Vertical directionality of a source observed by a short vertical line array in shallow water

Ambient noise in the ocean has been measured and studied for over half a century. Due to multipath effects, the signal will not always arrive from the horizontal direction. The directional response of a vertical line array (VLA) to a distant source can be expressed in terms of the modal beams weighted by their normal mode amplitudes. This modal representation offers a physical interpretation of the vertical directionality of the source in terms of normal modes. When a short VLA lies at the lower sound speed portion of the water column and a high frequency source is located at the higher sound speed portion of the water column, there always exists a notch in the horizontal direction. The vertical directionality of the source has been validated using Mediterranean Sea data, and it is shown that the vertical directionalities of the ambient noise and the distant source are quite different.