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LONG-TERM GOAL. The long-term goal of this project is to provide a numerically efficient and robust acoustic model for the propagation of waves in a three-dimensional fluctuating shallow water environment. OBJECTIVES. The scientific objectives of this work, as a first step toward the long-term goal, are: 1: Develop a new, more efficient 2D propagation model based on the coupled mode method which is more efficient but as accurate as the parabolic equation (PE) model. 2: Use this model to study how signal coherence in the ocean is affected by sound speed fluctuations caused by internal waves. APPROACH Coupled mode equations are generally derived from the wave equation by dividing a range dependent waveguide into range independent sections where in each section the wave equation can be solved as a sum of local normal modes [1]. The coupling between modes are expressed as integrals with respect to depth of the local modes and their range derivatives. The use of the wave equation in deriving the coupled mode equations results in two coupling matrices: one which involves first and another which involves second derivatives of the local modes with respect to range. Computation of the coupling matrices, particularly the one which involves the second derivative of the modes with respect to range, is impractical and inaccurate at best. It is therefore common to resort to approximations, which, among other things, destroy the anti-symmetry of the coupling matrices [2], [3], [4]. An energy conserving solution requires the coupling matrices to be anti-symmetric. Instead of using the wave equation, our approach is to use the equations of motion to derive the coupled mode equations. By this method only one coupling matrix is obtained. What is more important is that this coupling matrix is simple to compute since it involves the depth derivatives rather than the range derivatives of the local modes. Furthermore, this method guarantees energy conservation since the coupling matrix ob