This report documents the results of a three-year study of the interaction of sound with the sea floor. The investigation covered sea floor properties of interest in underwater acoustics, including velocity gradients in the sea floor, density, shear-wave velocities and other properties; research on acoustic propagation models, especially at low frequencies (2 to 200 Hz); and the development of accurate and efficient methods for coupling geoacoustic models to standard propagation models such as ray theory, normal mode theory and P.E. (a numerical method using the Parabolic Equation approximation to the wave equation). During this period numerous reports were distributed to the acoustic community. Various predictions for the surveillance community have been calculated using the geoacoustic and acoustic models, support was provided to others developing models and support was provided on a continuous basis to surveillance programs including the Indian Ocean, MSS, SURTASS, and others. An extensive list of references is included.