Modulation of acoustic signals in a shallow water using a normal-mode model

A signal transmitted into a shallow water waveguide is modulated whilst travelling through the medium. Because of the difference in group velocities for a set of propagating modes the transmitted signal is stretched in time and undergoes distortion. A special version of SACLANTCEN´s normal-mode propagation model has been developed to study the dispersion effects on CW-bursts and non-harmonic signals for a shallow water environment and for several source/receiver locations. Slightly irregular sea-surfaces are taken into account by the Clay model. It is shown that modulation sidebands are introduced by the discrete modes and group velocities, permitting the calculation of phase differences among them. Theoretical results are presented for frequencies below 150 Hz and as a function of range.