• Alpers, W., and I. Hennings, 1984: A theory of the imaging mechanism of underwater bottom topography by real and synthetic aperture radar. J. Geophys. Res, 89 , 10 52910 546.

    • Search Google Scholar
    • Export Citation
  • Calkoen, C. J., G. H. F. M. Hesselmans, G. J. Wensink, and J. Vogelzang, 2001:. : The bathymetry assessment system: Efficient depth mapping in shallow seas using radar imagery. Int. J. Remote Sens., in press.

    • Search Google Scholar
    • Export Citation
  • Caponi, E. A., D. R. Crawford, H. C. Yuen, and P. G. Saffman, 1988: Modulation of radar backscatter from the ocean by a variable surface current. J. Geophys. Res, 93 , 12 24912 263.

    • Search Google Scholar
    • Export Citation
  • Cooper, A. L., S. R. Chubb, F. Askari, and G. R. Valenzuela, 1994: Radar surface signatures for the two-dimensional tidal circulation over Phelps Bank, Nantucket shoals: A comparison between theory and experiment. J. Geophys. Res, 99 , 78657883.

    • Search Google Scholar
    • Export Citation
  • De Loor, G. P., 1981: The observation of tidal patterns, currents and bathymetry with SLAR imagery of the sea. IEEE J. Oceanic Eng, 6 , 124129.

    • Search Google Scholar
    • Export Citation
  • Donato, T. F., F. Askari, G. O. Marmorino, C. L. Trump, and D. R. Lyzenga, 1997: Radar imaging of sand waves on the continental shelf east of Cape Hatteras, NC, U.S.A.,. Contin. Shelf Res, 17 , 9891004.

    • Search Google Scholar
    • Export Citation
  • Greidanus, H., 1997: The use of radar for bathymetry in shallow seas. Hydrogr. J, 83 , 1318.

  • Hennings, I., 1990: Radar imaging of submarine sand waves in tidal channels. J. Geophys. Res, 95 , 97139721.

  • ——,. 1998: An historical overview of radar imagery of sea bottom topography. Int. J. Remote Sens, 19 , 14471454.

  • ——, Matthews, J. P., and M. Metzner, 1994a: Sun glitter radiance and radar cross-section modulations of the sea bed. J. Geophys. Res, 99 , 16 30316 326.

    • Search Google Scholar
    • Export Citation
  • ——, Stolte, S., and F. Ziemer, 1994b: Experimental method to measure surface signature generation by sea bottom undulations. IEEE J. Oceanic Eng, 19 , 3640.

    • Search Google Scholar
    • Export Citation
  • ——, Metzner, M., and C. J. Calkoen, 1998: Island connected sea bed signatures observed by multi-frequency synthetic aperture radar. Int. J. Remote Sens, 19 , 19331951.

    • Search Google Scholar
    • Export Citation
  • Holliday, D., G. St-Cyr, and N. E. Woods, 1986: A radar ocean imaging model for small to moderate incidence angles. Int. J. Remote Sens, 7 , 18091834.

    • Search Google Scholar
    • Export Citation
  • Hughes, B. A., 1978: The effect of internal waves on surface wind waves 2. Theoretical analysis. J. Geophys. Res, 83 , 455465.

  • Inoue, T., 1966: On the growth of the spectrum of a wind generated sea according to a modified Miles–Phillips mechanism and its application to wave forecasting. Rep. TR67-5, Geophysical Sciences Laboratory, New York University.

    • Search Google Scholar
    • Export Citation
  • Miles, J. W., 1959: On the generation of surface waves by shear flows, 2. J. Fluid Mech, 6 , 568582.

  • Mitsuyasu, H., and T. Honda, 1982: Wind-induced growth of water waves. J. Fluid Mech, 123 , 425442.

  • Nimmo Smith, W. A. M., S. A. Thorpe, and A. Graham, 1999: Surface effects of bottom-generated turbulence in a shallow tidal sea. Nature, 400 , 251254.

    • Search Google Scholar
    • Export Citation
  • Phillips, O. M., 1984: On the response of short ocean wave components at a fixed wavenumber to ocean current variations. J. Phys. Oceanogr, 14 , 14251433.

