Sea Surface Backscatter Research Articles

Surface scattering measurements using broadband explosive charges in the Critical Sea Test experiments

Extensive measurements of low-frequency (70–1000 Hz) sea surface backscattering strengths have been made as part of the Critical Sea Test (CST) experiments. These measurements were made during CST-1 through CST-5 for a variety of wind speeds from 1.5 to 13.5 m/s and for mean grazing angles from about 5 to 30 deg. These measurements have revealed several regimes in a frequency versus wind speed domain that probably correspond to at least two distinct scattering mechanisms. For relatively calm seas at high frequencies and for all wind speeds at lower frequencies, perturbation theory is found to give an adequate description of surface scattering. For rougher seas and higher frequencies, the Chapman–Harris empirical curves are adequate predictors of the levels of surface scattering. In between these two regimes is a transition region where the scattering strengths are more difficult to predict, as they depend on the details of the surface and wind characteristics. These observations lead to the idea that there are two mechanisms that dominate the scattering of sound from the surface. In the perturbation theory regime, air–water interface scattering is the dominant mechanism; in the Chapman–Harris regime, another mechanism such as scattering from subsurface bubble clouds must dominate the scattering process. The transition region is then the part of the frequency and wind speed domain where the two effects are competing.

Azimuthal variation of low‐frequency Bragg scattering from the ocean surface

Low‐frequency sound, backscattered from an annulus of ocean approximately 9 km wide at a nominal 43‐km range, was measured on a 128‐hydrophone horizontal line array (HLA) during an experiment in the Gulf of Alaska. Fourier analysis of the backscattered signal permitted separation of the Bragg‐shifted returns from the 250‐Hz carrier frequency. The Bragg‐reflected energy was resolved in azimuth by beamforming the HLA data. Azimuthally varying sea surface backscattering strengths, corresponding to measured wind speeds, were obtained using an exact solution for the backscattered field applied to a two‐dimensional Donelan water wave spectrum. Normal mode model predictions using the calculated scattering strengths agreed with the up‐ and down‐shifted Bragg energy measurements in both level and azimuthal dependence. [Work supported by SPAWAR PMW 192.]

An empirical bispectrum model for sea surface scattering

The properties of a surface bispectrum are found by generating a skewed surface on a digital computer and then evaluating its correlation function, bicoherence function, power spectrum, and bispectrum. The bispectrum is defined to be the Fourier transform of the bicoherence function. It is found that the surface bicoherence function and its first and second derivatives must all vanish at the origin. In general, the surface bispectrum is a complex function. Its real part is centrosymmetric, just like the surface spectrum, and its imaginary part is antisymmetric. A function with the above-stated properties is introduced to represent the imaginary part of the sea surface bispectrum. The unknown parameter in this function is calibrated using a data set from the FASINEX experiment. The sea surface backscattering model is based on an integral equation model which accounts for frequency, polarization, incident angle, azimuthal angle, and wind speed. It is found that the proposed bispectrum can be used to account for the up/down wind asymmetry. >

Measurements of low-frequency, low-grazing-angle sea-surface scattering and feature strengths

Direct-path measurements of low-frequency (200–1000 Hz) and low-grazing-angle (≤10 deg) acoustic surface scattering were made in the Gulf of Alaska in April of 1990 and in March of 1992. Both narrow-band (cw) and broadband (FM,PRN) transmissions were used to examine the strength of sea-surface backscatter as a function of frequency and environmental conditions. Results have revealed that in addition to a continuously distributed reverberation component, there are often strong spatially and temporally discrete scattering events. Scattering and feature strengths will be presented to quantify the relative contributions of these components to mean backscatter levels (such as were used in deriving the Ogden–Erskine scattering-strength curves), and to extend the scattering-strength results of Ogden and Erskine to lower grazing angles. Furthermore, the presented results have strong implications as to the types of bubble clouds that are responsible for each component (as will be discussed).

