Abstract:
Ray-travel-time nonreciprocity has been used in most tomography experiments to determine current velocity in the ocean by acoustic means at distances large compared to ocean depth. Although very successful in deep water, this approach is not applicable in the coastal ocean where ray arrivals are not separable and/or identifiable due to multiple bottom interactions. In this paper, other parameters of the acoustic field, including normal mode travel time, ray and mode horizontal refraction angles, and full field phase, are treated as possible data for the current velocity field. Some qualitative differences between acoustic fields in moving and motionless fluid are indicated and their importance for acoustic monitoring of ocean currents is emphasized. Existing mathematical models of low-frequency underwater sound propagation in a moving ocean are discussed. It is demonstrated that nonreciprocity of various acoustic field variables possess quite a different sensitivity to the flow velocity field and robustness with respect to unavoidable uncertainties in our knowledge of system and environmental parameters. Full-field inversion, based on an appropriate acoustic field variable, is concluded to be a promising technique for current velocity remote sensing in the coastal ocean environments. [Work supported by NSERC and RBRF.]