A Comparison of Ray Theory, Modified Ray Theory, and Normal Mode Theory for a Multiple Caustic Convergence Zone

Abstract

Ray theory calculations done for a multiple caustic convergence zone generated by an arbitrary constant gradient profile do not provide an adequate description' of the sound field. Due in large part to ray theory deficiencies near caustics, ray computations show poor agreement when compared with normal mode calculations done using a finite difference approach [A. V. Newman and F. Ingenito, NRL Memo. Rep. 2381 (1972)]. When results from modified ray theory (D. A. Sachs and A. Silbiger, J. Acoust. Soc. Am. 49, 824 (1971)] are incorporated into the ray theory calculation, agreement with the normal mode results improves considerably. The oscillations present in the typical propagation loss versus range curve for a convergence zone are shown to be not only due to double arrivals to the right of a particular range minimum caustic, but also due to interference to the left of the caustic between its shadow zone contribution and real rays associated with the preceding caustic. [Supported in part by ONR and DNA (Subtask V99QAXNA003-04).]