THE EFFECTIVE DEPTH APPROXIMATION FOR SOUND-PROPAGATION IN SHALLOW-WATER OVER A SEDIMENT LAYER AND A HARD-ROCK BASEMENT

This paper describes an effective depth approximation for a constant d epth ocean waveguide with a layered bottom. It is shown that a good ap proximation to the acoustic field can be obtained by using an effectiv e depth approximation based on the bottom impedance, and that attenuat ion of the modes propagating in the waveguide can be included if the c omplex form of the impedance is utilized. It has also been shown that the approximation of the field should include both effective depth mod es, which are similar to those in a pressure release waveguide and rig id bottom modes which are similar to those in a wave guide with a rigi d bottom. A simple criteria has been given to distinguish between thes e two types of modes depending on the bottom impedance, and the attenu ation factors for each type of mode has been shown to be of a differen t form. Finally, a formula is given for the calculation of the distanc e between nulls in the long-range transmission loss in terms of the ef fective depth of the water channel.