LABORATORY EXPERIMENTS FOR THE INTERPRETATION OF PHASE SHIFT IN SAR INTERFEROGRAMS
Interferometric data (amplitude, coherence, phase difference) may be used for general or thematic cartographic purposes. They provide information on surface state and surface changes that occur betwen different dates of view. Coherence and phase difference images bring on these points original information that usually used amplitude images do not necessary contain (change of superficial geometry at the wawelenght scale, phase effects of dielectric origin, measure of weak amplitude subsidence of mining exploitation, etc ..). In particular, rotations of phase observed in the interferograms take their origin all both in the displacement of the surface and in changes of dielectric properties that occur between different dates of view.
In this study, wetempt to evaluate the various contributions by confronting measures of phase rotation undertaken in the laboratory to interpretations of interferograms obtained on the globally stable site of Naizin in Brittany.
II. Test Site
The test site is the Naizin area in South Brittany , France ( size 50km * 50km, Lat 48°N Long 3°W) . Present day, the Naizin area is aseismic, without active deformation, in a standard agricultural area.
Fig 1 give also the rain periods over the Naizin basin situated in centre of the study area. The distribution of the rain events is not exactly know for the entire test site and it is expected that the Naizin basin is representative of the entire test site.This hypothesis seems reasonable for this kind of climate, devoided of local microclimate.
III. INSAR products
The ERS 1 images used were acquired during phase B (3 day repeat orbit) during February and March 1992 from ascending orbits. Out of 12 available images, 6 are in interferometric conditions. The corresponding acquisition dates are the following: 6, 9, 12, 15 February, 13 and 16 March, in a winter period where there is no variation in vegetal cover.
For each pair, we have two amplitude images , a coherence image and a phase difference image. All these products were corrected for the effects of distance and of known relief (ref 1).
Sign convention for phase differences:
The difference provided is algebraic, coded in 8 bits. The interval (0..255) corresponds to a complete 360° phase rotation and the result of the processing in CNES is given by:
where 1 indicate the master image and 2 the slave one.
The optical path is a one way antenna to target journey, l0 the wavelength in vacuum, j the phase effect introduced by ground reflection.
The purpose of the geometric corrections is to remove all the predictable effects on the phase (i.e. topography and orbits) from the interferogram.
The choice of (j1 -j2) follows from the use of the e jwt convention in the formulea for a progressive wave:
where n is the optical index (complex value) and u unitar vector in propagation direction.
It results from (3.1) and (3.2) that, with this convention, a negative j is equivalent to an increasing of the optical path. An area raised by a few mm between the two image acquisitions, for example, would look darker than its surroundings on the phase image (for black and white phase scale).
IV. interpretations of the phase images
In summary, the phase image would be completely uniform if the following conditions were fulfilled:
- exact geometric corrections,
-the atmospheres at the two image acquisition dates have no propagation inhomogeneities (they need not be identical since the differences are to within a constant),
- surface conditions unchanged or subject to a uniform movement or dielectric change.
In practice, the phase difference images show effects with different origins, related either to the preprocessing, or to changes in the conditions of the surface and the atmosphere.
Artefacts related to the precision of the DEM used
The correction performed to bring the phase differences to altitude zero takes into account the altitude provided by the DEM used. The imprecisions of the DEM have an impact on the values of the unwrapped phase according to the following algebraic relationship:
where Ha represents ambiguity in altitude .
This results in certain systematic artefacts, in particular for the low points, since the resampling of the DEM used at 40 m smoothes the thalwegs and makes them seem higher than they really are. An example of the preceding phenomenon is shown in Fig 2 for phase image PHA-10, corresponding to an altitude of ambiguity of + 21 meters. The lowest points in the network stand out in several places by a negative contrast that corresponds to an error in the DEM that is positive by several meters for the thalwegs.
Modifications of atmospheric states, dielectric changes in surface states, global displacements
The corresponding effects do not depend on the altitude ambiguity.
Atmospheric effects are linked with the variability of temperature, pressure and humidity in the air, and dielectric effects are linked with humidity, temperature of the soil and it vegetal cover.