    • Search Google Scholar
    • Export Citation
  • Plant, W. J., 1982: A relationship between wind stress and wave slope. J. Geophys. Res, 87 , 19611967.

  • Romeiser, R., and W. Alpers, 1997: An improved composite surface model for the radar backscattering cross section of the ocean surface. 2. Model response to surface roughness variations and the radar imaging of underwater bottom topography. J. Geophys. Res, 102 , 25 25125 267.

    • Search Google Scholar
    • Export Citation
  • Shuchman, R. A., D. R. Lyzenga, and G. A. Meadows, 1985: Synthetic aperture radar imaging of ocean-bottom topography via tidal–currents interactions: Theory and observations. Int. J. Remote Sens, 6 , 11791200.

    • Search Google Scholar
    • Export Citation
  • Snyder, R. L., and C. S. Cox, 1966: A field study of the wind generation of ocean waves. J. Mar. Res, 24 , 141178.

  • ——, Dobson, F. W., J. A. Elliott, and R. B. Long, 1981: Array measurements of atmospheric pressure fluctuations above surface gravity waves. J. Fluid Mech, 102 , 159.

    • Search Google Scholar
    • Export Citation
  • Stolte, S., 1994: Short-wave measurements by a fixed tower-based and a drifting buoy system. IEEE J. Oceanic Eng, 19 , 1022.

  • ——, and Stolte Jr., S., 1999: In-situ measurements of short wave modulation due to bottom topography. Proc. IGARSS'99 Symposium, Hamburg, Germany, Institute of Electrical and Electronics Engineers, 1661–1663.

    • Search Google Scholar
    • Export Citation
  • Terwindt, J. H. J., 1971: Sand waves in the southern bight of the North Sea. Mar. Geol, 10 , 5167.

  • Thompson, D. R., 1988: Calculation of radar backscatter modulations from internal waves. J. Geophys. Res, 93 , 12 37112 380.

  • Valenzuela, G. R., and J. W. Wright, 1979: Modulation of short gravity–capillary waves by longer-scale periodic flows—A higher order theory. Radio Sci, 14 , 10991110.

    • Search Google Scholar
    • Export Citation
  • Van der Kooij, M. W. A., J. Vogelzang, and C. J. Calkoen, 1995: A simple analytical model for brightness modulations caused by submarine sand waves in radar imagery. J. Geophys. Res, 100 , 70697082.

    • Search Google Scholar
    • Export Citation
  • Vogelzang, J., 1997: Mapping submarine sandwaves with multi-band imaging radar 1. Model development and sensitivity analysis. J. Geophys. Res, 102 , 11631181.

    • Search Google Scholar
    • Export Citation
  • ——,. 2000: A model comparison study to the imaging of submarine reefs with synthetic aperture radar. Int. J. Remote Sens., in press.

    • Search Google Scholar
    • Export Citation
  • ——, Wensink, G. J., C. J. Calkoen, and M. W. A. Van der Kooij, 1997: Mapping submarine sandwaves with multi-band imaging radar 2. Experimental results and model comparison. J. Geophys. Res, 102 , 11831192.

    • Search Google Scholar
    • Export Citation
  • Zimmerman, J. F. T., 1985: Radar images of the sea bed. Nature, 314 , 224226.

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Radar Imaging Mechanism of the Seabed: Results of the C-STAR Experiment in 1996 with Special Emphasis on the Relaxation Rate of Short Waves due to Current Variations

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  • 1 GEOMAR, Forschungszentrum für Marine Geowissenschaften der Christian-Albrechts-Universität zu Kiel, Kiel, Germany
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Abstract

During the field experiment of the Coastal Sediment Transport Assessment using SAR imagery project of the Marine Science and Technology program of the European Commission an Air–Sea Interaction Drift Buoy (ASIB) system was equipped with special sensors and instruments to measure the position, the water depth, the surface current velocity and direction, the modulation characteristics of short-wave energies, and relevant air–sea interaction parameters due to undulations in the seabed. The ASIB system was operated from on board a research vessel and the data were measured while the buoy drifted in the tidal currents across sand waves of the study area. All buoy measurements were analyzed by computing frequency spectra of low and high frequency waves (scalar spectra between 0.1 and 50 Hz). The whole range of short water waves was recorded by these in situ measurements on board the buoy, which is responsible for the backscattering of commonly used air- and spaceborne imaging radars. A comprehensive dataset of wave energy density spectrum modulations above sand waves was produced. Normalized Radar Cross Section (NRCS) modulations of a selected P-band airborne Experimental-Synthetic Aperture Radar (E-SAR) image were compared with wave energy density spectrum variations at the appropriate short surface gravity Bragg-wave frequency measured along the drift path of the ASIB system. The NRCS and wave energy density modulation depths agreed within a factor of 2.

