掩埋深度、沙层吸收系数等参数的变化规律.开展实心球的自由空间和浅掩埋条件下水池声散射实验,利用共振隔离技术处理实验数据,提取目标声散射的纯弹性共振特征进行分析,结果表明可将其用于掩埋目标识别和探测.最后利用总散射声场与理论计算结果进行对比,验证了理论仿真的正确性.

Acoustic scattering from objects buried in water-sand sediment is the foundation of target detection and identification. It is also a research hotspot in areas of acoustic scattering while the domestic research on scattered field from buried targets is not deep. This paper deduces an approximate Green＇s function of acoustic scattering from targets buried in water-sand sediment, which describes clearly the whole physical process during the propagation of scattered waves. Next, on basis of geometric acoustics, the corresponding Helmholtz-Kirchhoff formula of integration is presented.Complicated integration of the full wave number spectral representation of the Green＇s function is avoided by employing approximate formula derived from the method of ray acoustics. As a result of neglecting the influence from lateral waves,the Helmholtz-Kirchhoff integral given applies to supercritical incidence case. The function of COMSOL multiphysics software is expanded by writing this formula of integration into it. By means of finite-element method, numerical calculation models for two-dimensional axisymmetric targets are established on the software platform. The proposed model built in free field is verified through comparing numerical results obtained with the Rayleigh method which has been validated in previous research achievements of acoustics. The target strength of buried elastic solid sphere is calculated under different conditions in order to analyze the change regularity of buried scattered field. We provide a summary about the law of target strength of the elastic sphere varying with frequency, buried depth, and the attenuation of sand. Finally, we conduct acoustic scattering experiments in free space and shallow buried conditions and process the data with the method of isolation and identification of resonance to separate eliastic echoes from reverberation echo and specular echo. Results from the experiment of free field show that components of the scattered wave should include Rayleigh waves and whispering gallery waves. The processed data of objects buried inside layered fluid media indicate that characteristics of resonance spectra can be used to identify and detect the target effectively while echo signal is not available for identification of target. The proposed technique is verified through the comparison of data from total scattered field between experiment and theoretical prediction. This study has important guiding significance for detecting and identifying targets embedded within layered acoustic media in practical applications.