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G.j. Wensink - One of the best experts on this subject based on the ideXlab platform.

  • The Use of Radar Imagery to Assess the Bottom Topography of Shallow Seas
    International Hydrographic Review, 2015
    Co-Authors: C. J. Calkoen, G. H. F. M. Hesselmans, G.j. Wensink
    Abstract:

    Under favourable conditions features of the Bottom Topography of shallow seas are visible in radar images, which are nowadays obtained from satellites on a routine basis. A Bathymetry Assessment System (BAS) was developed to use these images in order to produce depth maps. This paper describes the principles behind the system, indicates for what type of applications it might be useful, notes on the accuracy and gives an example of an application.

  • Bottom Topography assessment from SAR images
    IGARSS '98. Sensing and Managing the Environment. 1998 IEEE International Geoscience and Remote Sensing. Symposium Proceedings. (Cat. No.98CH36174), 1998
    Co-Authors: C. J. Calkoen, G.j. Wensink, G. H. F. M. Hesselmans
    Abstract:

    It is well known that under favourable meteorological and hydrodynamic conditions features of the Bottom Topography of shallow seas are visible in synthetic aperture radar (SAR) images. Models have been developed to simulate SAR contrast variations for a given depth map. Inversion of these models makes it possible to assess the Bottom Topography from SAR images and a limited amount of measured depth information. This has great economic benefits as traditional bathymetric surveys are expensive and time consuming. At ARGOSS a Bathymetry Assessment System (BAS) has been implemented along these lines. The present operational version is based on simplified, one-dimensional models. It has been tested a dozen times in demonstration/validation projects in various locations. Depth maps were obtained with an accuracy between 20 cm and 30 cm root mean squared. This is sufficient for many applications.

  • sea Bottom Topography with x band slar the relation between radar imagery and bathymetry
    International Journal of Remote Sensing, 1992
    Co-Authors: J. Vogelzang, G.j. Wensink, G. P. De Loor, H.c. Peters, H. Pouwels
    Abstract:

    Abstract On 19 January 1988, and experiment was carried out to establish the positional relation between corresponding features in radar imagery and Bottom Topography. The radar measurements were done with the Dutch Digital SLAR, an X-band HH polarized SLAR system. The experiment look place in an area 30 km off the Dutch coast where the Bottom Topography is dominated by sand waves with a height between 2 and 6 m and a crest-to-crest distance of typically 500 m at a depth of 22 m. Ground data were recorded from two ships near the centre of the test area and from the Measuring Platform Noordwijk, 20 km from the test area. By comparing the radar images showing sand waves with digitized maps of depth and Bottom slope it is concluded that the extremes in the radar backscatter are located right above regions with maximum Bottom slope, within a positional accuracy of 30 m. This is in excellent agreement with predictions by simple relaxation models of the imaging mechanism.

  • Sea Bottom Topography imaging with SAR
    1992
    Co-Authors: M. W. A. Vanderkooij, G.j. Wensink, J. Vogelzang
    Abstract:

    It is well known that under favorable meteorological and hydrodynamical conditions the Bottom Topography of shallow seas can be mapped with airborne or spaceborne imaging radar. This phenomenon was observed for the first time in 1969 by de Loor and co-workers in Q-band Side Looking Airborne Radar (SLAR) imagery of sandwaves in the North Sea. It is now generally accepted that the imaging mechanism consists of three steps: (1) interaction between (tidal) current and Bottom Topography causes spatial modulations in the surface current velocity; (2) modulations in the surface current velocity give rise to variations in the spectrum of wind-generated waves, as described by the action balance equation; and (3) variations in the wave spectrum show up as intensity modulations in radar imagery. In order to predict radar backscatter modulations caused by sandwaves, an imaging model, covering the three steps, was developed by the Dutch Sea Bottom Topography Group. This model and some model results will be shown. On 16 Aug. 1989 an experiment was performed with the polarimetric P-, L-, and C-band synthetic aperture radar (SAR) of NASA/JPL. One scene was recorded in SAR mode. On 12 Jul. 1991 another three scenes were recorded, of which one was in the ATI-mode (Along-Track Interferometer). These experiments took place in the test area of the Sea Bottom Topography Group, 30 km off the Dutch coast, where the Bottom Topography is dominated by sand waves. In-situ data were gathered by a ship in the test area and on 'Measuring Platform Noordwijk', 20 km from the center of the test area. The radar images made during the experiment were compared with digitized maps of the Bottom. Furthermore, the profiles of radar backscatter modulation were compared with the results of the model. During the workshop some preliminary results of the ATI measurements will be shown.

