Suzanne T. McDaniel

Sea surface reverberation: A review

Measurements of reverberation from the sea surface and the accompanying development of theories to explain the experimental results are reviewed from a historical perspective. Because of the relative ease of measurement, significant amounts of sea surface reverberation data have been amassed that yield a clear picture of the dependence of measured reverberation levels on acoustic frequency, grazing angle, and wind speed. The evolution of theoretical treatments of rough surface scattering and scattering from the layer of wind‐generated microbubbles that exist below the surface is also reviewed, as are the empirical models that describe ocean surface roughness and subsurface bubble distributions. For acoustic frequencies of 3–60 kHz, the data are consistent with the view that the reverberation is due to rough surface scattering at moderate to high grazing angles, and at low grazing angles to scattering from resonant subsurface microbubbles. At frequencies below 1 kHz, measured reverberation levels at low gr...

An examination of the composite‐roughness scattering model

A perturbation technique is applied to derive a composite‐roughness theory for acoustic scattering from the sea surface. The leading term in the expansion obtained is the well‐known result obtainable using ad hoc arguments. Higher order terms are evaluated to assess their contribution to the high‐frequency monostatic backscattering strength of the sea. It is concluded that the leading term in the perturbation expansion provides an excellent approximation.

A finite‐difference treatment of interface conditions for the parabolic wave equation: The irregular interface

A finite‐difference solution to the parabolic wave equation is extended to treat an irregular interface. Interface conditions are developed to preserve continuity of pressure and continuity of the normal component of particle velocity at the boundary between media having different sound speeds and densities. A complete mathematical treatment for the case of an irregular interface is presented. To demonstrate the method, numerical results are obtained for sound propagation from deep to shallow water.

Parabolic approximations for underwater sound propagation

Several alternative parabolic approximations to the reduced wave equation and their numerical solution are investigated for underwater acoustics applications. Parabolic approximations are derived by splitting the wave equation into transmitted and reflected components. An exact splitting operator is found, and further parabolic approximations for the transmitted fields are based on approximations to this operator. For the case of a range‐independent environment, the resulting parabolic approximations are compared with normal‐mode theory. The efficiency with which numerical results can be obtained is discussed, and an example of propagation from shallow to deep water is given.Subject Classification: 30.20; 20.15, 20.40.

Seafloor reverberation fluctuations

Reverberation intensity fluctuations are not only important because they impact the performance of active sonar systems, but also because they may provide insight into the major scattering processes. In this article, a model for the fluctuations of signals backscattered from rough surfaces is developed, and its predictions are compared with high‐resolution bottom backscatter data. For reverberation due to Bragg scattering, the model predicts a unique dependence on beamwidth, which is supported by the data. A dependence on waveform, i.e., a frequency diversity effect is also predicted. This effect is, however, not unique to the Bragg scattering process.

Small-slope predictions of microwave backscatter from the sea surface

Abstract The small-slope approximation is applied to the prediction of microwave sea-surface backscatter. Numerical results are obtained assuming a non-directional surface wavenumber spectrum and compared with azimuthally averaged C- and Ku-band radar backscattering data. For vertical polarization, the predictions and measurements agree to within 3 dB at incidence angles of interest for remote sensing. Small-slope predictions of azimuthal scattering asymmetry do not agree as well with measurements. These predictions are found to be sufficiently dependent on the assumed wave directionality that they may be used to improve existing directional wave spectral models.

The influence of the physical properties of ice on reflectivity

A model for the plane‐wave reflection coefficient from a layered elastic solid bounded on either side by a fluid half‐space is developed and applied to study environmental factors affecting the reflectivity of smooth arctic ice. Experimental measurements of the internal friction in ice and snow are reviewed and applied to compute realistic attenuation profiles. An examination of the effect of ice layers conforming with measured temperature profiles in floe ice shows that the use of average values for sound speed and attenuation is an acceptable approximation for modeling purposes. A study of the reflectivity due to the absorption of shear and compressional waves demonstrates that shear wave attenuation is the most important loss mechanism from 20° to 60° incidence. The effect of an additional snow layer is to produce more attenuation without shifting the pattern of reflection nulls. Major results are presented for a frequency of 2 kHz. Data comparisons are performed from 0.5–3 kHz that show a limited qual...

Mode coupling due to interaction with the seabed

A simple mode‐coupling model is developed to investigate acoustic propagation in an ocean environment in which the water depth is range dependent. The model is applied to the case of a constant sloping bottom, and the results are compared with those obtained using other propagation loss models.

Microwave backscatter from non-Gaussian seas

Rough-surface scattering theory is applied to study microwave backscattering from seas characterized by a non-Gaussian wave-height distribution. The relationship of the geometrical optics limit of rough-surface scattering theory to the probability density of surface slopes is used to relate the coefficients of Gram-Charlier expansions, describing measured slope statistics, to the wavenumber spectra of non-Gaussian surface components. Functional forms for the spectra consistent with measured slope statistics are assumed, and the backscatter predicted by rough-surface scattering theory is compared with measured cross sections. The predicted upwind-downwind asymmetry of scattering cross sections is comparable to that observed, and a measurable dependence of cross sections on atmospheric stability is predicted.

Acoustic and radar scattering from directional seas

Abstract The small-slope approximation is applied to predict acoustic and electromagnetic scattering from directional seas. Results are presented for the scatter of high-frequency fields from fetch-limited seas for which the wavenumber spectrum is isotropic at high wavenumbers but highly directional near the spectral peak. Monostatic backscatter is found to display an upwind-crosswind dependence for a broad range of scattering angles due solely to the directionality of the large-scale waves.