Z. Y. Zhang
C. T. Tindle
Physics Dept., Univ. of Auckland, Auckland, New Zealand
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.