Three years of mining subsidence monitored by SAR interferometry, near Gardanne, France

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1 Three years of mining subsidence monitored by SAR interferometry, near Gardanne, France C. CARNEC (1) and C. DELACOURT (2) (1) BRGM, 117 Avenue de Luminy Marseille Cedex 09, France. (2) UCBL-ENS Lyon Bat 402 Bld du 11 Novembre Villeurbanne. delac@alto.univ-lyon1.fr Abstract Differential interferogram images derived from repeat-pass spaceborne Synthetic Aperture Radar (SAR) systems give the possibility to map surface deformations of small spatial extent and to monitor their spatiotemporal evolution. A slow and local phenomenon of subsidence caused by underground coal mining has been observed near Gardanne, France, from images acquired by both European ERS-1/2 satellites between 1992 and Interferometric monitoring revealed the migration of the subsidence halo caused by the advance of the coal working face. Comparison with field leveling data has shown that the amount of effective land subsidence is similar. The analysis and interpretation of the observed movements take into account the structural characteristics of the sedimentary basin, the micro-seismic activity of the region and the geometry of the underground workings. Such an application of SAR interferometry, under normal conditions of observation and at different periods, opens up wide application prospects, not only for research goal but also for industrial use. Keywords: differential SAR interferometry, land subsidence, underground mining and seismic activity. I. Introduction Differential SAR interferometry can strongly contribute to current programs dedicated to the assessment of natural risks and can be considered as a unique tool to obtain surface deformation measurements over large areas, over a dense grid and with a point-to-point accuracy of a few millimeters. This technique has been shown to enable accurate mapping of large-scale surface displacements (Massonnet et al., 1993; Zebker et al., 1994; Peltzer et al., 1995). More recently, its ability to monitor displacement fields of small spatial extent has been demonstrated (Fruneau et al., 1996; Rosen et al., 1996, Van der Kooij, 1997; Massonnet et al., 1997). A study of subsidence from underground coal mining was carried out on the Gardanne mine site with ERS-1 35-day orbital cycle images acquired in This study has confirmed the ability of interferometry to detect the impact of subterranean activity (Carnec et al., 1996). However, heavy precipitation occurring the day before one of the three acquisitions led to a loss of coherence in the interferometric pairs computed with this image, limiting the experiment to a single combination. The area of interest, affected by underground coal mining near Gardanne, covers some 10 km 2 in the Arc sedimentary basin, about 20 km north of Marseilles. The alternating carbonate and clay beds (Campanian to Eocene) constituting the fluvio-lacustrine Arc basin exceed 1500 m in thickness. Only the so-called "Grande Mine" bed of the early Fuvelian (Late Campanian) carbonate succession has been mined in the Gardanne sector. The regularity of the deposit has enabled high levels of mechanization; such coal extraction techniques are commonly used in so-called "high performance" mines. Behind the headings (or working faces), the beds immediately overlying the coal seam crack and break; they fall as blocks that fill the void left by the evacuated coal (a phenomenon known as caving). Some of the subsidence occurs gradually, associated with the collapse of cavities previously filled with coal. In this paper we present results of a long time monitoring surface deformation. We compare interferometric results with conventional geodetic techniques and relationship between micro-seismicity, geological setting and subsidence. 1

