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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Using a deep source and deep receivers, measurements of the acoustic reflectivity of the ocean bottom at 1 kc/sec were made at an area in the Atlantic Ocean. The data, that is, the direct and the first-order bottom-reflected arrivals, were acquired digitally at sea and have been reduced via a computer program. The program computes the reflection coefficients based on peak pressure as well as the time-integral square of the pressure. The computer analysis also furnishes the travel time of each arrival, horizontal range, and incident angle for each transmission. The total number of data points considered exceeds 4000. Computations based on velocities, densities, and attenuations that have been obtained from an analysis of cores taken in the general area of the acoustic tests have been performed, using a theoretical model of the bottom. This model consists of many absorbing liquid plane parallel layers over one absorbing semiinfinite solid. Comparisons of the experimentally derived reflection coefficient as a function of incident angle are made with those obtained from the model, and the relevance of the complicated layering structure and attenuation of the bottom is discussed.

Bottom-reflection measurements were made in four different geographically smooth, nonsloping bottom areas. Reflection coefficients were computed as a function of angle from data obtained at two single frequencies, two frequency bands, and several pulse lengths. The experimental design and first area results are discussed elsewhere [J. Acoust. Soc. Am. 38, 707–714 (1965)]. Vertical hydrophone arrays were used in three of the areas and it was possible to make comparisons based on hydrophone separation. Average values of reflectivity for data taken at the same angle but for different bottom-reflection points within a small area are observed to differ by as much as a factor of two. Cores taken within the same reflection areas as the observed acoustic results also showed variations in sediment properties. Results obtained at the four locations are presented to show the degree to which it is possible to characterize a large geographic area from measurements taken over a very small area within it.

Ocean bottom reflectivity is generally described by a single function, the reflection coefficient (R), defined as a ratio of reflected-to-incident intensities; −10 log(R) is bottom-reflection loss. Measuring bottom-reflection loss within the constraints of the definition is virtually impossible. In practice it is estimated by (1) measuring propagation loss of acoustic pulses transmitted via the path which interacted once with only the bottom, (2) calculating the water-region propagation loss only for an idealized bottom-reflection path by assuming a flat, single-interface bottom having a reflection coefficient of one, and then (3) comparing (measured minus calculated) values. This estimate is called bottom loss. The above assumption inherently ignores subbottom refraction and reflection of sound; important effects at low frequencies (<500 Hz). Sound returning from the subbottom can constructively interact with sound reflected from the water-sediment interface, yielding “negative bottom loss” results. This paper presents such results (obtained using a self-calibrating bottom-loss measurement technique) and suggests that the ocean bottom, by refracting 2nd reflecting sound, can partially focus energy over a considerable volume within the ocean. Three following companion papers discuss aspects of this premise. [This work was sponsored by Naval Sea Systems Command, SEA 06H1-4, A.P. Franceschetti.]