Interferometric Synthetic Aperture Radar InSAR – method for study and monitoring subsidence over mining areas Katarzyna Mirek AGH Unversity of Science and Technology, Department of Geoinformatics and Applied Computer Science [email protected] Abstract Interferometric synthetic aperture radar (InSAR) is powerful tool for mapping the Earth’s land, ice and even the sea surface topography. It is based on processing of the pair of images to map out the differences in the reflected signals over the area (typically 100km x 100km). This paper presents the ability of satellite interferometry technique for monitoring of manmade surface deformation on example of two areas: Upper Silesian Coal Basin (Poland) and Wieliczka (Poland). Keywords: SAR interferometry, change detection, surface deformation Introduction Spaceborne imaging radars began to play an important role in remote sensing in the late 1970s. Early missions demonstrated that synthetic aperture radar (SAR) is able to reliably map the Earth’s surface and acquire information about its physical properties, such as topography, morphology or roughness. SAR is a microwave imaging system. Operating at microwave frequencies SAR systems provide unique images representing the electrical and geometrical properties of a surface in nearly all weather conditions. It is an active system and SAR' s can image at daylight or at night. Currently, operational satellite SAR systems work in one of the following microwave bands: • C band – 5.3 GHz, 5cm (ERS, Envisat, Radarsat) • L band – 1.2 GHz, 25 cm (J-ERS, ALOS) • X band – 10 GHz, 3cm (X-SAR) The use of spaceborne SARs as interferometers (interferometric synthetic aperture radar, InSAR) became popular in 1980s (e.g. [1-4]). After the launch of ESA satellite ERS-1 in 1991 InSAR was investigated intensively and with success by many researches.

Interferometric synthetic aperture radar is powerful tool for mapping the Earth’s land, ice and even the sea surface topography. By bouncing signals from a radar satellite off the ground in successive orbits and looking at the differences between the images, interferometric synthetic aperture radar can detect small differences in the distance between its position and the ground as the land surface moves – whether up or down. InSAR shows spatial patterns of deformation and in combination with ground-based monitoring gives unprecedented insight into a wide range of earth science processes (e.g. [5-12]) A digital SAR image can be seen as a mosaic of pixels. Each pixel gives a complex number that carries amplitude and phase information about the microwave field backscattered by all the scatterers (such as rocks, buildings, vegetation) within the corresponding resolution cell projected on the ground. The amplitude depends on the roughness and typically, exposed rocks and urban areas show strong amplitudes, whereas smooth flat surfaces (like quiet water basins) show low amplitudes. The phase is directly linked to the distance between the observed terrain and the satellite sensor. By calculating the differences in phases (interferogram) between two sets of data, one can determine ground displacements that have occurred in the time between the data acquisitions. The Permanent Scatterer Interferometry technique (PSInSAR) is an upgrade of InSAR. This technique was developed to resolve problem of geometrical and temporal decorrelation [1314]. Furthermore, by using a large amount of data, atmospheric signal is estimated and corrected for. PSInSAR technique uses coherent radar targets (called Permanent Scatterers or PS) that can be clearly distinguished in all images and do not vary in their properties. The PS points usually correspond to such artificial objects like building, transportation infrastructure or industrial facilities, but may also correspond to some natural elements of the terrain, e.g. geological outcrops. One of the unquestionable advantages of the PSInSAR technique is its capability to measure displacements to the accuracy of a few millimeters. On the other hand such high accuracy is also a limitation of the method since fast deformations of the monitored terrain (rate of change above 5-6 cm per year) will remain undetected. Another limitation of the method is that it requires at least 3 PS points per square kilometre, which means that the method practically works only in highly urbanized areas. InSAR was applied for monitoring of mining subsidence in the Selby Coalfield (United Kingdom) for the first time [15]. In Poland InSAR for mining monitoring was applied with success in Upper Silesian Coal Basin [16-17] and Legnica-Głogów Copper Mining District [18-19]. For the first time, PSInSAR technique was used in Upper Silesian Coal Basin [20-22] and next it was developed in another regions [23-27].

This paper presents the ability of satellite interferometry technique for monitoring of manmade surface deformation on example of two areas: Upper Silesian Coal Basin and Wieliczka. The characteristic of study areas Upper Silesian Coal Basin is one of the world’s biggest mining centres. The negative aspect of such a magnitude of exploitation is visible on the surface in the form of surface deformation and subsidence. Since 1970 almost 40% of coal mining activity is located under cities and important infrastructures. A unique salt mine is located in Wieliczka, over 700 years old, which is one of the best known tourist attractions in Poland. Since the Middle Ages over 7,5 million m3 of underground passages have been excavated, extending from level I (64 m below the ground surface) to level IX (at 327 m depth). The salt mine is sited directly under Wieliczka and has influenced the ground and building stability in the town. Interferogram analysis Delft Object-oriented Interferometric Software (DORIS) was applied for processing of SAR images [28]. The DORIS is a software package which is freely available to the scientific community and has been developed at Delft University of Technology, Netherlands. Typical InSAR processing chain consists of: -

data input – read SLC data and precise orbits

-

pre-processing – oversampling and calibration

-

coregistration and resampling

-

data filtering

-

products – computation of complex interferogram and coherence image

-

reference phase – computation and subtraction of the interferometric phase correction of a reference body

-

phase unwrapping – reconstruction of the original phase from “wrapped” phase representation

-

geocoding – conversion of the unwrapped phase to height

Example 1. Upper Silesia Coal Basin The interferogram was processed from two SAR images acquired by ENVISAT satellite. SAR images were made on March and April 2008 (tab. 1), and covered south-west part of USCB.