  • Mapping of sea Bottom Topography
    1992
    Co-Authors: C. J. Calkoen, G.j. Wensink, G. H. F. M. Hesselmans
    Abstract:

    Under suitable conditions the Bottom Topography of shallow seas is visible in remote sensing radar imagery. Two experiments were performed to establish which remote sensing technique or combination yields optimal imaging of Bottom Topography and which hydro-meteorological conditions are favorable. A further goal is to gain experience with these techniques. Two experiments were performed over an area in the North Sea near the measuring platform Meetpost Noordwijk (MPN). The Bottom Topography in the test area is dominated by sand waves. The crests of the sand waves are perpendicular to the coast line and the dominating (tidal-)current direction. A 4x4 sq km wide section of the test area was studied in more detail. The first experiment was undertaken on 16 Aug. 1989. During the experiment the following remote sensing instruments were used: Landsat-Thematic Mapper, and NASA/JPL Airborne Imaging Radar (AIR). The hydro-meteorological conditions; current, wind, wave, and air and water temperature were monitored by MPN, a ship of Rijkswaterstaat (the OCTANS), and a pitch-and-roll WAVEC-buoy. The second experiment took place on 12 July 1992. During this experiment data were collected with the NASA/JPL polarimetric synthetic aperture radar (SAR), and a five-band helicopter-borne scatterometer. Again the hydro-meteorological conditions were monitored at MPN and the OCTANS. Furthermore, interferometric radar data were collected.

Edward D Zaron - One of the best experts on this subject based on the ideXlab platform.

  • Recent Developments in Bottom Topography Mapping Using Inverse Methods
    Data Assimilation for Atmospheric Oceanic and Hydrologic Applications (Vol. III), 2016
    Co-Authors: Edward D Zaron
    Abstract:

    The problem of identification and mapping of underwater Topography , in the form of river channel depth, littoral zone depth profiles, and spatially-resolved river, estuary , and ocean Bottom Topography, has received attention in recent years in tandem with the increasing availability of remotely-sensed data for hydrologic and hydrodynamic modeling. A variety of inverse methods have been successfully applied in order to estimate the Bottom Topography from diverse data, typically by using variants of the ensemble extended Kalman filter , but variational methods and non-parametric filters have also been used. The types of measurements used include remotely-sensed and in situ water level, surface currents, surface wave celerity, and measurements of surface wave direction and wave breaking. The dynamics employed to relate Bottom depth to the measured variables have, to date, been based on the vertically integrated shallow equations, the Saint-Venant equations , with either Chezy or Manning frictional representation; and coastal zone applications have additionally coupled these dynamics with the wave radiation stress and dissipation from models of phase-averaged surface waves. The relevance of three-dimensional dynamics associated with vertical shear and baroclinicity are recognized but not yet incorporated into inverse methods for topographic estimation. A scale analysis of the shallow water equations is proposed as a guide to understanding how the dynamics, spatial correlation scales, and data types are related to length and time scales of the given application.

