Two Modeling Approaches for Reverberation in a Shallow Water Waveguide Where the Scattering Arises From a Sub-Bottom Interface

Abstract

In shallow water environments where the uppermost sediment layer is a fine-grained fabric (e.g. clay or silty-clay), the observed reverberation may be dominated by scattering from the subbottom. Here, reverberation predictions from normal mode and energy flux models are compared for the case where the scattering arises from a sub-bottom half-space under a fine-grained sediment layer. It is shown that in such an environment, the position of the angle of intromission, in addition to the angular dependence of the scattering kernel, is a factor controlling the reverberation and its vertical angle distribution. It is also shown that the reverberation from a sub-bottom horizon is typically governed by higher grazing angles than the case where the scattering occurs at the watersediment interface. There was generally very close agreement between the models as a function of frequency (200-1600 Hz), layer thickness (0-8 m), and range (1-15 km). The model comparisons, showing some differences, illuminate the result of different approximations in the two approaches.

Low frequency seabed scattering at low grazing angles

“…This equivalent relationship offers a simple method to derive LF&LGA BBS from reverberation measurements [see Eqs. (26)- (29)]. The method is valuable because it allows seabed backscattering strength to be inverted at frequencies and angles where no practical method of measurements exists.…”

“…47 The theoretical expressions used to invert the BBS, Eqs. (24), (26)- (29), are limited to the three-halves law region where r 3 > r > r 2 . In addition to this limitation, it is often hard to get quality reverberation data at long range because of ocean background noise interference; particularly at lower frequencies.…”

“…[6][7][8][13][14][15][16] Due to its simplicity and intuitive nature, the energy flux method has further been developed and widely used for SW reverberation calculations, 17-30 mainly due to works by Harrison,Holland and Ainslie. 18,20,21,[23][24][25][26][27][28][29][30] All the closed-form expressions in isovelocity shallow water, derived by Zhou, 6 Harrison, 18 Ainslie 27,30 and Holland, 24 are almost identical, having only minor differences in constant coefficients. In this paper, Zhou's closed-form expressions for reverberation in isovelocity water are used to extract the frequency and angular dependence of the BBS at LF&LGA from long-range reverberation data.…”

“…If both wideband TL and RL are well measured, or if both RL data and the seabed geoacoustic model are available, then Eqs. (28) and (29) can be used to derive an angular dependence of the LF BBSðhÞ. In this section, the wideband RL data, obtained from three sites with sandy bottoms, and the average Q values shown in Fig.…”

“…3, are used in Eq. (29) to derive the BBS vs angle and frequency. The RL data from the YS site II are not used, because the RL data at this site exhibit an uncommon/unknown water volume scattering at long-range (around 20 s).…”