### ASA 125th Meeting Ottawa 1993 May

## 4pUW5. Approximate methods for shallow water acoustics with solid sea
beds.

**Z. Y. Zhang
**

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C. T. Tindle
**

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*Physics Dept., Univ. of Auckland, Auckland, New Zealand
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The reflection properties of representative shallow water solid sea beds
can be reproduced by several simple approximations. An ``equivalent fluid''
with real parameters can be found provided the shear wave speed in the bottom
is less than about 600 m/s. An equivalent fluid with complex density is useful
if the shear wave speed is less than about 1000 m/s. A perfectly reflecting
boundary placed at a ``complex effective depth'' gives a very simple
approximation, again for shear speeds up to 1000 m/s. The acoustic field at
long ranges is determined by reflection at low grazing angles and is reproduced
well by all three approximations. In all cases algebraic expressions are given
for the various parameters. If the complex effective depth is matched to each
mode by iteration it provides a rapid means of finding exact normal mode
eigenvalues for both the trapped and leaky modes. All normal modes are leaky
for a soft solid sea bed and have complex parameters even when there is no
attenuation.