Broadband measurements of sea surface backscattering strength in the Gulf of Alaska

Measurements of low-frequency (70–1000 Hz) sea surface backscattering strength were made in the Gulf of Alaska during February–March 1992 as part of a surface backscatter and air–sea interaction experiment. The measurements described here used explosive charges (SUS MK-59 MOD1a) as sources and a towed horizontal line array of hydrophones as receiver. The experiment objective was to measure surface backscattering strength as a function of frequency, grazing angle, and environmental conditions of the air–sea boundary. Scattering strengths were obtained during 18 tests (each of duration approximately 30 min) conducted from 24 February to 1 March for wind speeds that varied from approximately 4 to 18 m/s. The experiment uses a direct path test geometry for which the measured surface backscatter originated from surface/near-surface regions at ranges of several km or less. Surface grazing angles for these measurements varied from about 5 to 30 deg. Results are compared to earlier surface scatter measurements below 1000 Hz and are interpreted relative to concurrent air–sea boundary environmental measurements.

Sea echo in tropospheric ducting environments

Path loss formulas for sea surface backscatter are developed for horizontally stratified tropospheric ducting environments. First‐order Bragg backscatter results coupled with ray and waveguide formalism serve as the basis for the development. Two features of composite model formalism, namely shadowing and tilting of the surface normal in the plane of incidence, are included in the development. Though developed for hh polarization, conventional composite model modifications for vertical polarization and depolarization can be incorporated into the echo models. Comparisons are made with previously generated parabolic equation results and with empirical model results generated by Engineers Refractive Effects Prediction System. These comparisons are for a frequency of 9.6 GHz for the standard atmosphere and for 14‐ and 28‐m evaporation ducts. Results apply to wind speeds of 10, 20, 30, and 40 knots (≈ 5, 10, 15, and 20 m/s).

Experimental investigations into the anisotropy of centimeter radio-wave backscattering from the surface of the sea at small slip angles

The surface of the sea is anisotropic. The main factor responsible for this anisotropy is the wind. The anisotropy of the surface leads to the anisotropy of the radio-wave backscattering, i.e., the relationship between a specific cross section o of the surface and the azimuth angle ~ between the direction of the probe and the direction of the wind at the surface. In principle, this allows us to use the data from the measurement of the scattering anisotropy of a radar signal to obtain information regarding the structure of the surface as well as of the wind at the surface, for example, to determine the direction of the wind [i-3]. To make this possibility real, as well as to determine the accuracies which can be achieved by a noncontact radar wind-direction gauge, we need a detailed theoretical and experimental study into the relationship between the anisotropy of the reflected signal and the geometry of the experiment, the state of surface agitation, and the hydrometeorological conditions.* The radio-wave scattering anisotropy of the sea surface was observed over a period of fifty years [5] and was then studied by numerous authors, both experimentally and theoretically (see, for example, [3] and the bibliography cited in that reference). However, these investigations are even now still far from completion. The experimental data on sea-surface radio-wave backscattering anisotropy cited in the literature are still far from complete and quite inaccurate, which has been noted by a number of authors. This is governed primarily by the difficulties of carrying out natural experiments under conditions of substantial space-time variations in the characteristics of the surface, as well as by the shortcomings of the experimental methods and the processing of the resulting data. The most satisfactory data were obtained from measurements aboard an aircraft flying circular patterns over the surface of the sea at a fixed angle of irradiation [3, ii]. However, these cover only a region of comparatively large slip angles ~ from 30 to 50 ~ . Data for slip angles smaller than 20-30 ~ are extremely fragmented, incomplete, and at times simply contradictory. Thus, for example, in [4] we find the contention that the ratio of the scattering cross sections for surface irradiation into and with the wind - let us denote this as Ktl - in the three-centimeter radio-wave band is independent of polarization and does not exceed 1.5-2 dB, whereas in [9] for ~ ~ i ~ we have values of this parameter equal to 5 dB *We note that the information regarding the anisotropy of the reflected radio-location signal is needed also for the case in which the object being studied is not the wind, but certain other factors which affect the scattering cross section (contamination, rippling, internal waves, precipitation, etc.), i.e., when the wind variations in the radio-location signal are caused by noise.

