Acoustic bottom backscattering measurements and the corresponding geoacoustic properties of the ocean bottom are presented for an experiment conducted in the shallow waters of the North Atlantic. The bottom scattering strength data, which were obtained with a high resolution (narrow beamwidth) parametric sonar, were measured as a function of frequency (5–20 kHz), grazing angle (4°–10°), azimuthal angle (± 55°), and pulse length (0.4–10 ms). The supporting environmental measurements included box cores for determining the acoustic properties of the sediment and stereo photography for calculating the two-dimensional roughness spectrum of the sea floor. [Work supported by NAVSEA 63R.]

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.

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Bubble pulses generated by wide band explosive sources complicate the interpretation of the arrivals from the ocean bottom. However, these arrivals can be deconvolved to remove bubble pulse contamination and yield the impulse response of the ocean sediment. The deconvolution process is degraded by noise; but methods to reduce the noise sensitivity of the process have been developed as described in the previous paper. These methods were applied to ocean-sediment interacted acoustic signals acquired in a deep-water location. Bottom loss values obtained from the deconvolution process were in close agreement with values computed prior to deconvolution. Furthermore, the deconvolution process enhanced the clarification of the acoustic path structure within the sediment and thereby provided experimental verification of the contention in a previous paper that the interaction of ocean-bottom interface reflected arrivals with subbottom reflected and subbottom refracted arrivals can result in a “negative” bottom loss interpretation. Also, as a result of this interaction bottom loss can be (1) experiment configuration dependent, and (2) processing bandwidth dependent at a given frequency. [This work was sponsored by Naval Sea Systems Command, SEA 06H1-4, A.P. Franceschetti.]

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