OI² - A CLOSELY MONITORED VOLCANO
III - Interpretation How to read an interferogram ?
The phase differences which appear on an interferogram, resulting from ground displacements, generally appear in the form of more-or-less concentric interference fringes. Each fringe corresponds to a complete phase cycle, in other words a phase variation from 0 to 2π. In terms of displacement, each fringe is related to a variation in the Earth-satellite distance of λ/2 cm, where λ is the radar wavelength (3.1 cm for TerraSAR-X and COSMO-SkyMed, 5.6 cm for ASAR and RADARSAT, and 23.6 cm for PALSAR).
In order to convert the interferograms into images, the phase values are coded into one, or several, octets (the coding into one octet, which is most commonly used, theoretically provides a precision of π/128 or, in terms of displacement, λ/512). A different colour is assigned for each numerical count of the encoded image. The interferograms shown on this site, as well as on the CASOAR database, are represented using the « Wave15 » colour table (also known as the « Stern Special »). According to this colour table, low phase values (close to 0) are shown in red, intermediate phase values (around π) are shown in blue, and high phase values (around 2 π) are shown in yellow.
This interferogram, obtained by combining two ASAR images of the Piton de la Fournaise from the 27 and 31 August 2003, has captured displacements resulting from the injection of a dyke into the Central Cone of the Piton de la Fournaise. The dyke, as it neared the surface, caused a series of parallel eruptive fissures to be opened up which are represented by the long green trace on the interferogram. The style of displacement is clearly asymmetric on this trace - one part shows large displacements (numerous fringes) in a pear-shaped array, over the eastern part of the Central Cone, and the other part shows less extensive, smaller-scale displacement (fewer fringes) on the western part of the Cone.
There are around 11.3 fringes visible between the centre and the periphery of the eastern lobe (between the two white arrows). For each fringe there is a colour gradation from blue, through red to yellow (towards the centre of the lobe). This shows that, for each fringe, the phase decreases towards the centre of the lobe, in other words that the Earth-satellite distance decreases towards the centre of the lobe. Based on these fringes, we calculated that, between 27 July and 31 August, the centre of the lobe moved around 32 cm (11.3 x 2.8 cm) towards the satellite relative to the lobe periphery. In the same way 1.3 fringes can be seen between the periphery and the centre of the western lobe (between the two black arrows). For these fringes the colour gradation is from yellow, through red to blue (moving towards the centre of the lobe). This shows that, for each fringe, the phase increases towards the lobe centre, in other words the Earth-satellite distance increases towards the lobe centre. Thus it can be calculated that, between 27 July and 31 August, the centre of the lobe has moved around 3.6 cm (1.3 x 2.8) away from the satellite relative to the periphery of the lobe.
It is important to note that the estimated displacements are line-of-sight displacements (LOS) relative to the satellite. The East-West and vertical components can also be calculated by combining several interferograms with different acquisition geometries (see the section on « Displacement Calculations Based on Interferometry Data »).