Riemannian Geometric Modeling of Underwater Acoustic Ray Propagation · Basic Theory

Underwater sound propagation models are generally established from the extrinsic perspective, that is, embedding acoustic channels in Euclidean space with fixed coordinate system. Riemannian geometry is intrinsic for curved space, that can describe the essential properties of background manifolds. The underwater acoustic Gaussian beam was originally adopted from seismology. Till now it is the most important method used in acoustic ray based modeling and applications. Due to the advantages of Gaussian beam method over the traditional ray counterpart, it is the mainstream technology of ray propagation computational software such as the famous Bellhop. With the assumption of Euclidean space, it is hard to grasp the naturally curved characteristics of the Gaussian beam. In this paper, we propose the Riemannian geometry theory of underwater acoustic ray propagation, and obtain the following results : (1) The Riemannian geometric intrinsic forms of the eikonal equation, paraxial ray equation and the Gaussian beam under radially symmetric acoustic propagation environments are established, that provide a Riemannian geometric interpretation of the Gaussian beam. In fact, the underwater acoustic eikonal equation is equivalent to the geodesic equation in Riemannian manifolds, and the intrinsic geometric spreading of the Gaussian beam corresponds to the lateral deviation of geodesic curve along the Jacobian field. (2) Some geometric and topological properties of acoustic ray about conjugate points and section curvature are acquired by the Jacobi field theory, indicating that the convergence of ray beam corresponds to the intersection of geodesics at the conjugate point with positive section curvature. (3)The specific modeling method under horizontal stratified and distance-related environment is presented using the above theory. And we point out that the method proposed here is also applicable to other radially symmetric acoustic propagation environments. (4) Simulation and comparative analysis of three typical underwater acoustic propagation examples, confirms the feasibility of the Riemannian geometric model for underwater acoustic propagation. And shows that the Riemannian geometric model has exact mathematical physics meaning over the Euclidean space method adopted by the Bellhop model. The basic theory given in this paper can be extended to curved surface, three-dimensional and other complex propagation environments. And especially it lays a theoretical foundation for the further research of long-range acoustic propagation considering curvature of the earth.