Comparison of unified full-wave solutions for normal incidence microwave backscatter from sea with physical optics and hybrid solutions

Abstract The unified full-wave solution for electromagnetic wave scatter from a rough surface is expressed as an integral similar in form to physical optics solutions. However it correctly predicts return for small and intermediate roughness scales where the physical optics approach fails. This full-wave solution is used here to evaluate microwave sea surface backscatter at normal incidence for both the like-and cross-polarized linear components. Surface heights and slopes are assumed to be Gaussian, the sea is characterized by its surface height spectral density function and both perfectly and finitely conducting surfaces are considered. Results from this complete full-wave evaluation are compared with approximations, involving single-scale (specular point) and two-scale models. For both of these models, however, it is necessary to assume a spectral wavenumber, νd, where spectral splitting between the large and the small scales of the rough surface occurs. The full-wave solution is used as a guide in the...

The influence of bubbles on sea surface backscatter measurements

The results from a recent sea surface acoustic scattering experiment, which was conducted in the North Sea, are presented with accompanying sea surface roughness parameters and subsurface bubble information. The acoustic data were obtained utilizing a high‐resolution (narrow beamwidth) pulsed parametric sonar transmitter and conventional receiver. Scattering strength values were obtained as functions of frequency (3–18 kHz) for wind speeds from 2–45 knots. It appears that the backscattering strength at 30° grazing angle is caused by the high‐frequency wavenumber spectrum at low wind speeds and by subsurface bubbles at high wind speeds. The backscattering strength shows strong fluctuations in the intermediate region caused by both scattering mechanisms.

Omnidirectional and directional sea surface backscattering strength measurements at low frequencies and low grazing angles

Limited experimental data exists on surface backscattering strengths at low acoustic frequencies (below 300 Hz) and low grazing angles (below 20°) for deep ocean environments. We report here preliminary results of such measurements taken during four days of a sea test in the North Atlantic in September 1980 (sea states varied from 2 to 3). Four sets of approximately 20 small explosive charges (5‐lb TNT‐equivalent; bubble frequency of 77 Hz) were dropped beneath a towed acoustic array. We measured the direct path surface reverberation by processing the first 3 s of data, for mean grazing angles down to 8°, prior to receipt of the first bottom bounce at 6 s past detonation. The data were averaged over six frequency bands of 20‐Hz width from 150 to 270 Hz and were reduced to scattering strengths versus mean grazing angle and frequency by using the NRL reverberation model to remove propagation effects and to compute the grazing angles versus time. Typical omnidirectional surface scattering strengths decrease from −55 to −70 dB for mean grazing angles from 20° to 8° with little (<5 dB) frequency dependence over the measured frequency range. These results are consistent with previously published measurements. Further, the data have been beamformed to examine the azimuthal directionality of the surface backscatter (sea direction varied from 27° to 110° relative to array heading), and preliminary results will be reported. [Data acquisition funded by Defense Mapping Agency; analyses supported by Naval Electronic Systems Command, Code 612.]

Acoustic and radar sea surface backscatter

Theoretical predictions of sea surface reverberation are compared with representative acoustic and radar data to determine those mechanisms responsible for backscatter. Predictions of the composite roughness scattering model are compared with data in three frequency ranges, 2–5 kHz acoustic (428‐and 1228‐MHz radar), 20–25 kHz acoustic (4455‐MHz radar), and 58–60 kHz acoustic (8910‐MHz radar). The predicted backscattering strength is found to be in relatively good agreement with the radar data at all frequencies; however, only at the lowest frequency do the predictions match the acoustic data. At the two higher frequencies, the measured acoustic backscatter greatly exceeds the composite roughness predictions at low grazing angles. To account for this discrepancy, a simple model is used to compute the reverberation due to a near‐surface bubble layer. The computed bubble contribution is shown to provide good agreement with the acoustic data and also to predict the saturation that occurs with increasing wind speed.

