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Werner Alpers - One of the best experts on this subject based on the ideXlab platform.
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imaging of Ocean Waves on both sides of an atmospheric front by the sir c x sar multifrequency synthetic aperture radar
Journal of Geophysical Research, 1998Co-Authors: Christian Melsheimer, Werner AlpersAbstract:Radar images of an Ocean scene containing an atmospheric front and almost range-propagating Ocean Waves which were acquired by the multifrequency/multipolarization synthetic aperture radar (SAR) aboard the space shuttle Endeavour during the spaceborne imaging radar-C/X-band synthetic aperture radar (SIR-C/X-SAR) mission over the North Atlantic in 1994 are analyzed. The L-band SAR image spectra calculated from two areas located on opposite sides of the atmospheric front are quite similar, whereas the corresponding X- and C-band SAR image spectra differ significantly. It is shown that this is a consequence of the SAR imaging mechanism; at L band the SAR imaging mechanism depends weakly on the local wind field, and at X and C band it depends strongly on the local wind field. This is in agreement with earlier results obtained from the analysis of airborne multifrequency/multipolarization SAR images acquired over the North Sea during the SAR and X-Band Ocean Nonlinearities - Forschungsplattform Nordsee (SAXON-FPN) experiment. In this investigation it was found that at X and C band the phase of the real aperture radar modulation transfer function (RAR MTF) changes by almost 90°, when in the reference system moving with the group velocity of the dominant wave, the component of the wind velocity in the direction of the wave propagation changes sign. However, at L band such a change in local wind direction affects the phase of the RAR MTF only slightly. Using this phase behavior of the RAR MTF in simulations of the SIR-C/X-SAR image spectra, we show that the observed differences of the X- and C-band SAR image spectra measured on both sides of the atmospheric front are consistent with a change in wind speed and direction across the front. From this we conclude that for inverting X- or C-band SAR image spectra into Ocean wave spectra that contain Ocean Waves propagating near the range direction, it is quite important to have a good knowledge of the local wind field, whereas for L-band SAR image spectra this is of minor importance.
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imaging of Ocean Waves on both sides of an atmospheric front by the sir c x sar multifrequency synthetic
1998Co-Authors: Christian Melsheimer, Werner AlpersAbstract:Radar images of an Ocean scene containing an atmospheric front and almost range-propagating Ocean Waves which were acquired by the multifre- quency/multipolarization synthetic aperture radar (SAR) aboard the space shuttle Endeavour during the spaceborne imaging radar-C/X-band synthetic aperture radar (SIR-C/X-SAR) mission over the North Atlantic in 1994 are analyzed. The L-band SAR image spectra calculated from two areas located on opposite sides of the atmospheric front are quite similar, whereas the corresponding X- and C-band SAR image spectra differ significantly. It is shown that this is a consequence of the SAR imaging mechanism; at L band the SAR imaging mechanism depends weakly on the local wind field, and at X and C band it depends strongly on the local wind field. This is in agreement with earlier results obtained from the analysis of airborne multifrequency/multipolarization SAR images acquired over the North Sea during the SAR and X-Band Ocean Nonlinearities - Forschungsplattform Nordsee (SAXON-FPN) experiment. In this investigation it was found that at X and C band the phase of the relperture rdr modulation transfer function (RAR MTF) changes by almost 900 , when in the reference system moving with the group velocity of the dominant wave, the component of the wind velocity in the direction of the wave propagation changes sign. However, at L band such a change in locaJ wind direction affects the phase of the RAR MTF only slightly. Using this phase behavior of the RAR MTF in simulations of the SIR-C/X-SAR image spectra, we show that the observed differences of the X- and C-band SAR image spectra measured on both sides of the atmospheric front are consistent with a change in wind speed and direction across the front. From this we conclude that for inverting X- or C-band SAR image spectra into Ocean wave spectra that contain Ocean Waves propagating near the range direction, it is quite important to have a good knowledge of the local wind field, whereas for L-band SAR image spectra this is of minor importance.
Fabrice Ardhuin - One of the best experts on this subject based on the ideXlab platform.
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Atmospheric infrasound generation by Ocean Waves in finite depth: unified theory and application to radiation patterns
Geophysical Journal International, 2020Co-Authors: Marine De Carlo, Fabrice Ardhuin, Alexis Le PichonAbstract:Between 0.1 and 0.5 Hz, infrasound signals recorded in the atmosphere are dominated by Ocean-generated noise called microbaroms. Microbaroms propagate through the atmosphere over thousands of kilometers due to low absorption and efficient ducting between the ground and the stratopause. Different theoretical models have been developed to characterize the source of microbaroms, all based on the second-order non-linear interaction of Ocean Waves. While early theories considered an infinite Ocean depth and a source radiation depending on the acoustic wave elevation angle, other works have approximated the radiation pattern as a monopole, and found a considerable effect of the water depth. This paper reviews these models and extends the previous theories to the combined effects of both finite depth Ocean and source directivity in both elevation and azimuth angles. It is found that the water depth has a negligible effect for the near-horizontally propagating acoustic Waves that should dominate the measured microbarom records. Another important result is that the microbarom azimuthal variation can be highly directive locally, but it generally becomes isotropic when integrated over a realistic source region.
