118 EXPERIENCE OF APPLICATION INSAR TECHNOLOGY IN POLAND MINING INDUSTRY

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1 118 EXPERIENCE OF APPLICATION INSAR TECHNOLOGY IN POLAND MINING INDUSTRY Edward Popiolek, Artur Krawczyk, University of Science and Technology AGH, Krakow, Poland Summary: The article presents the experience of using InSAR method, applied to determine the subsidence of the area, caused by mining exploitation in Poland. The results of the carried out so far studies show that, based on this technology, it is possible to determine temporary subsidence of the area, zones of present threat to constructions and also verification of the surface deformation forecasts. It was proved that the InSAR method allows the assessment of current threat to the mining area and objects by defining the areas of intensive subsidence as well as the estimation of maximal values of deformation indexes by current verification of the parameters in Knothe s theory, based on so-called elementary troughs. No necessity to establish a measuring geodetic grid of points makes an advantage of this method. In paper was described using InSAR for taking measurements in the area of LGOM and Silesian region. Key words: mining, deformation, subsidence, interferometry, InSAR. 1. Introduction Underground mining exploitation of deposits causes negative effects on the surface of the area; seen as translocations and deformations. They cause unfavourable phenomena such as (among others) damage to construction and engineering objects and technical infrastructure. Because of the protection of objects and safety of people it is necessary to provide possibly on-line monitoring of the increase of surface changes and their impact on surface management. It is also crucial to make forecasts of any possible threats. Satellite Radar Interferometry InSAR is a modern, constantly developing, remote sensing method, widely applied in many scientific disciplines and practical applications. In the protection of mining areas it can be used in the description and forecasting of the changes in the surface of the area, caused by the underground exploitation of deposits. By using radar repeatable scanning of large areas of the Earth - this method provides new quality of information, unavailable with other techniques used so far. This is the first special method of the measurement of deformation, where surface measurement, not point measurement is applied. Providing data in a mass way; the method enables us to obtain a comprehensive picture of temporary area subsidence in a relatively short time. Capturing of interferomertic data relies on the process of digital calculation of the phase differences between two satellite radar images (radarograms). As a result of this interferometric processing we receives a new image the interferogram. This new image makes it possible to take an observation of the height change of the terrain surface. One of the first field where this technology was applied, was an observation of the earth surface movement [1], caused by earthquakes as well as the movement of tectonic plates. This application proved that the technology is a very useful method to make an observation of vertical ground movement. By this method we can measure displacements of the terrain surface smaller than one centimeter. A very important feature of the method is that since the SAR data are acquired frequently it is possible to use archived data to measure deformations over the last fifteen years for almost any place on Europe. The first research in Poland was done in 1997 in the Silesian Coal Region [2]. The research confirmed high credibility of interferometry technology, and its particular usefulness in environmental monitoring and in the observation of dynamic changes of land subsidence caused by underground mining exploitation. Since 2000 the studies of this kind have been carried out by the team of the Chair of the Protection of Mining Areas and Geoinformatics of AGH-UST in Krakow. They were carried out in the conditions of the Legnica-Głogów Copper District. The applied nowadays InSAR method has significant possibilities of application in many aspects of monitoring and forecasting of post-mining deformations of the terrain surface. First expertises have already been made (Chair of the Protection of Mining Areas and Geoinformatics) in the largest mining areas of Poland. They are based on the confirmed reliability of radar satellite measurements. Presented in this paper modern remote sensing technique is successfully becoming a subsequent method of deformation measurements carried out in mining areas to protect them and monitor them on-line. In this paper the results of new studies (on the InSAR method) are presented. The studies were focussed on the detection and measurement of dynamic inclinations of temporary subsidence troughs, observed in the areas of copper ores deposits in Poland. These results confirm the reliability and usefulness of the InSAR method in the monitoring of the changes occurring in the areas of mining deformations. 2. The method of measurement and interpretation The interferogram was processed from two SAR (Synthetic Aperture Radar) images acquired by ERS-1 or ERS-2 satellite (European Remote-sensing Satellite) during repeat-pass observations. During data acquisition the SAR satellite antenna generated the electromagnetic pulse that is radiated and propagates to the terrain. A fraction of incident field is reflected back towards antenna where it is gathered by receiving SAR antenna. The received signal, after digital processing is formed into SAR image the radarogramme. The digital SAR image contains information about the power of bacscattered signal, which is called scene intensity and the phase of the received signal. The intensity of the signal is used for land-use maps generation. The phase of the SAR signal is used then for an interferogram generation i.e. phasechange pattern between repeated SAR observation. This interferometric phase is represented on interferogram by the sets of colours called inteferometric fringes. Repeat-Pass Radar Interferometry is a technique to extract information relative to

