№ 2 (February 2008)
Space Radar Monitoring Identifies Earth Surface Subsidence Caused by Oil and Gas Field Development
Intensive earth surface subsidence takes place when hydrocarbon fields are developed. Subsidence is currently mapped with model calculations, with or without data from reference observations
By Yuri Baranov, Yuri Kantemirov, Evgeny Kiselevskiy
Even when these observations are taken regularly, however, generating a reliable continuous map of subsidence is impossible since reference marks only provide a discrete measurement net with general interpolation between its nodal points. Furthermore, model calculation of earth surface deformation requires significant assumptions (particularly, estimation of the compression modulus of the rock skeleton from the reservoir bed to the earth surface).
Differential Radar Interferometry Shows Displacement
Differential radar interferometry (DRI) directly and efficiently maps earth surface displacement and structural deformation (Fig. 1-4). The main advantage of DRI over other methods of vertical and horizontal deformation monitoring is direct measurement of the changes in relief that occur between surveys.
The picture of displacement obtained with interferometric processing usually gives an integral view of displacement, including both natural and industrial components.
Naturally occurring soil deformation results from the movement of crustal blocks due to the lunar tidal cycle, seasonal thawing of soil in permafrost areas, subsidence in areas of high thermokarst activity, and modern geodynamic processes.
Production induced displacements mostly occur due to a significant (by several times) pressure drop in reservoir beds accompanying hydrocarbon production, depletion of water-bearing horizons, and mineral recovery with a mining method that produces large cavities in the skeleton of the sedimentary mantle. Underground gas storage facilities (UGS) are associated with an approximately semi-annual cycle of alternating displacements of the earth surface. Radar interferometry enables the tracing of deformation of main pipelines caused by industrial impact on the soil.
Space radar monitoring of subsidence from hydrocarbon field development has been practiced abroad since the mid-1990s. No systematic study of applying radar interferometry to the gas industry had been done in Russia until now.
In 2006, VNIIGAZ specialists began to analyze foreign experience with radar interferometry in monitoring earth surface displacements.
Analysis of the results of NASA, ESA, etc., enabled the following conclusions:
1. Two-, three- and four-pass chains of radar photos are used for differential interferometric processing.
2. Minimum displacements recorded in the course of interferometric processing of non-simultaneous radar photos taken in X- and С-bands of the radio-wave electromagnetic spectrum are several millimeters.
3. The rate of the earth surface subsidence in hydrocarbon production areas can exceed 1 mm/day.
4. As production induced vertical displacements are due to the recovery of hydrocarbons from productive horizons, the outline of the subsidence area can correspond to the gas-water contact, the outline of actual reserves, or can show the location of these outlines (as in various standard cases).
5. Best results are achieved by combining results from radar survey interferometric processing and data from ground reference observations, and by setting radio signal corner reflectors.
Gazprom Refines DRI Technology
For practical refinement of differential radar interferometry technology, Gazprom is planning pilot projects at its fields. No comprehensive testing of this technology has been done in Russia. In 2006, VNIIGAZ started planning an interferometry radar survey of a complicated natural environment, specifically northern areas in Western Siberia. Initial results are presented in this article.
The researchers planned to analyze a five-pass 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;
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 the relief and deformation will not be accurate. These areas mask at the step of phase sweep, and common interpolation is performed in them.
Relief and deformations are evaluated only in areas with higher coherence. In this case, the image coherence of the interferometric couple is rather high, about 0.6-0.8 for most pixels of the coherence map.
Then, interferogram filtration with the adaptive algorithm was performed. Filtration was set at a minimum, as in this case the interferogram was almost ideal. Filtration results are presented in Fig. 7.
The filtered interferogram has undergone phase sweep. Direct sweep was done only in areas with sufficiently high coherence (the threshold value is selected interactively and is an expert judgment). In areas of low coherence, values were interpolated. Then, the swept phase was transformed into the geocoded absolute digital relief model shown in Fig. 8.
The length accuracy of the output DRM generated by analyzing the interferometric couple phase component is estimated in the first several meters. Still, these data could theoretically be used to trace subsidence in the Urengoi and Yamburg fields with centimeter or even millimeter accuracy.
An example of the generation of the earth surface deformation field based on the above photo couple is shown in Fig. 9. A pronounced slanting interference is visible, resulting from the inaccuracy of determining orbital parameters of the ERS-1 satellite after an accident on June, 5.
This example proves once again the importance of the exact determination of orbital parameters in interferometric processing.
Modern radar satellite systems (Radarsat-1 and especially ENVISAT) ensure accuracy sufficient to map centimeter and millimeter subsidence without the complicating effect seen in Fig. 9.
Thus, the intensity and amplitude of displacements in the Urengoi field can be determined reliably only after a survey using the aforementioned modern satellite systems with the positioning of radio signal corner reflectors