Using the obtained in situ measurements from the ASIB system the relaxation rate μ of short water waves due to current variations above submarine sand waves was calculated by applying a first-order weak hydrodynamic interaction theory. The relaxation rate μ dependence on several responsible hydrodynamic air–sea interaction parameters was calculated as a function of wavenumber k in the range of P-, L-, C-, and X-band radar Bragg waves for three different mean wind speed regimes of Uw = 0.8 m s−1, Uw = 3.8 m s−1, and Uw = 7.4 m s−1. Several published parameterizations of μ showed that this parameter increases with wavenumber and wind speed. Results show that μ increases also with wind speed Uw but decreases with wavenumber k. This can possibly imply that the wind growth relaxation rate μw is not equivalent with the relaxation rate μ of short waves due to current variations above submarine sand waves as a function of k. The analysis can also imply that the Bragg scattering mechanism seems to be insufficient to explain completely alone the NRCS modulation due to the seabed via surface current gradients especially at higher radar frequencies.

Corresponding author address: Ingo Hennings, GEOMAR, Forschungszentrum für Marine Geowissenschaften der Christian-Albrechts-Universität zu Kiel, Wischhofstraße 1-3, D-24148 Kiel, Germany. Email: ihennings@geomar.de

Abstract

During the field experiment of the Coastal Sediment Transport Assessment using SAR imagery project of the Marine Science and Technology program of the European Commission an Air–Sea Interaction Drift Buoy (ASIB) system was equipped with special sensors and instruments to measure the position, the water depth, the surface current velocity and direction, the modulation characteristics of short-wave energies, and relevant air–sea interaction parameters due to undulations in the seabed. The ASIB system was operated from on board a research vessel and the data were measured while the buoy drifted in the tidal currents across sand waves of the study area. All buoy measurements were analyzed by computing frequency spectra of low and high frequency waves (scalar spectra between 0.1 and 50 Hz). The whole range of short water waves was recorded by these in situ measurements on board the buoy, which is responsible for the backscattering of commonly used air- and spaceborne imaging radars. A comprehensive dataset of wave energy density spectrum modulations above sand waves was produced. Normalized Radar Cross Section (NRCS) modulations of a selected P-band airborne Experimental-Synthetic Aperture Radar (E-SAR) image were compared with wave energy density spectrum variations at the appropriate short surface gravity Bragg-wave frequency measured along the drift path of the ASIB system. The NRCS and wave energy density modulation depths agreed within a factor of 2.

Using the obtained in situ measurements from the ASIB system the relaxation rate μ of short water waves due to current variations above submarine sand waves was calculated by applying a first-order weak hydrodynamic interaction theory. The relaxation rate μ dependence on several responsible hydrodynamic air–sea interaction parameters was calculated as a function of wavenumber k in the range of P-, L-, C-, and X-band radar Bragg waves for three different mean wind speed regimes of Uw = 0.8 m s−1, Uw = 3.8 m s−1, and Uw = 7.4 m s−1. Several published parameterizations of μ showed that this parameter increases with wavenumber and wind speed. Results show that μ increases also with wind speed Uw but decreases with wavenumber k. This can possibly imply that the wind growth relaxation rate μw is not equivalent with the relaxation rate μ of short waves due to current variations above submarine sand waves as a function of k. The analysis can also imply that the Bragg scattering mechanism seems to be insufficient to explain completely alone the NRCS modulation due to the seabed via surface current gradients especially at higher radar frequencies.

Corresponding author address: Ingo Hennings, GEOMAR, Forschungszentrum für Marine Geowissenschaften der Christian-Albrechts-Universität zu Kiel, Wischhofstraße 1-3, D-24148 Kiel, Germany. Email: ihennings@geomar.de

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