2 vertical height changes on the Earth s surface. For the SAR sensors of ERS-1 and ERS-2 satellites with a wave length 5.6 cm, the inteferometric fringe marks a change of 2.8 cm in the amount of ground motion between the two data sets, in the direction towards the satellite. This value corresponds to 2.54 cm of vertical surface change. In mining subsidence, the interpretation of the fringes is different from other cases, e.g. earthquake effects where shifts can be expected subside in all directions. In mining subsidence close to vertical shifts can be assumed. For InSAR digital data processing the EarthView InSAR Workstation has been applied. Obtained interferogram and radarograms have been then imported into the Mining Areas Information System of [3] for further data processing and interpretation. The main problem at this stage of the project was a high error in co-registration of InSAR data with topographic and mining data stored in local coordinate system. These different data sets are also characterized by different pixel size: 20x20m for InSAR data and 5x5 m for raster topographic data. Further selection of additional control points and application of higher order of transformation allows to co-register data with 30 m of maximal error. Each interferogram consists subsidence troughs, one of them is presented in fig. 1. The image showing us digitized quite big subsidence growth of the subsidence trough no 23. It was supplemented by the fragment of the map background of completed exploitation. This exploitation was located in the area of the mining area Lubin I and was caused by the exploitation of copper ore deposit m thick, situated on the mean depth of 550 m. Subsidence growth was slightly above 8.6cm/60 days. Legend completed exploitation isoclines of the mm/60 days Fig.1: Isolines of trough subsidence on the background of the completed exploitation Based on the generated models of the growths in the subsidence of the area, a map of changes in the inclination of the area was made. One should keep in mind that manual data processing has certain errors and a defined accuracy. In the framework of the project it has been defined that the error in the determination of inclination is within the margin between 5% and 10% of the determined value, thus does not significantly deform the results of the studies. In the following step - the profiles for all the subsidence troughs were determined. Then for each subsidence trough maximum inclination was calculated. In figure no. 2 isoclines of the changes of inclination, occurring within 2 months in the area of subsidence trough no. 23 are presented. Legend completed isoclines of the changes of inclinations [mm/m Fig.2: Isolines of the changes in the inclinations on the background of the completed exploitation

3 Maximum change of inclination for trough no. 23 was 0.31 mm/m. Comparison of maximal inclination changes in other subsidence troughs showed that the change of inclination in this trough was the biggest. In figure no. 7 the course of the profile line (A-B) was presented. Combined profiles of the growth in the subsidence of the trough and changes of inclination were based on this line. Presented in fig. 2 spatial distribution of the changes in the inclination of trough no. 23 is typical. This example makes visible the occurrence of extreme values of the growth of the inclination in the relation to the direction of exploitation. The biggest growths in inclination occur between the front of exploitation and centre of the subsidence trough. Although in the centres of troughs the inclinations reach values close to zero, making flat saddles. The analysis of the changes in the inclinations of other subsidence troughs showed that in all the other cases similar type of the distribution of changes in inclination was observed. 3. Determination of parameters in Knothe s theory based on the observation of subsidence in the insar method Studies of dynamic inclination of temporary interferometric subsidence trough, in the aspect of the measurement of subsidence were used to determine parameters of Knothe s theory of influences. Their on-line verification, with the development of mining exploitation, allows effective monitoring of the deformation process by the determination of its maximal indexes. To determine the parameters of this theory, the calculation method was based on the assumption that subsidence trough is elementary in its longitudinal vertical section. Selected for the studies sub-elementary subsidence troughs, in selected sections (where cross-dimension of the exploitation field is significantly smaller than the depth of the deposit) approximate Gaussian curve quite well (Fig. 3); fulfilling the established assumption. The accepted calculation technique allows the determination of the value of the parameter of influence dispersion r, defined in the mentioned theory of influences. Making calculations the following formula was used [4]: r = 1,17 x2 lg( w1 : w2 ) (1) where: w 1 maximal value