2 2. Interferogram analysis Radar investigation campaigns were carried out in 1994, then in 1995, from the various orbital cycles (1, 3 and 35 days) of the ERS mission. From thirteen images acquired between July 1992 and November 1995 twelve pairs which ensured negligible topographic residual have been retained; their altitudes of ambiguity are much higher than the RMS precision of 10 m typical of the accurate digital elevation model from IGN (1990), using 50-by-50 m pixels and covering an area of 220 km 2. The detection of the subsidence halos observed over a period of 35 days during the summer of 1992 was confirmed by the 1994 series (maximum time span of 15 days). These results were made possible by the exceptional climatic conditions during this season: no recorded precipitation throughout the month. On the 3 interferometric combinations covering intervals of 3, 12 and 15 days, surface deformation can be observed (Fig. 1). When the underground workings map is superimposed (white polygons on Fig. 1), the southward halo (B) happened to be directly located above a heading face that has been active precisely since January 1994 (black polygons on Fig. 1). Maximum deformation is respectively 6 mm, 17 mm and 23 mm for the 3 days, 12 days and 15 days interferograms. The elliptical halo (major axis a=1100 m, minor axis b=650 m) covers about 100 ha. The major axis of the ellipse is parallel to the direction of heading advance. The limiting angle of influence,, defined by the angle between the underground heading face and the line of zero subsidence at the surface, can be directly estimated from the interferograms. Mar-07 / Mar-10 Mar-10 / Mar-22 Mar-07 / Mar-22 Figure 1: Interferometric monitoring of mining subsidence from 3 days ERS-1 repeat cycle (1994). In white, the underground working map since 1991 and in black, the blocks mined between 01/01/1994 and 31/03/1994. However, the deformation observed on the northern part of the concession (C on Fig. 1) does not coincide with an active working face of the mine. As this subsidence is located above an inactive working face, it may be due to the reactivation of subsidence caused by old operations or water pumping... As regards section A which is being actively mined, no deformation can be observed on the interferograms (Fig. 1). Topometric measurements show that the maximum recorded deformations do not exceed 20 mm over a period of 3 months covering the images acquisition. If we assume a subsidence with a stationary rate, maximum deformation expected is 3.5mm on the 15 days interferogram. The high noise level of the interferograms (probably due to atmospheric heterogeneities and/or modification of surface state) masks part of the deformation signal. So the expected deformation value may be under the detectability threshold. Consequence of geological setting on the behavior of this area will be discussed in the next section 2

3 In 1995, the ERS-1/ERS-2 TANDEM mission provided the opportunity of acquiring a regular sequence of radar images that enable monitoring of near-continuous deformations over 7 months. Detailed analysis of the TANDEM series (Fig. 2) reveals two subsidence zones, observed on several combinations and therefore cannot be related to atmospheric artifacts. Furthermore, despite different values for the altitude of ambiguities, the retention of fringe patterns shows that it cannot be a topographic artifact. The two halos represent surface indications of on-going underground operations (Table 1). a : April-14 / May-19 b : May-19 / Sept-02 c : May19 / Sept a c b d e f d : April-14 / Sept-01 e : July-28 / Oct-06 f : Oct-06 / Nov-10 Figure 2 :ERS1/ER2 TANDEM interferometric series. The black lines represents the geodetic network. 3

4 orbits dates 04/95-09/95 07/95-10/95 subsidence halo C z = 17mm max 500m z max = 18mm 500m subsidence halo E z = max 28mm z max = 14mm 1500m 1800m Table 1 - Characteristics of subsidence halos observed by interferometry - TANDEM series 1995 We note that the southward progression of the halo associated with the mine (denoted E in Fig. 3) is limited on its NE side by the Verdon canal. This canal marks a discontinuity of the Cretaceous beds composed, to the east, of Begudian limestone lenses and, to the west, of early Rognacian clays. The subsidence has largely spread to the neighboring panel exploited in 1992 and located about 500 m to the east. This boundary part of the concession, crossed from east to west by the Safre Fault, had been weakened by earlier mining operations (about 42 mm of subsidence measured between July and August 1992) (Carnec and al., 1996). It would mean that the geological context (early Rognacian clays) combined with the geometry of the mine workings has favored an extension of the subsidence. The deformation observed over the mining operations on panel D (Fig. 3) enlarges the potential of the interferometry technique. The area is characterized by a highly urbanized environment and is therefore favorable for interferometric radar monitoring. Man-made civil engineering-type structures ensure long term conservation of phase coherence. The limiting angles of influence measured for panels D and E were respectively 28 and 26 on October 10 for mining depths of 750 m and 1150 m. These values measured by interferometry above advancing faces are substantially underestimated compared with those for an abandoned face. Variations of surface state over large period have led to loss of coherence. Interferometric combinations generated over more than six months (for example between 1994 and 1995) have not enabled confirmation of intermediate observations. 4