Doppler spectra of sea surface backscatter at high acoustic frequencies

The initial transfer of energy from atmospheric wind to the ocean surface, which would be a significant factor in ambient noise, may occur via the so‐called Cat's Paw phenomena. To study this effect an experiment was conducted at Seneca Lake for a moderately disturbed (no whitecaps) air‐water interface. Doppler spectrum measurements of high‐frequency acoustic waves backscattered from the wind‐driven water surface indicate the presence of small‐scale roughness that is convected downwind by surface drift. For a slightly rippled surface there is a sharp resonant peak corresponding to a convection velocity approximately 0.4 m/s which is nearly independent of windspeed. The spectrum is broadened by phase modulation due to the orbital motion of larger scale gravity waves but remains skewed in the downwind direction. [Work supported by DARPA.]

A critical comparison of acoustic and radar sea surface backscattering

Sea surface backscattering of underwater acoustical signals has long interested both acousticians and oceanographers. More recently, airborne radar measurements have been employed to study surface backscattering. We have normalized pertinent environmental and data reduction parameters to enable a meaningful comparison of acoustical and radar backscatter. Data is presented in the frequency ranges 2–5 kHz acoustical (428‐ and 1228‐MHz radar), 15–30 kHz acoustical (4455‐MHz radar), and 58–60 kHz (8910‐MHz radar).

New, empirical expression for sea surface backscattering strength

A new, empirical expression for the sea surface backscattering strength has been developed. Previously developed empirical expressions considered only constant coefficients and therefore have quite limited ranges of validity. The new backscattering strength expression has variable coefficients and is valid for a wide range of frequencies, wind speeds, and grazing angles. In particular, the new expression saturates at high frequencies and/or wind speeds; this is in agreement with both theoretical and experimental considerations which indicate that the scattering strength should become frequency independent under these circumstances. Comparison of this new expression with experimental data shows good agreement for frequencies from 0.1 kHz to 60 kHz, wind speeds from 1 to 35 knots, and grazing angles from 5° to 80 °.

Backscattering of Sound from the Sea Surface: Its Measurement, Causes, and Application to the Prediction of Reverberation Levels

In many applications of underwater sound, the “reverberation,” or backscattered return, from the sea surface is the limiting background in which the desired echo appears. Little has been known about sea-surface backscattering in spite of its importance to the prediction of reverberation levels in practical applications. Using two transducers—one for transmitting, one for receiving—mounted so as to be rotatable in a vertical plane and suspended from a buoy, measurements have been made of the backscattering of sound pulses from the surface of the sea as a function of grazing angle between 5 and 90 degrees, wind speed between 3 and 18 knots, and pulse length. When converted to a scattering coefficient per unit area, the results provide some hints as to the processes by which sound is scattered by the ocean surface, and give the basic data for computing the reverberation to be expected with transducers of arbitrary beam width and orientation.

Backscattering of Sound from the Sea Surface: Its Measurement, Causes, and Application to the Prediction of Reverberation Levels

In many applications of underwater sound, the “reverberation,” or backscattered return, from the sea surface is the limiting background in which the desired echo appears. Little has been known about sea-surface backscattering in spite of its importance to the prediction of reverberation levels in practical applications. Using two transducers, one for transmitting and one for receiving, mounted so as to be rotatable in a vertical plane and suspended from a buoy, measurements have been made of the backscattering of sound pulses from the surface of the sea as a function of grazing angle between 5° and 90°, wind speed between 3 and 18 knots, and pulse length. When converted to a scattering coefficient per unit area, the results provide some hints as to the processes by which sound is scattered by the ocean surface, and give the basic data for computing the reverberation to be expected with transducers of arbitrary beam width and orientation.