James C. Preisig
Josko A. Catipovic
Dept. of Appl. Ocean Phys. & Eng., Woods Hole Oceanographic Inst., Woods Hole, MA 02543
The shallow water environment is characterized by multiple processes that cover a wide range of temporal and spatial scales and that have a significant effect on the propagation of acoustic signals [G. V. Frisk, ``Report on the Office of Naval Research Shallow Water Acoustics Workshop,'' 24--26 April 1991, WHOI-92-06]. The coupling between the modes of acoustic signals is potentially an effective means of monitoring these processes. Traditionally, a signal source is used to excite the acoustic medium. A receiver array is used to sort out the modes of the propagating signal and the coupling between particular modes is inferred from travel time differences. This approach provides relatively little control of the distribution of transmitted energy among the modes and adds uncertainty to the estimation of the coupling between particular modes. An alternative approach is proposed to use an array of sources to selectively excite modes of the medium. There are several advantages and challenges of using such an approach. First, this will improve the ability to estimate the actual coupling between different modes. Second, it is well known that propagation of and coupling between particular modes is most sensitive to environmental perturbations having particular spatial scales. [B. Cornuelle and B. M. Howe, ``High Spatial Resolution in Vertical Slice Ocean Acoustic Tomography,'' J. Geophys. Res. 92, No. C11 (15 October 1987)]. Therefore, by exciting particular modes and not exciting other modes, the emitted signal can be tuned to be most sensitive to environmental fluctuations on a particular spatial scale. A major challenge in this approach is the tuning of the source array to ensure that only the desired modes are excited.