5 April / Sept Figure 3 : underground workings map superimposed on the 14/04-01/09/1995 interferometric combination (black lines). The white lines represent the part of the concession mined during the radar acquisitions. 3. Interpretation Regular monitoring of mining subsidence has revealed the influence of the geological factors and of the mining operations geometry on ground behavior. One example is given by the lack of surface phenomena directly above the heading face denoted A (Fig. 1). First, the expected subsidence associated with panel A, located in a virgin mining area, is smaller than in case of juxtaposed headings; secondly, the nature of the overlying rocks has limited the surface repercussions of the caving. Indeed geology of the overburden effectively differs in the two cases: the northern panel A is located in limestone (Lutetian), whereas the subsidence observed farther south, on panel B, is associated with a succession of limestone, clay and marl layers and lenses (Begudian, Gaviglio, 1980). The overlying rocks relatively more competent in area A than in B, are not conducive to a continuous phenomenon, in which case the movement is more one of collapse (Josien 1980), which could have occurred outside the dates of radar acquisitions. The nature of the beds encasing the coal seam is of major importance. Indeed, the excess pressures resulting from mining lead to an accumulation of energy in the limestone beds, in particular those forming the hanging wall ("lintel"), and in the coal seam. With a great strength and a high modulus of material elasticity, the threshold of release for this stored energy is higher for panel A than for panel B. 5

6 The conjunction of natural factors (notably the stress and the quality of the material constituting the beds) and technical factors (mine geometry) at this location could explain the difference of ground behavior observed by interferometry. In addition, it should be noted that this interpretation is far from exhaustive and that other influencing factors need to be considered, such as fracturing level, thickness and dip of the mined seam (Josien, 1980). Comparison between field geodetic measurements (geodetic network is displayed in black lines on Fig. 2, 3, 5) and interferometric results has been realized. Due to low temporal sampling of topometric measurements (between six months and one year) a systematic comparison with interferometric data was difficult. The essential limitation of monitoring networks located along road and rail routes or sensitive structures, lies on the static representation of subsidence according to topometric profiles. In a first step location of the maximum deformation for each halo of subsidence is extracted from interferograms. Then the field data are plotted as a function of the distance between the measured points and this center of halo. This comparison with field data shows that the amount of effective land subsidence is similar over short periods; furthermore the symmetry of the graphs tends to show that the maximum deformation is near the interferometric halo center distance from benchmark to the maximum of subsidence C detected by interferometry (m) between 01/06/95 and 01/01/1996 Figure 4 : Elevation differences measured along the geodetic network plotted as a function of the distance from benchmark to center of halo subsidence D and E of Fig. 4.. The location of the center of halo subsidence is derived from interferogram displayed on Fig Micro-seismicity and surface subsidence One of the main characteristics of the Gardanne deposit is the exceptional rigidity of the formations which favors dynamic phenomena associated with sudden releases of the energy stored under the influence of the various natural or operational stresses (Senfaute, 1995). The local seismic network is composed of nine recorders (4 at the surface and 5 in old mine galleries). The uncertainty concerning the horizontal positions of the tremors is about 200 m; the average focal depth is less than 2000 km for an average magnitude of M Ω The goal of a comparative study of micro-seismicity and surface subsidence is to show the correlation between the advance of micro-tremors and the displacement of the subsidence basin, and to take into account the spatial distribution of the two phenomena. The satellite imaging technique tested in this study offers the particular advantage of a synoptic perception of effective movements, which enables direct comparison with the micro-seismicity. The amplitude and extent of the surface deformation observed by interferometry are larger over the southern sector of the mining area, where the most intense local seismic activity originates (74% of recordings), than those in the northern sector of the concession. This difference in seismic activity associated with active working faces in north (E) and south (D) is due to different lithological formations (as described in section 3). The higher rigidity of the limestone beds encasing the coal seam in the north may partly explain the lower level of local seismic activity. The country rock is better able to homogeneously redistribute changes in stress. The database of seismic recordings reveals, moreover, that the most intense periods of activity coinciding with the dates of radar observation are precisely those for which the largest deformation gradients were observed (about 40 mm/month at the center of the halo). 6

7 The superposition of the micro-seismicity spurts (Fig. 5) on the series of interferograms produced from the 1995 TANDEM acquisitions shows a concentration of seismic spurts on the working faces D and E, where work was started on 04/05/1995 and 28/02/1995 respectively. Comparison between the interferograms of April-September (Fig. 3d) and July-October 1995 (Fig. 2e) reveals an extension of the subsidence basin above the working face E by about 320 m to the northwest and 400 m to the north. As regards the seismic spread, the displacements measured in the same directions between August and October are respectively 150 m and 190 m (Fig. 5). The mine plan (confidential HBCM document) confirms an effective advance of the face by 200 m between August and October These results demonstrate the extension of the subsidence basin towards the northwest, whilst the working face D is heading roughly N.10 E. This lateral extension of the deformation is confirmed by a northwest displacement of the seismic activity. In addition, the delay observed between the advance of this activity (characterized by the spatial shift between seismicity and location of the maximum of deformation) and the subsidence illustrates perfectly the notion of "zone of influence" of the mining (Fig. 5). Interferogram : 1995 April-14 / Sept-01 Micro-seismicity : April + August 7