A vertical surface displacement +d will induce a negative change in phase that is inversely proportional to the wavelength used, equal to
- 360 * 2 * d * cos(a) / l where a is the beam incidence angle.
On the interferograms considered here several remarkable effects can be noted:
1-A general variability of 30-40° amplitude, which is neither systematic nor structured either by the relief or by man-made structures (fields, networks, urban areas), typically extending around 10 km, seems to be due to atmospheric effects (for example Fig 2). This type of effect is found on all the images with variable amplitudes.
2-Structured phase changes highlighting certain fields, in particular when the humidity conditions have changed between two acquisitions. (Fig 3).
When the contrast in humidity conditions is smaller (dry period or uniformly wet period), the phase change associated with the fields disappears (see for example Fig 2).
There may be various reasons for such positive or negative differentiation between the fields and their environment:
- a uniform elevation of the backscattering surface that leads to a shortening of the optical path and therefore a negative phase change (darker area).
- an increase in the mean value of the dielectric constant of the vegetation due to humidification, leading to a variation in the optical path and to a positive phase change if the dominant contribution in backscattering is given by the reflexion on the ground. Note that it is unlikely that cutting vegetation would increase the path, because this would make the speckle and the phases incoherent, since the basic reflectors would have changed.
Also note that fields probably corresponding to very rough bare soils, wich are characterized by their high radiometry (Fig 3), do not have a specific signature in the interferograms. The contrasts in phase with their environment are in general lower than 10°.
Note that for all the fields mentioned above, the phase noise is low and the coherence level high.
3- Phase changes on a scale of several kilometers correlated with the general shape of the observed relief when the climatic conditions change from a dry period to a wet one (see fig1 for rainy events).
Comparing the digital terrain model with the unwrapped phases PHA-1 and PHA-2 (6 (dry) and 9 (wet) February on the one hand and 6 and 12 February on the other) (Figs 4, 5) shows that the low areas react in a differentiated way, with phase difference DIFF-PHA, (see equation 3.1) of the order of ( - 80°), with respect to their surroundings (equivalent to moving the backscattering surface closer). This effect is equivalent to a reduction in the optical path, and the possible origins of the phenomen are: dielectric effect related to humidification (phase rotation equivalent to a reduction in the optical path between the second and the first date, with j2 -j1 = +80° (eq 3.1), ground swelling, localized atmospheric effect . Experimental study on the Naizin site (ref 2) shows that the level of humidity depends on the relative elevation with respect to the river and that the contrasts between low and high areas is larger for a wet period than for a dry period. It thus seems plausible that for the second date used for interferograms PHA1 and PHA2, the low points in the soil have a higher humidity level , and in consequence the air also.
V. laboratory experiments
First, let consider what phase rotation the Fresnel formula gives at normal incidence as result of the humidification of a flat homogeneous soil.
5.1. Phase rotation in Fresnel reflection coefficient
The Fresnel reflection coefficient for an incident wave normal to the interface plane of two homogeneous media is given by:
where nr is the relative index of the 2nd medium with respect to the first and j is the phase rotation associated with the reflection of the wave. This formula can be used for a greatly simplified initial quantitative evaluation of the effect of dielectric changes. We shaw in (ref 3) that the variation is always bounded to 90° and for typical values of dielectric constant is lower that 30°.
The evolution of the real and imaginary values of the dielectric constant as a function of the humidity leads, for example, to the following results:
Starting from a dry insulated medium (tropical soil) with dielectric constant e = 4.8 (real index m=2.2), subject to great humidification, we obtain:
-for dry ground, a phase rotation j = -180° for all frequencies
- after humidification,
in X-band; e = 15 - j * 9, nr = 4.03 - j * 1.11 (sign - for the imaginary part in accordance with formula 3-2), j= -187.8°,
in L-band; e = 24 - j * 4, nr = 4.91 - j * 0.4 , j= -182°,
i.e. phase changes of respectively -7.8° and -2°; the - sign corresponds to a phase rotation equivalent to an increase in the optical path travelled by the wave, in accordance with the formulae 3-2 , when this phase rotation is interpreted in terms of time delay.
