THE UNIDENTIFIED OBJECT PROBLEM IN A SHALLOW OCEAN

This work addresses the inverse problem of the identification of a pas sive three-dimensional impenetrable object in a shallow-water environm ent. The latter is assumed to have flat perfectly reflecting (sound-so ft top and sound-hard bottom! boundaries and therefore acts as a guide fat acoustic waves. These waves are employed to interrogate the objec t and the scattered acoustic wavefield is measured on the surface of a (virtual) vertically oriented cylinder (of finite or infinite radius, corresponding to near- or far-field measurements) fully enclosing the object. The direct scattering problem is resolved in approximate mann er by employing, in a local manner, the known separated-variable solut ion for a scattering by a vertically oriented cylinder in a perfect wa veguide. The inverse problem is resolved in the same manner (i.e,, wit h the same approximate field ansatz) by least-squares matching of theo retical fields !for trial objects) to the measured field. Examples are given of successful shape reconstructions for two types of immersed o bjects. This manner of solving approximately both the forward and inve rse problems is generalized to the case of a body of shallow water wit h an elastic seabed. (C) 1998 Acoustical Society of America. [S0001-49 66(98)05303-X].