Three-dimensional investigation of the effects of regular seafloor geometry on low frequency sound propagation using parabolic equations

Abstract

Underwater propagation of sound is constrained by multiple physical factors among which the physical boundaries of the underwater world add to the complexity of the numerical modeling of this phenomenon. The present study investigates the effects of the sea floor shapes on transmission loss of sound in low frequencies. To accomplish this task, a computer code is developed based on three-dimensional parabolic equations using implicit finite difference approach. This code is validated by two sets of published data, ensuring the high accuracy of the code in low frequency modeling. Furthermore, as extreme representatives of the arbitrary and complex sea floor shapes, three geometries of hemisphere (for rounded slopes), cubic and cone shapes (for the rocky sea floors) made of granite are assumed in tandem 3 × 3 arrangements with different heights and dimensions. For better understanding of three-dimensional effects of these obstacles on sound propagation, the results are extracted in three directions of 30°, 60° and 90°. The conducted parametric studies show that rounded and conical sea beds have the highest transmission loss which is not significantly affected by the arrangement of the obstacles. On the other hand, the cubic obstacles have a low mean transmission loss, which is highly affected by the arrangement of the obstacles. It should be pointed out that in some arrangements of the cubic obstacles, even sound intensification occurs. Moreover, it is shown that the effect of obstacle heights on the average transmission loss is highly dependent on the direction of the propagation. It has been demonstrated that maximum value of local transmission loss occurs for the cubic arrangements, when the heights of cubic arrangements are about 2, 1, and 4 times of the wave length in the directions of 30°, 60° and 90°, respectively. Study of the heights of the seamounts indicates that variation of the heights does not have significant effect on the average of transmission loss. The results provided in the present study establish a preliminary step towards the acoustical modeling of real sea floors with arbitrary shapes which provide an insight into the most important effects of the more common simplified shapes on the transmission loss of sound.