This result shows among other things a lower sensitivity to the effects of humidity when the frequency decreases but in general that a pure dielectric effect at reflection level is always very small and this is in agreement with the observations on the interferograms where for "bare soils" there is not very much differentiation. However, the speckle effect can produce larger variations as pointed out in the above mentioned report.
5.2. Experimental measurements
Several experiments have been carried out in the laboratory (at ESIEE and JRC Ispra) in order to approach the preceding interferograms quantitatively.
5.2.1. Experiments at ESIEE
At ESIEE, experiments were conducted in order to study moisture effects on the backscattering and phase shift of microwaves by different kind of soil. An HP 8510A spectrum analyser and two X-band antennas were used to perform the measurements. experiments were limitted to X-band waves because of the physical constraints due to the sample size (ref 4). These experiments provided useful orders of magnitude and helped to formulate, in a better way, the questions related to the phenomena under study.
A fIrst experiment consisted in the measurement of the phase rotation of two samples, sand and humus, at different moisture states. The results show that for sand sample (fig 6) the phase rotation is equivalent to an increase of the distance between the sand and the antennas, the real displacement (sinking) contributes for one third to the equivalent displacement and the dielectric effect for two third. For humus sample (fig 7), an important surface swelling can be observed but the pure dielectric effect is, as for sand sample, equivalent to the surface moving away (increase of the distance sample-antenna). These measurement have been conducted at normal incidence.
In a second experiment, the samples wer covered by vegetation to study the hase shift undergone by the wave upon its transmission through vegetation. The vegetation, dry or wet, consiste of numerous thin stems, forming a dense and homogenous (at the wawelenght scale) medium. The results show that the humidification of the leaves induce an attenuation of the amplitude of the backscattered wave and a phase rotation of -20°. An increase of the density of the vegetation (number of shoots) is also followed by a negative phase rotation. (sign - equivalent to an increasing of the optical path).
Humidification, like the increase in density, leads therefore on the one hand to attenuation and on the other to a phase rotation equivalent to an increase in the optical path. The magnitude of this results are to be compared with the phase changes observed over fields after rainy periods and allows them to be interpreted when the amplitude decreases and when the phase change corresponds to an increase in the path (positive contrast on the interfeograms).
5.2.2. Experiment at the EMSL
Measurements (monostatic, polarimetric) have been performed on two samples (1.0 x 0.7 m2) of sand in the frequency range 2 -12 GHz and for three incidence angles (18°, 23°, 28°). Progressive humidification of the samples has been performed by gently sprinkling water on the surface, from dry to saturated moist conditions. The experimental procedure and the data analysis is described in (ref 5).
The experimental results can be summarised as follows :
The experimental approach employed allows us to appreciate quantitatively some of the causes of the variability in differential interferograms. The main points are recalled below:
- For soil, the purely dielectric effects are in general superposed on the geometric effects of sinking or swelling that depend on the nature of the soil. The humidification of sand induces sinking by a fraction of a millimeter while that of topsoil can lead to swelling of several millimeters. In addition, also the effect of surface geometry , needs to be considered.
- In most of the experiments carried out, the purely dielectric effect associated with humidification is equivalent to the backscattering surface moving away. In the only case where the opposite observation was made, the phase rotation obtained was limited to 15 °. We would mention that neither the -35_ neither the +15_ can be explained by pure dielectric surface effects. May be a volumetric interaction takes place even if I can not image any volumetric mechanism when the whole soil layer is saturated of water.
- In transmission, the effects associated with the humidification of plant cover or an increase in biomass have the same type of signature - weakening of the signal and increase in the optical path.
If we compare these results with the observations made on the analysed interferograms from the Naizin site, we can make the following comments and propose the following conclusions:
- some important phase changes observed over fields can be interpreted in terms of changes in the optical path due to a modification of the density characteristics of the vegetation traversed. The effects of humidification on the fields are only visible through the associated phase changes when the humidity contrast is large and the delay between image acquisition very small.(3 or 6 days in our exemples).Thise effects are not visible for the bare soil fields.
