Adjoint-based shallow water tomography with partially known bottom geoacoustic properties

Recently, a numerical extension of the wide-angle PE (WAPE) adjoint-based inversion method has been presented that allows direct inversion of the geoacoustic parameters that are embedded in a discrete representation of nonlocal boundary conditions at the water-sediment interface [Hermand et al., ICTCA 2005]. In contrast to conventional ocean acoustic tomography (OAT), correct modeling of sound interaction with the bottom is particularly important for shallow water acoustic tomography (SWAT). Uncertainty or partial knowledge of the bottom acoustic parameters can significantly degrade SWAT performance. In this context the paper discusses an adjoint-based tomography approach that includes the geoacoustic parameters as additional unknowns in the parameter space.
A

priori
information about the bottom characteristics, e.g., the approximate thickness and composition of sediment layers and hard rock basement are often available from site surveys with sub-bottom profiling systems or in the form of previously archived
in

situ
data. This information can be used as an initial solution to start the inversion process or be included in an augmented cost function through the use of regularization. By means of adjoint modeling exact gradient information can be obtained to determine the sound speed profile of the water column and concomitantly adjust the partially known sediment and bottom halfspace parameters. A case study based on environmental data obtained in Mediterranean shallow waters is used to present first results.