July 18, 2011

1993: first interferometric map of an earthquake

1993 FIRST INTERFEROMETRIC MAP OF AN EARTHQUAKE Update: 07/18/2011
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The British newspaper The Independent's headline of the July 7th, 1993 issue was about results and images obtained at CNES, in partnership with the Observatory of Midi-Pyrénées (OMP).

It presented the displacement map resulting from the Landers' earthquake (California, magnitude of 7 on the Richter scale) one year earlier in June 28th, 1992. The result was just published by the weekly scientific magazine Nature, headline again.

The technique which was used consisted in the comparison of two radar images of the same area before and after an event, using a part of the signal which is usually ignored when processing standard images: the radar echo phase.
This phase is specific to radar satellites and does not have any equivalent on optical ones.

This technique only acquires a geometric signification under certain conditions: the two images must have been taken from near points of view and that is usually the case for Earth observation satellites which follow an orbital "cycle".

The comparison technique was proposed in the 1970's to calculate the Earth's reliefs with a metric precision and was then called "radar interferometry" (INSAR) while "interferogram" referred to the comparison image.

In 1985, a CNES' technical note described the method to obtain a map of the resulting deformations between two images with a precision up to the millimetre establishing a fine pattern of an interferogram and using a numerical pattern of the observed area. This method was then named "Differential Interferometry" (D-INSAR).

The data available coming from the ERS-1 satellite of ESA spectacularly showed on the Landers' earthquake, 8 years after the technical note, the method power by comparing separated data over several months.

CNES continued an activity of capacity demonstration of this technique over the following years, in particular with volcanoes, landslides, subsidences resulting from pumping, continental drifts, etc. using all the radar satellites available. The technique's limits were also illustrated, in particular the atmospheric artefacts' effect.

This activity was accompanied by a formation effort to train in the interferometry licensed program created by CNES, named DIAPASON, which is still being industrialized.

The radar displacement mapping technique is now usually used by geophysicists and almost every earthquake on Earth, for the last 15 years, has been analysed.

This processing method, complemented by GPS positioning measurements, stirred up the measurement of the constant displacement resulting from earthquakes which were, until now, limited to transitory components measured by seismometer. The measurement possibility of the displacements every six days with the Sentinel 1 satellites in 2013 opens up new prospects in both monitoring and comprehension of the seismic cycle and geological hazards in general.

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