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Large-Scale Forces Under Surface Gravity Waves at a Wavy Bottom: A Mechanism for the Generation of Primary Microseisms
Geophysical Research Letters, 2018Co-Authors: Fabrice ArdhuinAbstract:Primary microseisms are background seismic oscillations recorded everywhere on Earth with typical frequencies 0.05 < f < 0.1 Hz. They appear to be generated by Ocean Waves of the same frequency f, propagating over shallow bottom topography. Previous quantitative models for the generation of primary microseisms considered wave propagation over topographic features with either large scales, equivalent to a vertical point force, or small scales matching Ocean wave wavelengths, equivalent to a horizontal force. While the first requires unrealistic bottom slopes to explain measured Rayleigh wave amplitudes, the second produced Love Waves and not enough Rayleigh Waves. Here we show how the small scales actually produce comparable horizontal and vertical forces. For example, a realistic rough bottom over an area of 100 km(2) with depths around 15 m is enough to explain the vertical ground motion observed at a seismic station located 150 km away. Ocean Waves propagating over small-scale topography is thus a plausible explanation for the observed microseisms at frequencies around 0.07 Hz. Plain Language Summary Microseisms are background oscillations of the solid Earth. Most of these oscillations are caused by Ocean Waves and can thus be used to study their source, the Ocean Waves, or the medium in which they propagate, the solid Earth. Several theories have been proposed for how Ocean Waves going over shallow Ocean topography make microseisms in the band of periods 10 to 20 s, but they are not satisfactory because they either require unrealistic large slopes of the Ocean floor or they produce a ratio of different types of seismic Waves, Love and Rayleigh Waves, that is too large. We thus revise these theories to show that a plausible seismic source is the propagation Ocean of Waves over a wavy bottom, when the bottom has wavelengths that match those of Ocean wave. We particularly verify that the predicted Rayleigh wave amplitude is of the order of what is measured at a particular seismometer located in Ireland. Because the necessary details in bottom topography vary a lot between different Ocean regions, the new theory suggests that the spatial distribution of seismic sources is more heterogeneous than previously thought.
Alexis Le Pichon - One of the best experts on this subject based on the ideXlab platform.
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Atmospheric infrasound generation by Ocean Waves in finite depth: unified theory and application to radiation patterns
Geophysical Journal International, 2020Co-Authors: Marine De Carlo, Fabrice Ardhuin, Alexis Le PichonAbstract:Between 0.1 and 0.5 Hz, infrasound signals recorded in the atmosphere are dominated by Ocean-generated noise called microbaroms. Microbaroms propagate through the atmosphere over thousands of kilometers due to low absorption and efficient ducting between the ground and the stratopause. Different theoretical models have been developed to characterize the source of microbaroms, all based on the second-order non-linear interaction of Ocean Waves. While early theories considered an infinite Ocean depth and a source radiation depending on the acoustic wave elevation angle, other works have approximated the radiation pattern as a monopole, and found a considerable effect of the water depth. This paper reviews these models and extends the previous theories to the combined effects of both finite depth Ocean and source directivity in both elevation and azimuth angles. It is found that the water depth has a negligible effect for the near-horizontally propagating acoustic Waves that should dominate the measured microbarom records. Another important result is that the microbarom azimuthal variation can be highly directive locally, but it generally becomes isotropic when integrated over a realistic source region.
X D Xie - One of the best experts on this subject based on the ideXlab platform.
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energy harvesting from transverse Ocean Waves by a piezoelectric plate
International Journal of Engineering Science, 2014Co-Authors: X D XieAbstract:Abstract An Ocean wave energy harvester from the transverse wave motion of water particles is developed by the piezoelectric effects. The harvester is made of two horizontal cantilever plates attached by piezoelectric patches and fixed on a vertical rectangular column. To describe the energy harvesting process, a mathematical model is developed to calculate the output charge and voltage from the piezoelectric patches according to the Airy linear wave theory and the elastic beam model. The influences on the root mean square (RMS) of the generated power from the piezoelectric patches, such as the Ocean depth, the harvester location under the Ocean surface, the length of the cantilevers, the wave height, and the ratio of wave length to Ocean depth, are discussed. Results show that the RMS increases with the increase in the length of cantilevers and the wave height, and decrease in the distance of the Ocean surface to the cantilevers and the ratio of the wave length to Ocean depth. As a result, an optimum Ocean depth is obtained to achieve a maximum RMS at different harvester locations under the Ocean surface. A value of the power up to 30 W can be realized for a practical transverse wave with the values of the Ocean depth, wave length, wave height and harvester location under the Ocean surface to be 10.6 m, 21.2 m, 4 m, and −2 m, respectively. This research develops a novel technique leading to efficient and practical energy harvesting from transverse Waves by piezoelectric energy harvesters that could be easily fixed on an offshore platform.
