Abstract
An undulating bottom in shallow water has a significant effect on sound propagation. An acoustic propagation experiment was carried out in the East China Sea in 2020. Measurements along two separate propagation tracks with flat and undulating bottoms were obtained. Abnormal transmission losses (TLs) were observed along the track with the undulating bottom. By using the parabolic equation model RAM and ray theory, these abnormal TLs and the distribution of the sound field energy were analyzed. Numerical simulations indicate that under the shallow water condition with a negative thermocline and for a high frequency (1000 Hz), the incidence and reflection angles of sound rays on the sea bottom are changed due to the undulating sea bottom. The larger the inclination angle of the undulating bottom, the greater the grazing angle changes. These angles changes lead to different sound propagation paths for the undulating bottom and the flat bottom, resulting in the difference of TLs at a certain distance and depth. The undulating bottom will cause energy convergence in the mixed layer when the source and receiver locate above the thermocline.
Highlights
A sound wave is usually refracted downward in shallow water, and the sea bottom is an unavoidable boundary condition in sound propagation [1]
As early as 1968, Northrop et al [2] found in an acoustic propagation experiment that the transmission loss (TL) at the SOFAR depth would be reduced compared to the transmission losses (TLs) with the flat seafloor due to the sloping bottom
In 2016, by analyzing an acoustic propagation experiment phenomenon, Hu et al [7] found that the TL above the slope decreases by about 5 dB due to the reflection of the bottom and a high-intensity region appearing below the sea surface
Summary
A sound wave is usually refracted downward in shallow water, and the sea bottom is an unavoidable boundary condition in sound propagation [1]. Rousseau et al [4] investigated in 1985 the effects of a sloping bottom on acoustic transmissions by using ray theory. In 2021, Liu et al [10] reported that the undulating sea bottom could influence sound pulse propagation, mainly reflected in the energy conversion within sound rays with different angles. The incidence and reflection angles of the sound rays on the sea bottom are changed due to the undulating sea bottom. In this paper, based on the sound field data of an acoustic propagation experiment conducted in the East China Sea in 2020, we analyze the variation of TLs and the distribution of the sound field energy under the undulating sea bottom condition with a negative thermocline. The mechanism of abnormal sound propagation caused by the undulating sea bottom topography is explained using ray theory
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