Space Radar Monitoring Identifies Earth Surface Subsidence Caused by Oil and Gas Field Development

December 5, 2012

chain of radar photos from the ENVISAT satellite (taken in the summers of 2003 to 2006) to monitor the subsidence caused by the Urengoi oil and gas condensate field. The researchers are also considering analyzing archived radar photos of the Urengoi and Yamburg fields taken in the 1990s by the tandem satellites ERS-1 and ERS-2.

Relief of the area was mapped by the authors from the interferometric couple of ERS photos taken June 24-25, 1996. Archived interferometric chains of space radar photos taken at the Zapolyarnoye field are being searched for as well.
Radar surveys are conducted in the ultra-short-wave (microwave) radio-wave region, divided into X-, C-, L- and P-bands (Table 1).

Surveying in each of these bands has advantages and disadvantages. X- and С-bands are suited to differential interferometry, as wavelengths in these bands enable the tracing of displacements of several millimeters. On the other hand, some data indicate that radar survey in the L-band helps to solve the problem of time decorrelation. By some expert estimates, radar survey in the Р-band enables “penetration” of the explored surface to a depth of several dozen meters.

In general, interferometric processing includes several basic steps:
1) Superposition of the main and auxiliary radar images of the interferometric couple (in automatic mode or with manual entry of reference points);
2) Generation of an interferogram, the result of item-by-item multiplication of the main image and an image complex-conjugated to the auxiliary one;
3) Obtaining a coherence file for the region of overlap of the two photos, making an interferometric couple, between 0 and 1 for every couple of corresponding pixels;
4) Interferogram filtration to reduce phase noise (interference) by the upscale of the output digital relief model (DRM) or displacement file;
5) Phase sweep (transferring from relative to absolute values);
6) Transformation of absolute phase values:
6-А) into relative or absolute altitude values in meters with the final generation of DRM;
or
6-B) into values of deformation of the photographed surface which occurred between the acquisition of the two images of the interferometric couple, or the three or four images in an interferometric chain.

Satellites Help Build a Digital Relief Model

In 2007, based on interferometric processing of the radar photo couple from satellites ERS-1 and ERS-2, VNIIGAZ generated a DRM for part of the Urengoi and Yamburg fields.

The relief mapped by interferometric data can serve as a reference for subsequent differential interferometric observations of the earth surface displacement in the region.

After the co-registration (superposition) of the photos of the interferometric couple, an interferogram was constructed, from which the reference ellipsoid phase was subtracted (Fig. 5).

The high quality of the interferogram is typical for the case of insignificant time base (1 day). The relief, in particular, is already noticeable.
A coherence map is shown in Fig. 6. Coherence characterizes the correlation between the main and auxiliary images, from 0 (dark areas) to 1 (light-colored areas).

Areas with low coherence can be subject to spatial or time decorrelation, and estimates of