### ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

## 4pUW7. Bathymetrically refracted modal eigenrays and bottom attenuation: A
paradoxical relation.

**B. Edward McDonald
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*Naval Res. Lab., Washington, DC 20375
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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.]