We study an inverse problem of reconstructing a 3D object in an ocean waveguide from far-field measurements of scattered field.An indicator sampling method for fast imaging of scattering object is presented.The method does not depend on any acoustic and geometric properties of the scattering object.It is valid for obtaining good imaging of location and shape of the scattering object,even as far-field data are measured in a limited aperture and corrupted by noise.Generally,multiple scattering between object and waveguide boundaries has positive influence on reconstruction of the scattering object.

We study multiple acoustic scattering of three dimensional multiple obstacles in shallow water waveguides. A method coupling finite-element and natural boundary-element is developed to solve acoustic scattering based on Galerkin variational principle and DtN mapping. The method avoids adoption of absorbing boundary used in conventional finite element method. It is validate by numerical study of 3D multiple obstacles with various shapes.