Broadband Models for Predicting Bistatic Bottom, Surface, and Volume Scattering Strengths

Abstract

Standard Form 298 (Rev. 8-98)Prescribed by ANSI Std. Z39.18Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Approved for public release; distribution is unlimited. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S ACRONYM(S) 9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S REPORT NUMBER(S)Multistatic active system performance can be driven by reverberation from the ocean boundaries and biologics. Providing accurate sonar performance predictions of reverberation, in turn, relies on providing accurate estimates of bistatic scattering strengths. This report presents new three-dimensional models that provide physics-based estimates of the dependence of scattering strength on the incident and scattered grazing angles, the bistatic angle, the acoustic frequency (10 to 10000 Hz), and physical descriptors of the environment (such as bottom properties for the bottom model, wind speed for the surface model, and fish properties for the volume model). The bottom model describes scattering from rough, elastic interfaces, while the surface model describes scattering from both the rough air-sea interface and subsurface bubbles. The volume models describe scattering from dispersed bladdered fish, including boundary-interference effects. For all, parameter studies along with data-model comparisons demonstrate the importance of using physics-based scattering models to describe the complex acoustic interaction processes at the ocean boundaries. These broadband models can enhance sonar performance prediction capabilities through their inclusion as submodels in both active performance/reverberation models

A Low- and Mid-Frequency Bistatic Scattering Model for the Ocean Surface

“…Direct-path measurements have revealed a strong dependence of backscattering strength at low grazing angles on wind speed and frequency (Fig. 1), and when wave breaking is significant, the backscattered levels are highly elevated over that expected solely from interface scattering [1]. This is consistent with scattering from subsurface bubbles generated by the breaking waves.…”

“…The CST data were particularly valuable in that the acoustic measurements were accompanied by complementary environmental measurements (such as wind speed, wave-directional spectrum, and subsurface bubble density) and, in view of the potential contribution that fish could make to the backscatter, dedicated measurements of fish scattering were also conducted. Because acoustic signatures that identify fish scattering have been developed [5,6], it has been possible to isolate their contribution from the CST data, crucial to determining the relative contributions of the air-water interface and sub-surface bubbles, especially at low-to-moderate wind speeds, and at low grazing angles and frequencies.…”

“…In previous work [1], the authors have developed a semi-empirical approach to modeling surface scattering strength applicable to open-ocean environments. This approach provides physics-based estimates of the surface scattering strength (SSS) due to both the rough air-sea interface ( int σ ) and subsurface bubble clouds ( bub σ ), given the acoustic frequency (< 5 kHz), incident and scattered grazing angles, bistatic angle, wind speed at 10-m elevation, and sound speed in the water.…”

Stochastic Cross-Sections Based on the Small Slope Approximation: Theory

“…The newly expanded technique has recently been used to develop improved algorithms for scattering from the air-sea interface [17,18], and to study the scattering behavior of the ocean bottom [2,19,20]. An overview of some of the material discussed below is published in reference [21].…”

“…In this case (interjecting a factor Since no assumptions are made about the spectral peak, the results in this section are particularly useful in problems such as scattering from the ocean bottom, where the nature of the spectral peak is site specific and rarely well-known. References [2,17,19] provide further information concerning the background and application of the expression for the stochastic cross-section derived here in Section 3.…”