J. Vogelzang

The Bathymetry Assessment System: Efficient depth mapping in shallow seas using radar images

With the operation of the European Remote Sensing (ERS) satellites and RADARSAT, radar images are now readily available. One of the new applications of radar images is their use for bathymetric mapping in shallow seas. The Bathymetry Assessment System (BAS), described in detail in this paper, constructs accurate depth maps from radar images and a limited number of echo soundings. The BAS consists of a forward imaging model and an inversion part. The system needs a first guess depth map that may be derived from echo soundings or an old map of the area. The forward model calculates a simulated radar image. This is compared with the actual radar image by evaluating a penalty function. The penalty function also contains a term that compares model depths with measured depths and a term that contains a smoothness criterion, prohibiting speckle noise to be interpreted as depth variations. The inversion part of the system consists of optimization of the penalty function. This leads to an iterative procedure in wh...

Mapping submarine sand waves with multiband imaging radar - 2. Experimental results and model comparison

On August 16, 1989, and on July 12, 1991, experiments were performed to study the mapping of submarine sand waves with the airborne imaging radar, a polarimetric (and, in 1991, interferometric) airborne P, L, and C band synthetic aperture radar system. The experiments took place in an area 30 km off the coast of the Netherlands, where the bottom topography is dominated by sand waves with a height between 2 and 6 m and a crest-to-crest distance of about 400 m at an average depth of 22 m. Ground measurements were recorded on a nearby platform and on a ship in the test area, which also acted as a position fix. On August 16, 1989, the wind was 5 m/s directed toward the northeast, while the surface current velocity was around 0.5 m/s directed toward the southwest. One overflight was made, with the flight direction parallel to the sand wave crests and the radar looking upwind. At P band, the sand waves are clearly visible as dark bands, as predicted by theory, while at L band the sand waves show up as sawtooth-shaped modulations. On July 12, 1991, wind and surface current had the same (opposite)-directions as in 1989, though the wind was much higher (10 m/s). Three flights were made, with the radar pointing upwind, cross wind, and downwind. The upwind and downwind images are very similar. Despite the high wind speed, the sand waves are clearly visible as sawtooth-shaped modulations at P band and vaguely visible at L band. At C band, only wind streaks can be seen. All cross wind images show the sand waves as dark bands, now with the highest modulations at C band. The wind streaks that dominated the upwind and downwind images at C band are much less pronounced in the cross wind images. The images are compared with predictions from a new model of the imaging mechanism which includes contribution to the radar cross section of waves moving both from and to the radar. Wave blocking or wave reflection is treated in an approximate manner. For the radar looking upwind or downwind, the predicted modulations at P and L band agree well with the observations, while those at C band are too high. For the radar looking cross wind, the model severely underestimates the modulations. It is questioned whether a local relaxation source term can describe such a situation. The interferogram shows some structure caused by bottom-induced surface current variations.

Mapping submarine sand waves with multiband imaging radar: 1. Model development and sensitivity analysis

In 1989 and 1991, unexpected modulations caused by submarine sand waves were observed with the airborne imaging radar (AIR), a P, L, and C band synthetic aperture radar. The measurements were performed in a sand wave area off the Dutch coast. The sand waves have an asymmetric, sawtooth-shaped profile with the steep slope oriented toward the northeast. During the experiments, wind and current had opposite directions: the wind was directed toward the northeast and the current toward the southwest. Under these conditions, existing models for the imaging mechanism predict that the modulation depth decreases with increasing radar frequency and that the steep slopes show up as dark lines in radar images. The AIR images show a relation between modulation depth and radar frequency as predicted. However, only the P band image of 1989 shows the steep slopes of the sand waves as dark lines. The P band image of 1991 and the L band images of 1989 and 1991 show a sawtooth-shaped modulation which cannot be explained by existing theory. A new model is developed that is one dimensional in position space and fully two dimensional in wavenumber space. The radar cross section is calculated using both first-order Bragg and an integral equation scattering model in which the cross section originates from a spectral band rather than a single value. The new model contains two improvements over previous ones. First, wave blocking or, better, wave reflection is included, though in an approximate manner. It is shown to be of importance at P band only. Second, the contribution to the radar cross section from waves moving both to and from the radar is included, whereas some previous models consider only one of these contributions. This explains the observed modulations, at least qualitatively, under the assumptions that the angular distribution of short waves is almost constant and that waves moving against the wind have small relaxation rates.

A simple analytical model for brightness modulations caused by submarine sand waves in radar imagery

Presented here is a simple analytical model based on established physics of the magnitude of the hydrodynamic modulations caused by sand waves. The model describes the modulations of the radar backscatter when first-order Bragg scattering is assumed. The major difference between this model and existing analytical models is that the specific shape of sand waves is incorporated. An assessment is made of the key parameters (shape, oceanographic, meteorological and radar) that influence the radar backscatter modulation. It is shown that depth, steepness of the slope, and height of the sand wave are the shape parameters that determine the radar backscatter modulation. The maximum backscatter modulation that can be found for sand waves in nature is approximately 3 dB. It is shown that sand waves in the North Sea near the Dutch coast have a linear relation between their heights and slopes. Implementation of this relation simplifies the model further. Furthermore, backscatter modulations calculated with two radar backscatter models are compared and discussed. The correlation between predictions and measurements with the airborne imaging radar of NASAs Jet Propulsion Laboratory is considered encouraging. Measurements from the images indicate a relation between sand wave height and brightness modulation similar to that predicted by the model.

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 model comparison study to the imaging of submarine reefs with synthetic aperture radar

Signatures of submarine reefs near Heligoland in the North Sea were observed in airborne radar images recorded at L-, C- and X-bands on 14 November 1990 during rather high wind speed of 9 ms-1. Predictions from various models of the imaging mechanism were compared to these observations. One of the models is the so-called weak hydrodynamic interaction theory (WHIT) model. It is fully two-dimensional in position as well as wavenumber space, so any surface current variation can be handled. Also more sophisticated scattering models than first-order Bragg scattering can be included. The model contains a number of parameterizations for the roughness length, the equilibrium wave height spectrum and the relaxation rate as well as different forms for the local relaxation source term. In the model intercomparison, the WHIT model performed not very well. It is shown here that this is due to the choice of the radial relaxation rate. In a sensitivity analysis it is shown that also the form of the relaxation source term is important. A linear source term may lead to unrealistically high positive hydrodynamic modulations (up to 50 dB) at some positions over the reefs for waves with a wavelength of about 0.6 m. Such effects do not occur in quadratic or cubic source terms, which are therefore to be preferred. The parameterizations chosen for the roughness length and the angular relaxation rate have little influence on the model results. Also shoaling may be neglected. A scattering model based on first iteration of the Stratton-Chu equation gives results similar to that of an improved two-scale model. When compared to the observations, good agreement is obtained at L-band, but at C- and X-bands the model underestimates the modulations. A number of possible causes is discussed, but additional data are needed to settle this question.

Evaluation Of A Two-scale Model Using Extensive Radar Backscatter And Wave Measurements In A Large Wind-wave Flume

One of the aims of the VIERS-1 project is to develop improved models for radar backscatter from water surfaces, based on a description of the underlying physical phenomena instead of an emperical parameterization. To this end, extensive wind, wave and radar backscatter measurements have been compared with model results.

Intercomparison and validation of bathymetry radar imaging models

Multi-frequency airborne SAR data over a submerged reef, with very large associated surface current variations, are compared with model calculations, using a wide variety in models and parametrizations. It is concluded that all models still underestimate the measured contrasts, that detailed differences in models cannot be validated due to speckle and non-bathymetric features, and that L-band is more suited for bathymetry applications than C- or X-band.

Comparisons of Backs Cantering Calculations with Measurements Made in a Large Wind/wave Flume

In preparation of tlie launch of tlie ERS-1 satellite, a group of Dutch and German laboratories started a study to improve the wind extraction algorithms for the scatterometer. T h e aim of this s tudy is t o create a model for wind retrieval based on physics. In this paper results of microwave backscattering calculations with three different models will be compared with measurenients in a large wind/wave flume. T h e three models are a two-scale model and two models based on approximations of the integral equation. T h e input for the bnckscattering models are measured wave parameters so the comparison permits a direct evaluatiQn of the accuracy of the model within the experimental accuracies.

The bathymetry assessment system

In the presence of current and wind, submerged topographic features of the sea bed produce contrasts in radar images. These contrasts can be quantitatively understood and modeled, based on hydrodynamics and electromagnetic scattering theory. A suite of models has been developed based on the generally accepted imaging mechanism, which consists of three steps: (1) surface current modulation by the bathymetry, (2) modulation of the (small) wave spectrum by wave-current interaction, and (3) radar backscattering by the sea surface. In order to invert this depth-radar backscatter relation, a data assimilation scheme has been developed. These numerical models have implemented, leading to the Bathymetry Assessment System (BAS). An example of an application in the Dutch coastal waters is presented.