  • on the observability of Bottom Topography from measurements of tidal sea surface height
    Ocean Modelling, 2016
    Co-Authors: Edward D Zaron
    Abstract:

    Abstract The question of whether features of the ocean Bottom Topography can be identified from measurements of water level is investigated using a simplified one-dimensional barotropic model. Because of the nonlinear dependence of the sea surface height on the water depth, a linearized analysis is performed concerning the identification of a Gaussian bump within two specific depth profiles, (1) a constant depth domain, and, (2) a constant depth domain adjoining a near-resonant continental shelf. Observability is quantified by examining the estimation error in a series of identical-twin experiments varying data density, tide wavelength, assumed (versus actual) topographic correlation scale, and friction. For measurements of sea surface height that resolve the scale of the topographic perturbation, the fractional error in the Bottom Topography is approximately a factor of 10 larger than the fractional error of the sea surface height. Domain-scale and shelf-scale resonances may lead to inaccurate Topography estimates due to a reduction in the effective number of degrees of freedom in the dynamics, and the amplification of nonlinearity. A realizability condition for the variance of the Topography error in the limit of zero Bottom depth is proposed which is interpreted as a bound on the fractional error of the Topography. Appropriately designed spatial covariance models partly ameliorate the negative impact of shelf-scale near-resonance, and highlight the importance of spatial covariance modeling for Bottom Topography estimation.

  • Bottom Topography mapping via nonlinear data assimilation
    Journal of Atmospheric and Oceanic Technology, 2011
    Co-Authors: Edward D Zaron, Marieaude Pradal, Patrick Miller, Alan F Blumberg, Nickitas Georgas, Julia Muccino Cornuelle
    Abstract:

    AbstractA variational data assimilation method is described for Bottom Topography mapping in rivers and estuaries using remotely sensed observations of water surface currents. The velocity field and Bottom Topography are related by the vertically integrated momentum and continuity equations, leading to a nonlinear inverse problem for Bottom Topography, which is solved using a Picard iteration strategy combined with a nonlinear line search. An illustration of the method is shown for Haverstraw Bay, in the Hudson River, where the known Bottom Topography is well reconstructed. Once the Topography has been estimated, currents and water levels may be forecast. The method makes feasible 1) the estimation of Bottom Topography in regions where in situ data collection may be impossible, dangerous, or expensive, and 2) the calibration of barotropic shallow-water models via control of the Bottom Topography.

J. Vogelzang - One of the best experts on this subject based on the ideXlab platform.

  • sea Bottom Topography with x band slar the relation between radar imagery and bathymetry
    International Journal of Remote Sensing, 1992
    Co-Authors: J. Vogelzang, G.j. Wensink, G. P. De Loor, H.c. Peters, H. Pouwels
    Abstract:

    Abstract On 19 January 1988, and experiment was carried out to establish the positional relation between corresponding features in radar imagery and Bottom Topography. The radar measurements were done with the Dutch Digital SLAR, an X-band HH polarized SLAR system. The experiment look place in an area 30 km off the Dutch coast where the Bottom Topography is dominated by sand waves with a height between 2 and 6 m and a crest-to-crest distance of typically 500 m at a depth of 22 m. Ground data were recorded from two ships near the centre of the test area and from the Measuring Platform Noordwijk, 20 km from the test area. By comparing the radar images showing sand waves with digitized maps of depth and Bottom slope it is concluded that the extremes in the radar backscatter are located right above regions with maximum Bottom slope, within a positional accuracy of 30 m. This is in excellent agreement with predictions by simple relaxation models of the imaging mechanism.

  • Sea Bottom Topography imaging with SAR
    1992
    Co-Authors: M. W. A. Vanderkooij, G.j. Wensink, J. Vogelzang
    Abstract:

    It is well known that under favorable meteorological and hydrodynamical conditions the Bottom Topography of shallow seas can be mapped with airborne or spaceborne imaging radar. This phenomenon was observed for the first time in 1969 by de Loor and co-workers in Q-band Side Looking Airborne Radar (SLAR) imagery of sandwaves in the North Sea. It is now generally accepted that the imaging mechanism consists of three steps: (1) interaction between (tidal) current and Bottom Topography causes spatial modulations in the surface current velocity; (2) modulations in the surface current velocity give rise to variations in the spectrum of wind-generated waves, as described by the action balance equation; and (3) variations in the wave spectrum show up as intensity modulations in radar imagery. In order to predict radar backscatter modulations caused by sandwaves, an imaging model, covering the three steps, was developed by the Dutch Sea Bottom Topography Group. This model and some model results will be shown. On 16 Aug. 1989 an experiment was performed with the polarimetric P-, L-, and C-band synthetic aperture radar (SAR) of NASA/JPL. One scene was recorded in SAR mode. On 12 Jul. 1991 another three scenes were recorded, of which one was in the ATI-mode (Along-Track Interferometer). These experiments took place in the test area of the Sea Bottom Topography Group, 30 km off the Dutch coast, where the Bottom Topography is dominated by sand waves. In-situ data were gathered by a ship in the test area and on 'Measuring Platform Noordwijk', 20 km from the center of the test area. The radar images made during the experiment were compared with digitized maps of the Bottom. Furthermore, the profiles of radar backscatter modulation were compared with the results of the model. During the workshop some preliminary results of the ATI measurements will be shown.

