Abstract:
The purpose of the work reported here is to determine the domain of validity in frequency and range of ray-based acoustic wave field expansions under typical deep ocean conditions. This is accomplished by comparing transient wave fields computed using the ray-based MaCh1 propagation model to those computed using the mode-based SNAP propagation model. Although MaCh1 is ray-based, it correctly treats caustics of arbitrary complexity. Attention is confined to range-independent environments so that potential ray theory deficiencies unrelated to ray chaos can be isolated; the latter problem will be investigated separately. Both high lattitude (constant gradient) and temperate (canonical) environments are considered. Two types of errors associated with the ray-based wave field expansion are identified. These errors may be attributed to the omission of: (1) complex rays (tunneling phenomena) near the source; and (2) beam displacement effects for near-grazing rays. Ray-based errors, which are generally isolated in time, grow with decreasing frequency and increasing range. Overall, however, the agreement between the ray- and mode-based wave fields is surprisingly good with, say, f[inf 0]=20 Hz, (Delta)f=25 Hz, r=300 km, or f[inf 0]=80 Hz, (Delta)f=100 Hz, r=1000 km. [Work supported by ONR.]