Christian Melsheimer - One of the best experts on this subject based on the ideXlab platform.
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imaging of Ocean Waves on both sides of an atmospheric front by the sir c x sar multifrequency synthetic aperture radar
Journal of Geophysical Research, 1998Co-Authors: Christian Melsheimer, Werner AlpersAbstract:Radar images of an Ocean scene containing an atmospheric front and almost range-propagating Ocean Waves which were acquired by the multifrequency/multipolarization synthetic aperture radar (SAR) aboard the space shuttle Endeavour during the spaceborne imaging radar-C/X-band synthetic aperture radar (SIR-C/X-SAR) mission over the North Atlantic in 1994 are analyzed. The L-band SAR image spectra calculated from two areas located on opposite sides of the atmospheric front are quite similar, whereas the corresponding X- and C-band SAR image spectra differ significantly. It is shown that this is a consequence of the SAR imaging mechanism; at L band the SAR imaging mechanism depends weakly on the local wind field, and at X and C band it depends strongly on the local wind field. This is in agreement with earlier results obtained from the analysis of airborne multifrequency/multipolarization SAR images acquired over the North Sea during the SAR and X-Band Ocean Nonlinearities - Forschungsplattform Nordsee (SAXON-FPN) experiment. In this investigation it was found that at X and C band the phase of the real aperture radar modulation transfer function (RAR MTF) changes by almost 90°, when in the reference system moving with the group velocity of the dominant wave, the component of the wind velocity in the direction of the wave propagation changes sign. However, at L band such a change in local wind direction affects the phase of the RAR MTF only slightly. Using this phase behavior of the RAR MTF in simulations of the SIR-C/X-SAR image spectra, we show that the observed differences of the X- and C-band SAR image spectra measured on both sides of the atmospheric front are consistent with a change in wind speed and direction across the front. From this we conclude that for inverting X- or C-band SAR image spectra into Ocean wave spectra that contain Ocean Waves propagating near the range direction, it is quite important to have a good knowledge of the local wind field, whereas for L-band SAR image spectra this is of minor importance.
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imaging of Ocean Waves on both sides of an atmospheric front by the sir c x sar multifrequency synthetic
1998Co-Authors: Christian Melsheimer, Werner AlpersAbstract:Radar images of an Ocean scene containing an atmospheric front and almost range-propagating Ocean Waves which were acquired by the multifre- quency/multipolarization synthetic aperture radar (SAR) aboard the space shuttle Endeavour during the spaceborne imaging radar-C/X-band synthetic aperture radar (SIR-C/X-SAR) mission over the North Atlantic in 1994 are analyzed. The L-band SAR image spectra calculated from two areas located on opposite sides of the atmospheric front are quite similar, whereas the corresponding X- and C-band SAR image spectra differ significantly. It is shown that this is a consequence of the SAR imaging mechanism; at L band the SAR imaging mechanism depends weakly on the local wind field, and at X and C band it depends strongly on the local wind field. This is in agreement with earlier results obtained from the analysis of airborne multifrequency/multipolarization SAR images acquired over the North Sea during the SAR and X-Band Ocean Nonlinearities - Forschungsplattform Nordsee (SAXON-FPN) experiment. In this investigation it was found that at X and C band the phase of the relperture rdr modulation transfer function (RAR MTF) changes by almost 900 , when in the reference system moving with the group velocity of the dominant wave, the component of the wind velocity in the direction of the wave propagation changes sign. However, at L band such a change in locaJ wind direction affects the phase of the RAR MTF only slightly. Using this phase behavior of the RAR MTF in simulations of the SIR-C/X-SAR image spectra, we show that the observed differences of the X- and C-band SAR image spectra measured on both sides of the atmospheric front are consistent with a change in wind speed and direction across the front. From this we conclude that for inverting X- or C-band SAR image spectra into Ocean wave spectra that contain Ocean Waves propagating near the range direction, it is quite important to have a good knowledge of the local wind field, whereas for L-band SAR image spectra this is of minor importance.