  • SEA Bottom Topography WITH IMAGING RADAR
    1991
    Co-Authors: J. Vogelzang, G.j. Wensink, D. Van Halsema, G. Van Der Burg
    Abstract:

    Mapping sea Bottom Topography with imaging radars or optical sensors can add valuable information to traditional bathymetric surveys. This paper describes a series of (already performed or planned) experiments in which the Dutch Bottom Topography Group is involved. It is shown how radar and optical imagery can be used to map the sea floor.

  • Sea Bottom Topography Imaging With Polarimetric P-, L-, And C-band Sar
    [Proceedings] IGARSS'91 Remote Sensing: Global Monitoring for Earth Management, 1
    Co-Authors: J. Vogelzang, G.j. Wensink, M.w.a. Van Der Kooij, G. Van Der Burg
    Abstract:

    On August 16, 1989, an experiment has been performed to study the imaging of Bottom Topography 2. Modulations in the surface current velocity give rise to variations in the spectrum of windgenerated waves, as described by the action balance equation. with the polarimetric P-, L-, and C-band SAR of NASA/JPL. The experiment formed a link in a number of experiments and modelling efforts of the Dutch Sea Bottom Topography Group. It took place in the test area 30 km of the Dutch coast where the Bottom Topography is dominated by sand waves. In-situ data were gathered by a ship in the test area and on "Measuring Platform Noordwijk", 20 km from the centre of the test area. The radar images made during the experiment were compared with digitized maps of the Bottom. Furthermore profiles of radar backscatter will be compared with the results of an imaging model.

  • Mapping of sea Bottom Topography in a multi sensor approach
    Proceedings of IGARSS '94 - 1994 IEEE International Geoscience and Remote Sensing Symposium, 1
    Co-Authors: J. Vogelzang, Ingo Hennings, G.j. Wensink, M.w.a. Van Der Kooij, Werner Alpers, J.p. Matthews
    Abstract:

    Three remote sensing methods for obtaining information on sea Bottom Topography have been investigated: passive optical bathymetry, sun glint observation and radar observation. Optical and microwave remotely sensed data as well as extensive in-situ data, including a detailed bathymetric map, were gathered in a sand wave area off the Dutch coast. These data were compared with each other and with model predictions. The models are based on the current state-of-the-art, with some extensions. Passive optical bathymetry has limited use above the North Sea because of its limited depth range. Sun glint observation of Bottom Topography is possible, but its practical applicability is limited by the requirement of low wind speeds and cloudless weather. Radar observation with an imaging radar operating at long wavelengths has the highest potential. The agreement between radar data and model predictions is not always good, due to lack of knowledge on the basic processes. However, in cases where there is good agreement, the imaging model can be inverted numerically to retrieve depth information from radar images. >

Rudy Magne - One of the best experts on this subject based on the ideXlab platform.