of subsidence in a cross-section of the subsidence trough (in place x 1 = 0); w 2 value of subsidence in the wing of the subsidence trough (in place x 2 distance from x 1 ); Value x 2 and respective subsidence w 2 (relation 2 fig. 3b), were obtained from the spatial picture of the subsidence trough from selected cross sections. Based in calculated r, knowing depth H of the deposit, the value of the parameter of rock mass tgβ can be calculated from the linear function: 1 r = H tgβ = tgβ H r (2) M. E. "Rudna" - temporary subsidence trough (S1) P P IV III II Conditions of mining depth - about: m thickness: 4,0 m to 6,1 m 1989 P2 Value of subsidence [cm] w 2 w 1 x 1 = 0 x 2 Section P1 Gaussian curve Distance [m] Local system of coordinates"pieszkowice" Fig 3. a) courses of interferometric sections of a sub-elementary subsidence trough b) vertical profile of area subsidence along section P1

4 To define parameter tgβ, for underground exploitation of copper ore in the LGOM area, six subsidence troughs, generated by InSAR technology were selected. The distribution of these troughs, numbered subsequently from S1 to S6 with the determined values of the parameter is presented in figure 4. Values of calculated parameters tgβ do not differ from the accepted so far values in the influence forecasts. The differentiation of ultimate values tgβ within the range , in our opinion, shows the changeability of the rock mass in the area of LGOM. To characterize the rock mass in LGOM in a very general way, one can calculate the mean value for the area of LGOM with selected for the analysis elementary subsidence troughs (fig. 4). This value is tgβ r This is close to the values obtained in direct surveying observations (tgβ between 1.4 and 1.7). Fig. 4. Distribution of subsidence troughs 4. Coal exploitation under motorway A4 Case study The A4 motorway partly is located in the Upper Silesian in Poland. Before the start of the motorway construction, agreements were made between the coal mine and the motorway construction company, to define and regulate the rules of exploitation. Between 2001 and 2004, a 16 km long section of the A4 motorway was constructed. This part of the motorway is crossing the province of Upper Silesia, where many areas of underground hard coal mining are located. For such types of mining, huge terrain surface deformations are very common, reaching several centimeters per month and affecting large areas above the mined coal fields. The mining exploitation continued during the motorway construction and special improved constructions for the road have been applied. In the central section of the motorway, some 10 km west from the city of Katowice, a high road embankment to link two local hills was constructed. To improve the resistance of the retaining embankment against in relation to potential mining deformation, it was necessary to apply special conditions for its construction. The coal mining continued even during the construction of the motorway. At that time 8 longwalls were exploited directly under embankment construction site. Seven of them do not give any unexpected deformations but in September 2004 on one part of newly constructed motorway some deformations were discovered. Their location was directly above longwall 4/c, which was mined at the depth of 588 m This longwall was exploited by the Kompania Weglowa S.A. Polska -Wirek Coalmine Division. That exploitation caused surprisingly significant deformations like cracks and step-faults on the road surface and within the road embankment. Due to lack of in-situ deformation measurements the question remained whether the road embankment construction was too weak or if the deformations caused by underground exploitation were significantly stronger than predicted. The answer to this liability question was evidently very important, to determine which party was responsible for the cost of repairing the infrastructure. To help to answer this question, satellite radar interferometry was applied. To obtain information about the extent and magnitude of the deformation caused by the exploitation of longwall 4c, the SAR interferometric method was chosen due to the lack of other measurements. Standard leveling was not available, as the construction works destroyed the benchmarks. For the project purposes 10 Envisat ASAR images from two tracks (415 and 143) were selected. All images were acquired between 2004 and Due to the large temporal baselines and significant signal decorrelation only one out of the 11 processed interferograms appeared coherent enough to perform a quantitative interpretation of subsidence caused by coal exploitation from the longwall 4c. All interferograms were generated with the public-domain Delft ObjectOriented Radar Interferometric Software (DORIS). On two other interferograms, deformation was detectable but the information about the rate of subsubsidence was not interpretable. One of the reasons for the difficulty in interpreting the interferograms was that the rate of subsidence was very high, up to 20 mm/day, leading to high fringe rates. To improve the interpretability of interferograms we oversampled the data in range direction instead of the typical multilooking in azimuth direction. The oversampling method was used only for the most coherent interferogram to avoid misinterpretation of phase noise. The resulting oversampled interferogram shows the detailed information about the rate of subsidence along azimuth. The high resolution interferogram is then geocoded and imported into the Mining Areas Information System for further interpretation.

