B. Edward McDonald
Naval Res. Lab., Washington, DC 20375
Shallow ocean acoustic propagation generally results in bathymetric refraction toward deeper water, and is accompanied by bottom attenuation. In modal eigenray calculations involving small horizontal grazing angle interactions with bathymetry, cases have been found [Heaney et al., J. Acoust. Soc. Am. 90, 2586--2594 (1991)] in which path integrated bottom loss actually decreases with increasing mode number. This apparent paradox is resolved by showing that the increase in modal ray turning rate with mode number can exceed the accompanying increase in bottom attenuation rate. Thus fixed angular deflection can imply attenuation which decreases with mode number. Investigation of acoustic eigenmodes leads to an algebraic relation between bathymetrically induced rates of horizontal ray turning and modal energy loss. This relation gives the modal attenuation rate per increment of horizontal ray turning, and is expressed in dB per radian. The dB per radian relation is in general not a monotonic function of mode number. Implications of this result may be relevant to observations in the Heard Island Feasability Test. [Work supported by NRL and Scripps Institute of Oceanography.]