In the first stage of processing, the whole SAR scene was processed (100km x 100km). Obtained interferogram made it possible to locate where exactly subsidence occurred. Finally the area of Knurów and Bieru were selected for detailed processing and analysis. Analysis of coherence demonstrated high value. Coherence is a measure for local interferogram quality and it provides valuable information about the scatterer. There are two main factors which determine image coherence, firstly the perpendicular baseline (Bperp) separation of the image acquisition (a critical baseline is about 1100 m) and secondly changes in ground scattering characteristic between image acquisition (changes in vegetation, freezing, thawing or human activities such as ploughing). Two images were carefully selected to avoid coherence decorrelation: the perpendicular baseline separation did not exceeds 500 m and there were no rainfall during acqusition. Figure 1 shows the 35 day interval differential interferograms. The interferometric phase images of two selected areas show a number of distinctive concentric fringe features. In the area of Knurów two subsidence troughs were located. Both of them consist of two interferometric fringes, correspond with 5 cm subsidence. In the area of Bieru , 5 subsidence trough were observed. Subsidence patterns show from 1 to 5 cycles of deformation, corresponding with 2,5 cm to 12,5 cm subsidence. Table 1. The characteristic of data used for interferometric processing Temporal separation Upper Silesian Coal Basin 31531 2008.03.11 35 days 32032 2008.04.15 Orbit

Master Slave

Date

Master

43391

Wieliczka 2003.08.08

Slave

53912

2005.08.12

2 years

Bperp [m]

366

258

54

52

50

16

18

20

22

Figure 1. Differential interferograms of the area of Upper Silesian Coal Basin 11.03.200815.04.2008 (35 days). One full colour cycle corresponds to a change of range of half a wavelength, i.e. 2.5 cm. Example 2. Wieliczka Two SAR scenes acquired by ERS-2 satellite were processed to obtain the interferogram (tab. 1). Those pair was selected according to the following criteria: interferometric baseline – Bperp - (i.e. the distance between satellite position during repeated observation) did not exceeds 500 m and dry weather condition (i.e. no rainfall). After preliminary full scene SAR processing, the region of interest was limited to approximately 20 km x 20 km.

Despite of long temporal separation (2 years) coherence demonstrated relatively high value. The influence of temporal decorrelation is clearly visible in the interferogram (fig. 2), diminishing the coverage of reasonable coherent data. Nevertheless, although the data consist of isolated patches, it is still possible to follow the main fringes visually. Figure 2 shows one fringe pattern in the area of Wieliczka. Each fringe in a subsidence feature represents a movement of about 2,5 cm. The rate of subsidence in study area is about 1,25 cm / year. The similar results were obtained by Wasowski [27]. Authors used PSInSAR technique to determine magnitude of subsidence in area of the town of Wieliczka. Data set cover a period of time of 1992 to 2000 and analysis shows subsidence rate of about 1 cm to 2 cm per year. Ground topographic measurements documented the maximum subsidence over 2,96 m (i.e. over 3 cm per year) in the period 1926-2006 [29]. The lower rate of displacements revealed for the period 1992-2000 and 2003-2005 by the SAR results may be in part related to the decreasing mining activity, which terminated around the end of the last century.

54

52

Wi eliczka 50

16

18

20

22

Figure 2. Differential interferogram of the area of Wieliczka 8.08.2003-15.08.2005 (2 years). It is visible one fringe pattern (indicated by elliptical frame). Conclusions Presented examples demonstrates the ability of InSAR technique for monitoring of man-made surface movements. SAR interferogram can be considerable source of information of subsidence dynamics. The number of elliptical fringes represents a multiple value of 2,5 cm of real magnitude of the terrain subsidence.

The paper presented interferograms for different time separation: 35 days and 2 years. Subsidence patterns were clearly visible in the 35 day repeat data but worse results were obtained from 2 years interferogram. There is clearly visible strong influence of mining activity on forming subsidence troughs in USCB. Subsidence patterns show deformation rate from 2,5 cm to 12,5 cm per 35 days. In area of Wieliczka subsidence rate is much slower than in USCB and it has value of about 1,25 cm per year. Problems in the interferogram analysis mainly occur due to temporal decorrelation. The temporal decorrelation leads to isolated patches of coherent phase information, surrounded by decorrelated areas. However in urbanized areas it is still possible to follow the main fringes visually. Acknowledgments The research was supported by the AGH University of Science and Technology in Krakow, project no. 11.11.140.561 References [1]

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