5 Finally 3 subsidence trough were analyzed for further interpretation. Two of them have 35 days time periods and a third has 140 time period. The resulting information about the spatial and temporal extent of the subsidence enables us to adjust the calculated land deformation model and the interferometrically derived subsidence bowl was used to determine the parameters for the Knothe model, that describes the influences of underground exploitation. Those estimated parameters were then used for subsidence modeling which allows to recalculate land deformation with a higher precision level. For calculations the MODEZ software has been used. MODEZ includes algorithms for calculating horizontal deformation, subsidence, inclinations and other factors of ground changes caused underground exploitation. In our case the calculation of deformation was performed based on evaluated value of tan. Within the motorway area the horizontal deformations reached maximal value of max=3.3 mm/m. It means that they exceed the marginal value of predicted deformation of about 0.3 mm/m. For the method used in this research the evaluated accuracy was estimated to be = ±0.4 mm/m of max. It means that deformations reached maximal permissible values, as mentioned in the agreement between the Coal Mine company and the Construction company. Finally the Mine Company paid only small amount of the costs of the motorway embankment repair. 5. Final Remarks and Conclusions The carried out studies once more confirmed the usefulness of the InSAR method in the monitoring of mining areas. We presented the possibilities of supplementing current analyses of area subsidence with the analysis of the growths of area inclinations connected with the movements of exploitation fronts and the possibilities to determine parameters of the theory of forecasting from the dynamic front of exploitation. The analysis of several observed subsidence troughs defining the growth of area subsidence in the period between radar images showed that they are clearly visible in the form of centric fringe, being isoclines of temporary subsidence and their situation is compatible to the fields of the exploited deposit. Determined based on sub-elementary interferometric troughs, parameters of Knothe s theory of influences enable online verification of the forecasts of surface deformations. Their values do not significantly differ from the values determined in a classical way in LGOM, from asymptotic states of subsidence troughs over large exploitation fields. They are correlated with the results of the analyses of the growths in the inclination of the area, showing milder inclinations of subsidence troughs outwards the exploited part of the deposit. Carried out analyses and speculations, the results of which are presented in this article, indicate great opportunities of satellite methods of radar interferometry InSAR in solving the problems in the monitoring of the dynamics of negative influence of underground mining exploitation of deposits on the surface of the area and objects on the surface. This modern method of satellite remote sensing extends the possibilities of the monitoring of the influence of mining exploitation in the whole mining area. Its application has also important economical aspects, for it allows obtaining valuable spatial information on current mining influence, with a smaller cost. 6. Acknowledgement This publication is financed by KBN, project: 4T12E ASAR SLCI data used in this work are the courtesy of ESA/EURIMAGE Literature [1] Gabriel A. K., Goldstein R. M., Zebker H. A. Mapping small elevation changes over large areas: differential radar interferometry Journal of Geophysical Research: Solid Earth and Planets, Vol. 94, No. B7, pages ; July 10, [2] Perski Z.: Applicability of ERS-1 and ERS-2 InSAR for Land Subsidence Monitoring in the Silesian Coal mining region, Poland. International Archives of Photogrametry and Remote Sensing, Vol 32, No. 7, [3] Piwowarski W., Krawczyk A.: Koncepcja Geoprzestrzennego Systemu Informacji o Terenie Górniczym Mat. V Konferencji - "Dni Miernictwa Górniczego i Ochrony Terenów Górniczych" Szczyrk 1999 [4] Batkiewicz W., Popiołek E. Prognozowanie wpływu eksploatacji górniczej na powierzchni terenu w warunkach LGOM. Prace Komisji Górniczo-Geodezyjnej PAN, Geodezja 14, Kraków 1972 r.