有限元/边界元耦合方法在海洋生物目标强度特性研究的应用*

目标强度特性是海洋生物声学识别与资源量评估的重要依据，其中，基于近似几何体和声阻抗特性的理论模型法是研究海洋生物目标强度的重要手段。由于对几何形态近似处理以及数值求解方法的限制，传统理论模型对声波频率、入射方位以及目标声阻抗、形态尺寸等均有各自不同的适用范围，单一模型难以满足不同种类或同一种类但不同尺寸海洋生物的目标强度求解。本文尝试将逐渐见诸应用的有限元/边界元耦合方法用于海洋生物目标强度特性研究，分别以球形生物、纺锤形鱼类尾明角灯鱼(Ceratoscopelus warmingii)和细长形浮游动物南极大磷虾(Euphausia superba)为例进行仿真计算，并与相适应的经典理论模型进行对比分析。结果表明，对于球形生物，有限元/边界元耦合方法与解析模型的目标强度频响曲线完全吻合；对于纺锤形鱼类，有限元/边界元耦合方法可有效弥补基于模态级数解的形变圆柱体模型在中低频和两端入射时的准确性问题；对于细长形浮游动物，有限元/边界元耦合方法与畸变波玻恩近似模型高度吻合。综上，有限元/边界元耦合方法对多种海洋生物目标强度求解均具有较好的适用性，未来有待进一步结合实验测定进行验证。

Application of the coupled FEM/BEM in predicting the acoustic target strength of marine organisms

The target strength (TS) is critical for the acoustic identification and biomass estimation of marine organisms. According to their morphologies and acoustic impendence, the theoretical acoustic scattering models accompanied with approximate geometry representations are widely used for predicating the TS of fish and zooplankton. However, due to various limitations of different conventional scattering models on acoustic frequency, incident angle, target impedance and morphology requirements, it is usually difficult to predict the TS of different species or the same species with different sizes using a single model. In this paper, the coupled finite element method and boundary element method (FEM/BEM) was introduced to study the TS of marine organisms including the spherical target, prolate spheroid fish and elongated zooplankton. The TS of gas-filled and fluid spherical target, Ceratoscopelus warmingii and Euphausia superba were analyzed and compared with those of conventional scattering models. The results show that the frequency response of TS calculated from the coupled FEM/BEM is consistent with the analytical model for the spherical marine organism. For the prolate spheroid fish with a swimbladder, the coupled FEM/BEM can effectively solve the inaccuracy at low frequencies and end-on incidence compared with the mode series-based deformed cylindrical model (MB-DCM). For the elongated zooplankton, the coupled FEM/BEM is in good agreement with the distorted wave born approximation (DWBA) method. Therefore, the coupled FEM/BEM has a good applicability in studying the TS of different marine organisms, which needs to be further validated with in-situ and ex-situ measurements.