ON THE MEASUREMENT OF MODAL GROUP TIME DELAYS IN THE DEEP-OCEAN

The modal description of sound propagation in deep ocean environments is considered. Recently published inversion algorithms have assumed th at modal group time delays can be measured. Such a measurement is stra ightforward to make if either: (1) the modal group arrival of interest is resolved in time; or (2) the wave field is sampled on a dense vert ical array which spans much of the water column, thereby enabling the orthogonality of the modes to be exploited. In order to temporally res olve modal group arrivals using measurements made on a single hydropho ne, the frequency bandwidth must: (a) be sufficiently broad that the t emporal separation between successive modal arrivals exceeds the recip rocal bandwidth; and (b) be sufficiently narrow that, across the band, the group slownesses of neighboring mode numbers do not overlap. To s atisfy both conditions [(a) and (b)] the ratio of the range to the cen ter frequency must be large. Unlike ray arrivals, modal group arrivals broaden as range increases due to dispersive spreading. To minimize d ispersive spreading so that accurate group delay time estimates can be obtained, the ratio of range to center frequency should be kept small . Thus the requirements for temporally resolving modal group arrivals conflict with the conditions that minimize dispersive spreading. Numer ical results are presented which give quantitative estimates of which combinations of range, center frequency, bandwidth, and mode number pr oduce conditions which are favorable for temporally resolving modal gr oup arrivals in six regions of the deep ocean at mid and low latitudes . These results suggest that it is extremely difficult to find conditi ons which allow modal group arrivals to be temporally resolved and sim ultaneously allow group delay times to be measured with sufficient acc uracy to be useful for tomography. The situation is further complicate d by internal waves which appear to cause mode coupling and significan t broadening of modal group arrivals at frequencies above approximatel y 50 Hz. The combined effects of dispersive spreading and internal wav e-induced mode coupling suggest that, without employing mode filtering techniques, modal group time delay-based inversion schemes in the dee p ocean do not appear to be promising. (C) 1996 Acoustical Society of America.