Heinz Herwig

Simultaneous acoustic and microwave backscattering from the sea surface

Simultaneous and coincident measurements of acoustic and microwave backscatter from the air/sea interface were obtained during Phase II of the SAXON-FPN experiment in December 1992 and again in March 1993. The acoustic and microwave grazing angles were both set to 17°, and the wavelengths were matched, being set to 2.14, 3.00, and 5.66 cm, corresponding to, respectively, acoustic frequencies of 26.5, 50, and 70 kHz and microwave frequencies of 5.3, 10, and 14 GHz. Backscattering cross sections normalized by ensonified area for the acoustic (σ0a) and microwave (σ0m) returns were determined, and their dependence on wind speed was investigated. The acoustic scattering strength is defined as 10 log10(σ0a) and the microwave scattering strength is defined as 10 log10(σ0m)−10 log10(4π). The results of these experiments show that the two scattering strengths are comparable at wind speeds below about 3 m/s but that the acoustic scattering strength increases much faster than the microwave scattering strength with i...

A two-frequency hydroacoustic scatterometer for bubble scattering investigations

High-frequency bubble layer scattering investigations require the measurement of the intensity of backscattered sound and the corresponding depth of the scatterers below the moving surface. Especially at high sea state conditions and high acoustic frequencies, bubbles acoustically mask the surface, i.e., the surface return cannot be detected. However, this environmental condition is the most interesting one in bubble scattering investigations and a reliable method is required to determine the range of the scatterers to the surface displacement. A method for the determination of the vertical profiling of acoustic scattering in the presence of bubbles at high sea state conditions is presented. It is based on the transmission of a low-frequency signal alternately to the high-frequency signal at which the scattering investigations are performed. The only information that is extracted from the low-frequency echo is the onset of the surface return. It is used to compute the true depth of scatterers at the high frequency. Experiments were conducted to determine the optimum low frequency at which the detection of the surface onset in the presence of a high bubble concentration is ensured. A screening ratio is defined to give a measure of the acoustic masking of the sea surface. It is depicted for an extreme wind condition (20 m/s) for the frequency range of 5-25 kHz and as a function of wind speed for 50 kHz measurements. Selected results of subsurface bubble scattering at 50 kHz from experiments under open sea conditions are presented for the wind speed regime from 9 to 22 m/s. Additionally, the two-frequency scatterometer is used to measure sea state characteristics simultaneously to the scattering investigations by remote sensing techniques. >

Acoustic backscatter measurements in the North Sea: 3–18 kHz

Measurements were conducted in the North Sea to investigate the influence of near‐surface bubbles on acoustic backscatter in the 3‐ to 18‐kHz frequency range. At normal incidence, the scattering caused by bubbles contributes to reverberation. A partial ‘‘screening’’ (or masking) of the surface was observed for short time periods when bubble plumes were introduced into the insonified volume. At lower grazing angles, the contribution to scattering by bubbles increases with wind speed and frequency such that, for a 30‐deg grazing angle, the bubbles become the dominant backscattering mechanism at wind speeds of 35 and 18 kn for 10 and 18 kHz, respectively, and mark the onset of saturation. A comparison with other experimental data shows good agreement.

The Influence of Bubbles on Acoustic Propagation and Scattering

Several experiments had been conducted in the North Sea to investigate the influence of bubbles on acoustic propagation and scattering within the near-surface layer. The data were obtained in the frequency regime from 3 kHz to 100 kHz for different environmental conditions. The results show acoustic screening of the sea surface due to bubble clouds with increasing frequency and sea state. The excess attenuation decreases with increasing depth below the surface and shows extremely high values within bubbles clouds.

Measurements of Acoustic Backscattering of the Near-Surface Layer

A selcted set of results from monostatic acoustic near-surface scattering experiments conducted in the North Sea is presented. Scattering strength values were measured as a function of frequency (3 kHz – 80 kHz) and grazing angle (10° to 90°) for a large variety of environmental conditions.

A further investigation of acoustic scattering from the sea surface

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 receivers. Scattering strength values were obtained as a function of frequency (3–18 kHz) and grazing angle (10°–90°) for differing sea surface roughness and wind speed conditions. The mean scattering strength values illustrate a strong dependence on large and small scale sea surface roughness. Comparisons with theoretical and semiempirical models indicate the appropriateness of using composite roughness modeling theories.

Measurements Of Acoustic Backscattering Of The Near-surface Layer During Saxon-fpn

The results from acoustic scattering measurements conducted in the North Sea during the experiment SAXON-FPN are presented. Scattering strength values were obtained as a function of frequency (7.5 kHz - 50 kHz) and for windspeeds from 3 kt to 40 kt. The acoustic measurements were done simultaneously to electromagnetic measurements. The backscattering strength is caused by the small-scale surface roughness at low wind speeds and by subsurface bubbles at high wind speeds. It depends only weakly on wind in the intermediate wind speed regime. The acoustic data will be subject to further correlation with electromagnetic backscattering results.

Backscattering measurements in the presence of bubbles

The results from a recent acoustic scattering experiment, which was conducted in the North Sea, are presented with supporting environmental information. The acoustic data were obtained utilizing a high‐resolution (narrow beamwidth) pulsed parametric sonar transmitter and conventional receiver. The measurements were done in the frequency regime from 20 to 80 kHz at grazing angles of 30° and 90° (normal incidence). The results show that the contribution of bubble clouds to total backscattering from the near‐surface layer is negligible at normal incidence. However, bubbles can acoustically screen the surface so that backscattered energy from the surface can no longer be used to compute backscattering strength because of increased attenuation within bubble clouds. At a grazing angle of 30°, the backscattered energy is significantly influenced by bubble clouds when breaking waves occur.

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.

Channel reverberation in shallow water

The results of experiments designed to measure the characteristics of channel reverberation in shallow water (30‐m water depth) are presented along with supporting environmental data. The acoustic data were obtained using long continuous wave pulses with a monostatic transmitter and receiver system from a fixed platform in the North Sea. The characteristic values of channel reverberation such as frequency shift, linewidth, and reverberation time are described as a function of frequency (500 Hz to 10 kHz) and the wind‐induced surface roughness. It is also shown for a more limited set of environmental data that channel reverberation is dependent on the angle between the acoustic propagation and the direction of the surface waves. The linewidth and the reverberation decay time show a strong dependence on frequency and surface roughness. These dependencies are presented for various sets of environmental parameters. [Work supported by MOD. West Germany.]