Studies on the interaction of low‐frequency acoustic signals with the ocean bottom

Abstract

Understanding the mechanisms by which the ocean sediment redirects impinging sound back into the ocean is necessary in developing propagation models for sonar performance prediction. The Naval Underwater Systems Center (NUSC) has (1) conducted controlled, self‐calibrating acoustic measurements where the ocean bottom interacted signal is isolated in time for analysis, (2) developed deconvolution processing techniques to aid in describing the impulse response of the ocean sediment, and (3) performed modeling to study the interaction of acoustic waves at the ocean bottom. This paper presents a synopsis of studies showing the necessity of considering the refraction of sound by the ocean sediment when predicting low‐frequency propagation loss. Constructive interference between nonplanar wave sediment refracted sound and sound reflected by the ocean‐sediment interface and subbottom layering can cause negative values of bottom loss when using plane‐wave models to interpret measured data. These models cannot account for all possible acoustic arrivals at a receiver. In addition, for a given frequency and constant ocean bottom grazing angle, bottom loss can be dependent upon both processing bandwidth and source/receiver depth. Deconvolution has aided in time resolution of signals that make up the bottom‐interacted signals. Resolution of these signals aids in interpreting results. A modeling effort utilizing the Fast Field Program (a computer technique for evaluating the field integral by the fast Fourier transform) provides quantitative evidence for the necessity of accounting for the refraction of sound by subocean sediments to interpret properly low‐frequency propagation loss measurements.

OSS IV: Noise Levels, Signal-to-Noise Ratios, and Noise Sources

“…The BRL values obtained for several frequency bands are shown in Figure 15. Bottom reflection loss is a well known quantity in the literature and our BRL results are similar to results found in other deep ocean sites (e.g., Santaniello et al, 1979;Mitchell et al, 1980;Chapman, 1983). Bottom transmission loss (BTL) was computed in a way similar to BRL above using the same symbols, terms and conditions (with the addition that both TV and M must be greater than one) in the following expression, BTL(Δf,g,N) = 10 × log 10 ((NWW(Δf,g)…”

Effective Low-Frequency Geoacoustic Properties Inferred from Measurements in the Northeast Atlantic

“…Dicus [28] measured bottom impulse responses for a gradient bottom and demonstrated precise quantitative modeling of the impulse responses and associated bottom-loss functions. Santaniello et al [29] discussed the focusing of sound rays in a gradient bottom, the contribution of lateral (head) waves, and the effect of different measurement geometries in connection with bottom-loss data that showed significant negative losses over a restricted grazing angle range. Stickler [30,31] pointed out the inadequacy of analyzing data with the assumption of incident plane waves, and showed numerical examples of the shift in apparent critical angle and occurrence of negative losses associated with different measurement geometries.…”

Evaluation of Experimental Techniques for Determining the Plane Wave Reflection Coefficient at the Sea Floor

“…Stickler [4) and Santaniello et al [5) demonstrated that the simple interpretation technique will give wrong reflection coefficients when the bottom is upward refracting or has deeper, reflecting interfaces, because of the interference between the different arrivals. Non-physical effects like negative bottom loss and source/receiver position-dependent results arise.…”