On the feasibility of modal group delay‐based tomography in the deep ocean

Abstract

The modal description of sound propagation in deep ocean environments is considered. Several recently published inversion algorithms have assumed that modal group time delays can be measured. Such a measurement requires that the frequency bandwidth: (1) be sufficiently broad that successive modal arrivals are resolved in time, and (2) be sufficiently narrow that, across the band, the group slownesses of neighboring mode numbers do not overlap. These conflicting requirements can be reconciled only at long range. In the deep ocean, the late arriving, low‐order modes are most likely to satisfy condition (2). Unfortunately, recent measurements and numerical simulations suggest that these modes are susceptible to internal wave‐induced mode coupling—especially at long range. The following pessimistic conclusion is made: Modal group delay‐based inversion schemes can be applied to measurements of acoustic wavefields in the deep ocean only for a carefully selected choice of experimental parameters (range, center frequency, and bandwidth) which may not be experimentally accessible. [Work supported by ONR.]