Ingo Hennings

Sun glitter radiance and radar cross-section modulations of the sea bed

Aircraft and satellite-borne multispectral sensors such as ocean color scanners, spectrometers, and scanning Lidars have proved to be effective in detecting submarine shallow-water bottom topography in clear coastal waters. For such studies the blue-green band of the visible electromagnetic spectrum (wavelength between 400 and 580 nm) is used, because natural light in this range has the deepest penetration into the water column. However, if the water becomes turbid, the reflection from the submarine sea bed disappears. In this case the only possible mechanism available in the optical range of the electromagnetic spectrum for detecting surface signatures of shallow water bottom topography is through the observation of direct sunlight specularly reflected from a roughened sea surface, known as sun glitter radiance. As the tidal flow over irregularities on the submarine sea bed creates surface roughness variations, sun glitter imagery can be used to detect such features. In this paper a first-order theory of the sun glitter imaging mechanism of submerged sand waves is presented. The results of sun glitter radiance modulations are compared with simulations of P band radar cross-section modulations and with experimental data. Calculations of both the constant background sun glitter radiance and the sun glitter radiance modulation show that these parameters are very sensitive to wind speed, to view angle with respect to acquisition time, and to observation geometry as a whole.

Comparison of submarine relief features on a radar satellite image and on a Skylab satellite photograph

Abstract We compare satellite imagery obtained by an optical sensor and by synthetic aperture radar for a shallow ocean area showing submarine relief features. A Skylab photograph and a Seasat radar image of the North American cast coast (Nantucket Shoals) taken at different dates, but at the same tidal phase and under comparable weather conditions, are analysed. It is shown that the radar imaging as well as the optical imaging is caused by roughness variations of the water surface due to tidal flow over submarine relief. It is investigated whether optical imaging is affected by backscattering by suspended sediment in the water column, by reflection from the sea floor or by variations of the surface roughness associated with wind and tidal flow over underwater bottom topography. We conclude from the analysis of the densities in the blue, green and red layers of the Skylab colour film that specularly reflected sunlight at the rough ocean surface is the dominant imaging mechanism. In cases where the underwa...

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

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 da...

Mapping of sea bottom topography in a multi sensor approach

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.<<ETX>>

A theory of the Kaband radar imaging mechanism of a submerged wreck and associated bed forms in the southern North Sea

The Ka band radar imaging mechanism of the submerged wreck/sand ribbon of the motor vessel (M/V) Birkenfels in the southern North Sea is investigated by applying the quasi‐specular scattering theory and considering the capillary as well as the gravity wave ranges of the wave energy density spectrum. For the imaging of wrecks and other oceanographic and meteorological phenomena at the sea surface it is assumed that quasi‐specular scattering becomes dominant at higher radar frequencies like Ka and X band and wind speeds ≥ 7–8 m s−1. Multibeam echo sounder images of the Birkenfels wreck and associated sand ribbons as well as other available environmental in situ data have been analyzed. The formation of sand ribbons at the sea bed and the manifestation of its radar signatures at the water surface are caused by an elliptical vortex or helical flow cell triggered by unidirectional tidal current flow interacting with the wreck. The difference between simulated and measured normalized radar cross section (NRCS) modulation as a function of the space variable is less than 31.6%. Results are presented for NRCS simulations dependent on position for different effective incidence angles, unidirectional current speeds, wind speeds, and relaxation rates. The calculated current gradient or strain rate of the imaging theory has the same order of magnitude as those obtained for marine sand waves. This implies that the responsible hydrodynamic interaction mechanism is able to produce radar signatures of submerged wrecks/sand ribbons and make them visible at the sea surface.

Observed suspended sediment dynamics during a tidal cycle above submerged asymmetric compound sand waves

The data from Acoustic Doppler Current Profiler (ADCP) of the three-dimensional current-field, echo intensity, modulation of Suspended Sediment Concentration (SSC), and related water levels and wind velocities have been analyzed as a function of water depth above submerged asymmetric compound sand waves during a tidal cycle in the Lister Tief of the German Bight in the North Sea. Signatures of vertical current component, echo intensities and calculated SSC modulations in the water column depend strongly on wind and current velocity. Bursts of vertical current component and echo intensity are triggered by sand waves itself as well as by superimposed megaripples due to current wave interaction at high current 1.0 ms-1 and wind speeds 10.0 ms-1, preferably of opposite directions, measured at high spatial resolution. The magnitude of currents and SSC modulations during ebb and flood tidal current phases are only weakly time dependent, whereas the local magnitudes of these parameters are variable in space above the sand waves. Some hydrodynamic parameters are further investigated and analyzed, showing a consistence of ADCP measurements in the applied theory.