  • Scattering of surface gravity waves by Bottom Topography with a current
    Journal of Fluid Mechanics, 2007
    Co-Authors: Fabrice Ardhuin, Rudy Magne
    Abstract:

    A theory is presented that describes the scattering of random surface gravity waves by small-amplitude Topography, with horizontal scales of the order of the wavelength, in the presence of an irrotational and almost uniform current. A perturbation expansion of the wave action to order η 2 yields an evolution equation for the wave action spectrum, where η =m ax(h)/H is the small-scale Bottom amplitude normalized by the mean water depth. Spectral wave evolution is proportional to the Bottom elevation variance at the resonant wavenumbers, representing a Bragg scattering approximation. With a current, scattering results from a direct effect of the Bottom Topography, and an indirect effect of the Bottom through the modulations of the surface current and mean surface elevation. For Froude numbers of the order of 0.6 or less, the Bottom Topography effects dominate. For all Froude numbers, the reflection coefficients for the wave amplitudes that are inferred from the wave action source term are asymptotically identical, as η goes to zero, to previous theoretical results for monochromatic waves propagating in one dimension over sinusoidal bars. In particular, the frequency of the most reflected wave components is shifted by the current, and wave action conservation results in amplified reflected wave energies for following currents. Application of the theory to waves over current-generated sandwaves suggests that forward scattering can be significant, resulting in a broadening of the directional wave spectrum, while back-scattering should be generally weaker.

  • Current effects on scattering of surface gravity waves by Bottom Topography
    arXiv: Atmospheric and Oceanic Physics, 2005
    Co-Authors: Rudy Magne, Fabrice Ardhuin
    Abstract:

    The scattering of random surface gravity waves by Topography of small amplitude, and horizontal scales of the order of the wavelength, is investigated theoretically in the presence of a an almost uniform irrotational current. This problem is relevant to ocean waves propagation on shallow continental shelves where tidal currents are often significant. Defining the small scale Bottom amplitude normalized by the mean water depth, � = h/H, a perturbation expansion of the wave action to order � 2 yields an evolution equation for the wave action spectrum. Based on numerical calculations for sinusoidal bars, a mixed surface-Bottom bispectrum, that arises at order �, is unlikely to be significant in most oceanic conditions. Neglecting that term, the present theory yields a closed equation with a scattering source term that gives the rate of exchange of action between spectral wave components that have the same absolute frequency. This source term is proportional to the Bottom elevation variance at the resonant wavenumbers, and thus represents a Bragg scattering approximation. With current, the source term formally combines a direct effect of the Bottom Topography with an indirect effect of the Bottom through the modulation of the surface current and mean surface elevation. For Froude numbers of the order of 0.6 or less, the Bottom Topography effects dominate. For all Froude numbers, the reflection coefficients for the wave amplitudes that are inferred @�

Huang Weigen - One of the best experts on this subject based on the ideXlab platform.

  • A review about shallow sea Bottom Topography mapping by SAR
    2009
    Co-Authors: Fan Kai-guo, Huang Weigen
    Abstract:

    Synthetic Aperture Radar (SAR) has become one of the important tools for mapping shallow sea Bottom Topography. This technique has significant economic efficiency compared with the traditional techniques. Numerical models have been developed to simulate shallow sea Bottom Topography SAR images. Inversion of these models makes it possible to assess the water depths from SAR images. This paper reviews these numerical models of SAR technique for mapping shallow sea Bottom Topography, and examples are illustrated including in the coastal areas of China. Some issues about SAR technique available and the research orientation in the future are also discussed and concluded.

  • Simulation study on optimal currents and winds for the spaceborne SAR mapping of sea Bottom Topography
    2000
    Co-Authors: Huang Weigen
    Abstract:

    A simulation model for the radar backscattering cross section of the sea surface has been developed based on the synthetic aperture radar (SAR) imaging mechanism of the sea Bottom Topography. The relationship between tidal currents, sea surface winds and the SAR mapping of the sea Bottom Topography has been analyzed using the results of the simulation. It is shown that the sea Bottom Topography can be observed by spaceborne SAR more easily at high current speeds. The optimal direction of the currents for mapping the sea Bottom Topography is the direction perpendicular to the bathymetric features while the direction parallel to the bathymetric features is the worst. The optimal range of wind speeds for mapping the sea Bottom Topography is between 3 and 9 m/s. The wind directions between 30?and 89?are preferred although the effect of the wind direction on SAR mapping of Bottom Topography is complicated.