- the phase rotation of the order of 50° to 80° (equivalent to moving closer to the surface) observed on the lowest areas when going from a dry period to a wet period is larger than the purely dielectric effects that we found elsewhere and are, furthermore, (for our measurements) generally equivalent to the backscattering surface moving away. The hypothesis of a swelling of the most humid low areas should not therefore be discarded. In our case, this swelling, corresponding to 80° in C-band, would be of the order of 7 mm (result obtained by neglecting the purely dielectric effect). For a conventional use of interferograms in geodesy, DTM production for example, the previous effet will be responsible for an error of a quarter of altitude ambiguity.
It is clear that at this stage, additional experiments would be welcome to complete this initial approach to measuring the phase changes associated with changes in surface state, in order to expand the use of radar interferometry in the field of environmental monitoring.
The humidification experiments carried out with backscattering on different soil types (variable depending on their nature and roughness), in backscattering and transmission over different types of vegetation (variable in structure and biomass) would allow the phenomena involved in the generation of differential interferograms to be better quantified. The EMSL's anechoic chamber at Ispra is ideally suited to this type of measurement.
2. Effect of saturated areas on backscattering coefficient of the ERS 1 synthetic aperture radar: first result, Merot P., Crave A., Gascuel-Odoux C., Louahala S. Water Resources Research , vol 30, (2), p 175-179, 1994.
3. Mesure au laboratoire d'effets de phase lies a des changements dielectriques
d'etats de surface: integration a l'echelle du pixel sol. Rudant J.P. Rapport
92 / CNES / 0421, 1992
4. Caracterisation d'un sol par la rotation de phase d'une onde centimetrique , Calonne R., Arnaud V Memoire d'Ecole d'Ingenieur, ESIEE, March 1995
5. Decorrelation of Backscattered Signal due to soil moisture changes Nesti G., Tarchi D., Rudant J.P IGARSS 95, pp 2026-2028
This work was founded by CNES and PNTS, and the ERS1 data given by ESA for the scientific project AO F 07 (Responsable Jean Chorowicz). Thanks to Patrick Gigord for offering technical assistance.
Fig 1: Rainy periods over the Naizin basin in February-March 1992 .
Figs 2: Phase differences obtained for an altitude of ambiguity of +21m, before and after unwrapping based on a simple model. Before unwrapping, a poor estimation of the altitude of ambiguity provoked fringes parallel to the satellite's direction of motion. The hydrographical network correspond to a negative contrast since, because of the smoothing performed on the DEM, the thalwegs appear higher than they actually are and the phase correction due to the relief is too large. The thalwegs would correspond to a positive contrast for a negative altitude of ambiguity.
Figure 3 : Specific effects due to the presence of fields
Zoom (30km*20km) on phase difference image PHA-2 and amplitude images (6 and 12 February,).
Phase is given before (1) and after (2) unwrapping, the geometric structures corresponding to the fields are very clear. These effects are more tenuous in the absence of a contrast in humidity.
Bare rough soils are visible on the amplitude images, (3) 6 february and (4) 12 february. No important specific phase shitfs are associated to this bare surface.
Figure 4: Representation in grey levels of the topography; lower values at 20 meters (dark) and highest values at 309 meters (white).
Figures 5a and 5b: Phase changes structured by the hydrographical
Fig 5 a : Phase image PHA-1 (6 and 9 February) shows the phase changes
structured by the network in the center and to the east (The darkening
is equivalent to a reduction in the antenna to ground distance). The fact
that such structuring is not found on all the data and is not linked to
the altitude of ambiguity excludes an artefact related to DEM imprecisions.
Fig 5 b : Phase image PHA-2 (6 and 12 February) shows, less
clearly, the same type of effects that are seen in Fig 5 a.
Figure 6: Reflection phase rotation and surface displacement
observed for sand (ESIEE, X-VV band). The phase rotation is equivalent
to the surface moving away. The sinking of the surface contributes one
Figure 7: Reflection phase rotation and surface displacement
observed for humus (ESIEE, X-VV band) The phase change is mainly due to
the swelling of the surface. The purely dielectric effect is equivalent
to the surface moving away.
Figure 8 : Backscattered phase rotations induced by the humidification of a sandy sample. (EMSL data).
a: The phase shift as a function of the humidification at 5 and 9 GHz ( incidence 18_, VV polarisation).
b: Phase change after humidification as a function of frequency for
two different profiles and three incidence angles.
Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry
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