8 Interferogram : 1995 July-28 / Oct-06 Micro-seismicity : August + October Figure 6 Figure 5 :Micro-seismicity superimposed on the 1995 temporal series (in black, geodetic network) 5. Conclusion The series of radar images used for the interferometric processing demonstrates the ability of the technique to map surface movements in standard orbital configurations (altitude of ambiguity~100 m), on the basis of variable intervals (from a few days to six months) and in environmental contexts with low coherence rates. The migration of the subsidence halos observed between 1992 and 1995 correlates closely with the underground operations and the various lithologic formations. For the first time, a comparison of the advance of the micro-seismicity and the surface deformation has been made, confirming the relationship between subsidence and micro-seismic activity. The combined representation of the interferometric and field measurements yielded the shape of the subsidence basin and the position of maximum subsidence. Direct validation with in situ data is made difficult by the fact that the periods of radar observation do not exactly coincide with those of the topometric measurements. The main limitations are related to the unidirectional component of the movement and the loss of coherence caused by variations of surface state (the interferometric combinations generated over more than six months, for example between 1994 and 1995, have not enabled confirmation of intermediate observations). Monitoring of this type of deformation by interferometry could usefully complement other commonly used methods of investigation by providing a synoptic perception of the effective movements, permanent day-and-night operability, overcoming the limitations due to the inaccessibility of the sites which prohibit reconnaissance work. The monitoring networks could effectively be installed in zones that are actually subsiding. 8

9 Acknowledgments We thank the Houillières de Basin du Centre et du Midi (HBCM) and its Unité d'exploitation Provence for providing the ground data, as well as G. Senfaute from INERIS for seismic data. The study was supported by the European Space Agency (ERS1/ERS2 TANDEM experiment code AOT.F303). References Carnec, C., Massonnet, D., and King, C., Two examples of the use of SAR interferometry on displacement fields of small spatial extent. GRL, 23 (24): Fruneau, B., Achache, J., and Delacourt, C., Observation and modelling of the Saint-Etienne-de-Tinée landslide using SAR interferometry. Tectonophysics, 265: Gaviglio, P., 1980, La Fracturation dans le gisement de lignite du bassin de l Arc. Rev. Ind. Miner., Suppl. n juin: IGN, Description of the topographic Data Base. Technical report, Annexe 2B. Josien, J.P., 1980, Le comportement des terrains autour de l'exploitation - Les coups de terrain du Bassin de Provence. Rev. Ind. Miner., suppl n juin: Massonnet, D. and Rabaute, T., Radar interferometry: limits and potential. IEEE Trans. Geoscience & Rem. Sensing, 31: Massonnet, D., Rossi, M., Carmona, C., Adragna, F., Peltzer, G., Feigl, K. and Rabaute, T., The displacement field of the Landers earthquake mapped by radar interferometry. Nature, 364: Massonnet, D., Holzer, T. and Vadon, H., Land subsidence caused by the East Mesa Geothermal field, California, observed using SAR interferometry. GRL, 24 (8): Peltzer, G., and Rosen, P., Surface displacement of the 17 May 1993 Eureka Valley, California, earthquake observed by SAR interferometry. Science, 273: Rosen, P.A., Hensley, S., Zebker, H., Webb., F.H., Fielding, E.J., 1996, Surface deformation and coherence measurements of Kilauea Volcano, Hawai, From SIR-C Radar Interferometry, J. Geophys. Res., 101 (E10): Senfaute, G., La surveillance microsismique de l'exploitation souterraine du charbon aux Houillères de Provence, contribution à la détection des risques de coups de terrain par écoute microsismique, Ph.D. Thesis, Institut National Polytechnique de Lorraine, Nancy, France. Van der Kooij, M., Land subsidence measurements at the Belridge oil fields from ERS InSAR data. Proc. 3rd ERS Symposium. ESA, Florence, Italy. Zebker, H.A., Rosen, P.A., Goldstein, R.M., Gabriel, A., and Werner, C.L., On the derivation of coseismic displacement fields using differential radar interferometry : the Landers earthquake. J. Geophys. Res., 99, 19: