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firstname.lastname@example.org IPGS-UMR 7516, CNRS-Université Background: Several centimetres of uplift were observed extending over a de Strasbourg, 5 rue René square-kilometre area around the geothermal site of Landau, Germany. Descartes, Strasbourg 67084, France Full list of author information is Methods: This observation is based on the interpretation of a geodetic survey using available at the end of the article radar satellite images of the Upper Rhine Graben recorded between April 2012 and April 2014. Observations are based on two data processing methods for synthetic aperture radar acquisitions: synthetic aperture radar interferometry (InSAR) and permanent scatterer InSAR (PS-InSAR) monitoring. Results: The inferred time evolution shows that the displacement began in July 2013 and that the displacement rate reaches its maximum (about 16 cm/year) during the summer period (from July to September 2013). We observe a surface displacement of 3.5 cm during this period. Conclusions: A preliminary inversion of the source of the deformation based on a simple elastic model of a buried cavity suggests that a significant injection of fluid occurred at a depth of approximately 450 m below the geothermal plant. Keywords: PS-InSAR; InSAR; Surface displacement; Geothermy Background The Upper Rhine Graben has high potential for deep geothermal energy owing to a high temperature gradient combined with a large fluid source (Pribnow and Schellschmidt 2000). Since 1987, a geothermal research laboratory in the French area of the Upper Rhine Graben, the GEIE of Soultz-sous-Forêts (France), has provided considerable knowledge on deep geothermal energy and its production (Gerard et al. 2006; Genter et al. 2010). One development of lessons learned from Soultz-sous-Forêts is the geo- thermal power plant of Landau, which is located south of the city of Landau and 35 km northeast of Soultz-sous-Forêts. The power plant has a doublet configuration in which one well is the production well and the second well is used for the geother- mal water reinjection. The production well was bored in 2005 to 3,300 m deep. The well deviates to the west, with an inclination as high as 29°. The reinjection well was bored in 2006 to 3,170 m deep, with an inclination as high as 25° to the east. The power plant had been running successfully since 2007, but the production stopped on 14 March 2014, when a major surface deformation was observed in the vicinity of the plant. A link between the geothermal production and the surface displacement was established (www.bodenhebungen-landau.de). © 2015 Heimlich et al.; licensee Springer. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http:// creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Heimlich et al. Geothermal Energy (2015) 3:2 Page 2 of 12 To understand the geophysical processes that might be responsible for this surface deformation, a regional geodetic survey will be important. Indeed, previous geodetic analyses of geothermal regions have already been performed and have shown that verti- cal and horizontal displacements have been caused by geothermal exploitation (e.g. Massonnet et al. 1997; Carnec and Fabriol 1999; Fialko and Simons 2000; Nishijima et al. 2005; Glowacka et al. 2010; Jeanne et al. 2014). Note that surface displacement can also be observed during the drilling phase (Lubitz et al. 2013). In this study, we present results from recent geodetic radar surveys using synthetic aperture radar (SAR) images. The images cover the Landau region from April 2012 to April 2014. First, we describe the methods in the Methods section, i.e. SAR interferom- etry (InSAR) and permanent scatterer InSAR (PS-InSAR) monitoring. In the Results section, we present a map of the surface displacement around the geothermal plant of Landau and discuss the evolution of the displacement along specific geographical pro- files. Finally, in the Discussion section, we discuss our results, we compare our InSAR results with levelling measurements and introduce a simple model that provides a pre- liminary inversion of the observed surface deformation. The Conclusion section is de- voted to conclusions. Methods InSAR and PS-InSAR Our dataset consists of 57 SAR acquisitions from the TerraSAR-X satellite of the German Space Agency (DLR) from April 2012 to April 2014, with a repeat time between successive acquisitions of 11 days. Some acquisitions were not, however, performed by the DLR; thus, measurement gaps of up to 55 days occurred at the end of the summer of 2013. The X-band (3.1-cm wavelength) SAR images are acquired in descending orbit with an incidence angle of 21° and a 2-m ground resolution. Surface displacement is measured by exploiting the phase variation between consecu- tive SAR measurements using both conventional InSAR (Gabriel et al. 1989; Massonnet and Feigl 1998; Rosen et al. 2000; Hanssen 2001) and PS-InSAR (Usai 1997; Ferretti et al. 2001) techniques. InSAR is sensitive to the change in distance between the ground and the satellite in the line of sight (LOS) direction and is, for a spaceborne SAR system with a small look angle such as used here, mainly sensitive to vertical ground displacement and, to a lesser extent, to horizontal ground displacement. Conventional InSAR has the advantage of maximising the spatial coherence for short temporal baselines but is limited by surface coherence, and PS-InSAR has the advan- tage of resolving deformation in regions of low coherence, such as the region surrounding the geothermal plant in Landau. InSAR techniques have been applied to study surface deformation related to the earthquake cycle (Massonnet et al. 1993; Zebker et al. 1994; Fialko et al. 2005; Burgmann et al. 2006; Peltzer et al. 2001), magmatic activity (Amelung et al. 2000; Pritchard and Simons 2002; Hooper et al. 2004; Wright et al. 2006; Wicks et al. 2006; Doubre and Peltzer 2007; Biggs et al. 2009) and subsurface reservoirs (Jonsson et al. 1998; Massonnet et al. 1997; Amelung et al. 1999; Fialko and Simons 2000; Bawden et al. 2001; Newman et al. 2006; Gourmelen et al. 2007; Vasco et al. 2008). For the PS-InSAR approach, we use algorithms developed at Sejong University (Kim et al. 2010) following the procedure described by Ferretti et al. (2001). A stack of SAR Heimlich et al. Geothermal Energy (2015) 3:2 Page 3 of 12 images coregistered to a common master is used to compute a stack of interferograms from which orbital and topographical phase components are removed using orbit infor- mation provided by the DLR and the SRTM DEM, respectively (Farr et al. 2007). Per- manent scatterers are then selected based on the temporal characteristics of the measured signal. The phase of the permanent scatterers is then spatially and temporally identified prior to evaluating the DEM errors, the atmospheric phase screen, the sur- face deformation time series and the mean velocity from linear regression of the de- formation time series at each persistent scatterer (PS). Levelling The local government produces regular levelling measurements in Landau (the mea- surements are published at www.bodenhebungen-landau.de). Some points have been measured since 1993, but most points have only been measured since October 2013. The levelling measurements provide information on the vertical movement over time. Since October 2013, the city of Landau has completed the levelling measurements with Global Navigation Satellite System (GNSS) measurements of horizontal displacement at sites close to the geothermal power plant. We selected the levelling measurement sites that are collocated with PS-InSAR pixels to compare both results. Results Figure 1A shows the PS-InSAR mean velocity in the area of Landau between 8 July 2013 and 9 April 2014. Despite a lack of information where the temporal coherence is too weak (black domains), we observe a quasi-circular displacement pattern south of Landau centred on the geothermal plant of Landau (white circle, Figure 1A). The aver- age uplift rate reaches 5 cm/year at the plant site. A highly coherent short-term inter- ferogram between 24 February 2014 and 7 March 2014 (Figure 1B) clearly shows the spatial relationship between the deformation field and the location of the geothermal plant. The radius of the whole area affected by the surface deformation between April 2012 and April 2014 centred on the power plant extends from 800 to 1,000 m. We also observe a larger north-south elongated area of moderate uplift north of Landau close to the village of Nussdorf (Figure 1A - point A), where oil exploitation is ongoing. A north-south profile extending from the oil fields located north of Landau to the geothermal power plant south of Landau (Figure 2; Figure 1 for the locations of the profiles) shows the temporal and spatial evolution of the cumulative displacement. De- tails on the evolution at six locations (A, B, C, D, E and F from north to south) along the N-S profile are plotted in Figure 3. Figure 2 displays two displacement profiles along the west-east direction: one through the centre of the city of Landau (crossing the N-S profile close to point D) and one close to the geothermal plant (crossing the N-S profile at point F). High-rate uplift from July 2013 around the Landau geothermal plant Figures 2 and 3F do not show significant displacement surrounding the geothermal plant from April 2012 to July 2013. The increase in the displacement rate was initiated at the beginning of July 2013 in the vicinity of the geothermal power plant. After July 2013, the displacement exhibited a major rate change that is clearly visible in the time Heimlich et al. Geothermal Energy (2015) 3:2 Page 4 of 12 Figure 1 Map of the mean displacement velocity and interferogram showing uplift in south Landau. (A) Map of the mean displacement velocity (cm/year) over Landau. Mean displacement velocity (cm/year) in the LOS direction (positive towards the satellite) over the city of Landau (Germany) between 8 July 2013 and 9 April 2014 as obtained from the PS-InSAR results. White lines: profiles of Figure 2. White stars: location of the time series of Figure 3. White circle: location of the power plant. (B) Interferogram showing uplift in south Landau. Uplift centred on the geothermal power plant between 24 February 2014 and 7 March 2014 as obtained from the InSAR analysis. The white cross marks the power plant location. One colour cycle corresponds to a deformation of 1.55 cm in the LOS direction (positive towards the satellite). Here, the displacement reaches a maximum of 2 cm. series (Figure 3F). The displacement amplitude reached 2.4 cm between 30 July 2013 (lower purple curve) and 23 September 2013 (middle black curve). During this period, the uplift reached more than half of the total uplift that had occurred since July 2013. The profiles of the displacement show a clear maximum at point F, i.e. the intersection of the profiles at 0.29 km from the power plant (Figures 1 and 2). Note that the loca- tion of the maximum displacement is slightly shifted from the surface location of the geothermal plant in the direction of the injection well (to the east). Heimlich et al. Geothermal Energy (2015) 3:2 Page 5 of 12 Figure 2 Profiles of the LOS displacement (cm). Displacement (cm) in the LOS direction (positive towards the satellite) for the three profiles (white lines in Figure 1). The coloured curves show the displacement at relevant dates (see legend) from 2 April 2012 (lower red line) to 9 April 2014 (black top line). The results are averaged over a set of PS-InSAR along the profile at a maximum of 100-m distance from the selected point. The results in the grey boxes suffer from a loss of coherence. The vertical black lines in the north-south profile (x-axis) identify the six points of the time series presented in Figure 3. The red point on the x-axis is the nearest point of the profile to the geothermal power plant. Displacement after March 2014 in the vicinity of the geothermal plant location In the southern west-east profile in Figure 2, we can see a decrease in the displacement approximately 100 m east of the nearest point of the geothermal plant location between 18 March 2014 (purple curve) and 9 April 2014 (black curve). We can also see this change in the displacement direction in the north-south profile. However, the results are affected by the spatial decorrelation in the pixel in the profile, so the results are less constrained than in the west-east profiles. The inversion of polarity of the deformation is more visible in the levelling results (Figure 4), where the time series extends a month later (until May 2014) than the PS time series, particularly at the sites closest to the geothermal plant (sites 5 and 6). Continuous uplift in the north during 2012 to 2013 North of Landau, we observe a region of continuous uplift that culminates in the vicin- ity of the village of Nussdorf (Figure 1). The uplift (Figures 2 and 3A) has an average rate of 1.5 cm/year. The area affected by this continuous displacement extends from point A to C, over a distance of 3 km, and reaches the centre of the city of Landau. The amplitude decreases from north to south, where the maximum displacement is at point A at a distance of 0.3 km from the northern limit of our profile and the minimum displacement along the profile is close to point C. On the regional map (Figure 1A), the uplift is maximal east of the profile between Nussdorf and Walsheim at the oil fields. Heimlich et al. Geothermal Energy (2015) 3:2 Page 6 of 12 Figure 3 Time series from the north-south profile. A, B, C, D, E, F: time series corresponding to the points located Figures 1 and 2 from the north (A) to close to the geothermal site location (F). Each time series represents the LOS displacement towards the satellite (cm). Discussion PS-InSAR We observe two patterns of uplift: one in the north and one in the south. Point C (Figures 1 and 2) seems to be at the transition between the two strain fields: the southern field related to the geothermal plant that was initiated in July 2013 and the northern field related to the oil exploitation that was initiated before April 2012, i.e. the beginning of our InSAR survey. Indeed, point C (see Figure 3C) exhibits both a slow, continuous displace- ment increase at the beginning of the monitoring in 2012 and then a clear acceleration in July 2013 to a higher rate that is synchronous with the onset of the geothermal strain. Therefore, the geothermal strain field is significantly distinct from the regional strain. The maximum PS-InSAR displacement for points in the proximity of the geothermal plant for the July 2013 to March 2014 time period reaches 4 cm in the LOS direction in the north-south profile and 4.2 cm in the west-east southern profile (Figure 2). Note that the PS-InSAR displacement at the geothermal power plant indicates a cumulative displacement of less than 3.3 cm. This value likely reflects error during the phase un- wrapping procedure rather than real behaviour as the region near the geothermal plant contains sparse coherent PS and SAR acquisitions during the period of maximum de- formation. Subsequently, at the geothermal plant, PS-InSAR underestimates the real deformation, as shown in the Comparison between InSAR and levelling section for the levelling observations. Comparison between InSAR and levelling We compare the geodetic PS-InSAR results to the ground levelling measurements at ten sites around the geothermal plant between mid-2013 and 2014 (see Figure 4). The PS-InSAR time series is the result of the mean value of PS-InSAR points within a 50-m Heimlich et al. Geothermal Energy (2015) 3:2 Page 7 of 12 Figure 4 PS-InSAR and levelling time series. Comparison of PS-InSAR displacement time series (black points) and levelling measurements (red points). The first levelling measurement is adjusted to the PS-InSAR measurements. The PS-InSAR time series is the mean of the vertical projection of the PS-InSAR results of the pixels located in the vicinity of the levelling points; the maximum distance of the points is 50 m. The location of the points is shown in the joined map. The white and red stars identify the geothermal power plant. For site 5, we propose a correction of the PS-InSAR results (green points). distance. We introduced an arbitrary offset to the first levelling measurement to fit the PS-InSAR estimates of the time series to compare the relative variations between both techniques. The levelling measurements of sites 1 to 4 and site 8 began in 2011, with a second measurement in October 2013. During this time lapse, all of the sites were af- fected by an uplift that decreased with the distance to the geothermal site. The uplift was 1.7, 1.6, 0.7 and 0.1 cm for sites 4, 3, 2 and 1, respectively. The difference between the levelling results at these site locations (2011 to 2013) and the InSAR results (April 2012 to September 2013) is less than 0.3 cm. Further, the main displacement before September 2013 arguably occurred during July to September 2013 and could be linked with the geothermal site. The first levelling measurements at the other sites began in October 2013. Differences between the levelling and PS-InSAR results are typically on the order of a millimetre, except at site 5, where the difference is on the order of a centimetre. An Heimlich et al. Geothermal Energy (2015) 3:2 Page 8 of 12 excellent agreement is obtained after the offset subtraction for all the sites over the time period. The displacement is higher close to the power plant and decreases with the distance from the power plant. The maximum of the displacement amplitude for the PS-InSAR measurement is on the eastern side of the power plant at site 6 (4.5 cm), and the max- imum for the levelling measurements after the offset subtraction (i.e. adjustment to the PS-InSAR results) is on the western side (5.8 cm). The levelling measurements show that the amplitude of deformation is higher at site 5 and is clearly visible in 2014. Spe- cifically, site 5 is closer to the geothermal plant and affected by higher ground inflation. The PS-InSAR results at site 5 suffers from unwrapping errors, and this is clearly vis- ible in Figure 4, for the dates comprised between 20 December 2013 and 22 January 2014 (four dates); here, the PS-InSAR results are shifted by half a satellite wavelength relative to the previous and following results. We use the levelling results to estimate and correct the phase shift (Figure 4, site 5). We corrected also the four last PS-InSAR results at site 5 of half a satellite wavelength for 7 March 2014 and of one wavelength for the dates between 18 March and 9 April 2014 (three last dates). At site 6, where the variations in the time series are similar using the two methods, an uplift through mid-March 2014 (2014.2 decimal years) is followed by subsidence. Note that the variation in the amplitude is lower for the PS-InSAR results than for the in situ measurements. At site 5, both the PS-InSAR and levelling methods show uplift from the beginning of the time series to mid-March 2014 (2014.2). After the 24 March levelling measure, the levelling result shows subsidence, whereas the PS-InSAR results show uplift after 18 March 2014. The difference between the two methods can be attributed to unwrapping errors and/or to the lack of PS-InSAR at the levelling measurement point 5. So the mean of the PS- InSAR displacement can result from PS-InSAR with a higher rate of displacement. A simple model for strain inversion We model the surface displacement measured by PS-InSAR between 30 July and 23 September 2013 to evaluate the characteristics of the first period of events. As a start- ing point, we shall assume that the deformation has a circular shape around the geo- thermal plant. The displacement measured by levelling and PS-InSAR increases towards the location of the power plant, suggesting a source located below the geother- mal plant. The uplift occurs continuously from the onset of the well incident in July 2013, and the subsidence begins when the plant is shut down in March 2013. The sur- face displacement seems therefore to linearly respond to the behaviour of the deform- ation source. Accordingly, we suggest that the dominant response underneath the geothermal plant is elastic. We tested two models commonly used in volcano studies: a point source model (Mogi 1958) and an ellipsoidal model (Yang et al. 1988). The dis- placements modelled by the Mogi point source model can be compared to a displace- ment induced by the volume variation in a buried spherical cavity with a uniform internal pressure in an elastic half space (McTigue 1987). In this case, the vertical and radial displacements due to a volume variation of the sphere are well known (Mogi 1958; McTigue 1987; Segall 2009). The width of the displacement area is a function of the depth of the source. Its amplitude is a function of the depth and volume of the Heimlich et al. Geothermal Energy (2015) 3:2 Page 9 of 12 injected fluid. When the depth is fixed, the amplitude is only a function of the volume of injected fluid. We also tested an ellipsoidal model (Yang et al. 1988). The ellipsoidal model is more realistic for modelling the injection of water in sedimentary layers due to the anisotropy characteristic of the sedimentary rocks. The Mogi model has four de- grees of freedom: the two horizontal coordinates, the depth and the variation of vol- ume. The elliptical model has eight degrees of freedom. We tested the following six parameters: the two horizontal coordinates, the depth, the pressure and the elliptical parameters (semi-major and semi-minor axes). We have not tested the plunge and the strike angle of the major axis. We fixed the Poisson's ratio value to 0.25 and the Young moduli to 10 GPa. We calculated the LOS displacement in the models from the radial and vertical components and compared them to the PS results. We first do a systematic research of the best fitting model, which allows to define the range of possible values for all the parameters. The source location parameters are tested both in the latitude and longitude direction from −180 to +200 m within an interval of 50 m. The depth is tested between 200 and 600 m with an interval of 50 m. The volume variation is tested 3 3 between 1,000 and 45,000 m with an interval of 5,000 m . In a second step, we do for- ward modelling using the parameter ranges defined during the first step. Then we look at all the residual maps between the true and synthetic data. We finally select the model showing the minimum residuals around the geothermal site. We obtain results with a good constraint on the depth, approximately 450 m. The lo- cation of the centre of the deformation is also well constrained and is in the vicinity of the geothermal plant (around ±150 m). Figure 5A presents the results of the best fitting Mogi model (blue curve) and Yang model (red curve) along the west-east and north-south profiles with a source at a 450-m depth. The volume variation is 30,000 m with the Mogi model. The Yang model pro- duces a pressure change of 10 MPa and elliptical dimensions of 17 and 1,300 m for the semi-minor and semi-major axes, respectively. Figure 5B represents the residuals between the Mogi model and the PS-InSAR re- sults. The residuals are very small, indicating that the model is in good agreement with the PS-InSAR results, except at the geothermal plant where the phase is not correctly un- wrapped. The residuals in the south correspond to an area of Landau with many new buildings, which causes the SAR signal to decorrelate. A circular pattern of deformation remains in the north. This pattern can result from a second source of deformation. The reinjection well of the Landau power plant is not cemented between depths of 479 and 751 m. A defective joint has already been observed at this well (Ministry of Economic Affairs, Climate Protection, Energy and Regional Planning press talk, 9 April 2014, www. lgb-rlp.de/uploads/media/Pressegespraech-Gelaendeveraenderungen-Landau.pdf). Thus, a geothermal water leakage at a depth of approximately 450 m is very consistent with our observations. Conclusions The PS-InSAR geodetic measures show that we can distinguish four main periods: 1) First, a continuous uplift north of Landau related to oil field exploitation. 2) In July 2013, a new displacement field appeared south of Landau in the vicinity of the geother- mal power plant. This event is clearly visible in the PS-InSAR time series between July Heimlich et al. Geothermal Energy (2015) 3:2 Page 10 of 12 Figure 5 Models of displacement. (A) Modelled LOS displacement (cm) along the west-east and north-south profiles. The red curve is the ellipsoidal Yang model; the blue curve is the Mogi model; the black dots are the PS-InSAR results. The source location of the models is at a 450-m depth. (B) Residuals between the ellipsoidal Yang model and the PS-InSAR results at a source location at a 450-m depth. 2013 and September 2013. The maximum displacement occurs in this period near the power plant. The amplitude of the displacement decreases with increasing distance from the power plant. 3) The displacement is a multi-centimetre uplift that continues through March 2014. The region of displacement extends up to the centre of the city of Landau. 4) In the last period, from March 2014, we detect subsidence in the vicinity of the geothermal power plant. Despite the lack of PS-InSAR measurements south of the geothermal power plant owing to the poor coherence of the signal over time, we can assume that the surface Heimlich et al. Geothermal Energy (2015) 3:2 Page 11 of 12 deformation has a circular pattern around the geothermal power plant. The levelling data show that the maximum displacement is on the east side of the power plant in the drift direction of the reinjection well. This observation is consistent with the origin of the deformation located around the reinjection well and is related to a leakage in the well. The modelling with the Mogi model and the Yang model suggests that this leak- age occurs at a depth around 450 m, which is consistent with the existence of a non- cemented zone in the reinjection geothermal well. Our study demonstrates the importance of geodetic monitoring of geothermal power plants. Indeed, geodetic monitoring can help determine the history and the origin of in- cidents. Moreover, the PS-InSAR method has the ability to delineate the border of the deformed area. However, the PS-InSAR method is unable to provide information on vegetated areas, such as those south of the power plant. Complementary methods, such as levelling and GNSS, are necessary. These complementary methods are also helpful for evaluating the amplitude of the displacement when the displacement between the time lapse and/or adjacent pixels is greater than half of the wavelength of the radar sig- nal. These methods are also necessary for retrieving the vertical and horizontal compo- nents of the displacement. In the future, data will be analysed with shorter baselines, and GNSS measurements will contribute to estimating the displacements. Abbreviations GNSS: Global Navigation Satellite System; InSAR: interferometric synthetic aperture radar; LOS: line of sight; PS: persistent scatterer; SAR: synthetic aperture radar. Competing interests The authors declare that they have no competing interests. Authors' contributions NG and SWK conducted the InSAR data processing. CH, NG, FM and JS designed the study. CH and FM conducted the analysis, the interpretation and the figure design. CH conducted the analysis and interpretation of the levelling measurements. FM, JS, CH and JA conducted the modelling. CH wrote the manuscript, along with contributions from NG, FM and JS. All authors read and approved the final manuscript. Acknowledgements This work has been conducted under the framework of the LABEX ANR-11-LABX-0050_G-EAU-THERMIE-PROFONDE, and it is supported by funding from the state managed by the French National Research Agency as part of the Investments for the Future program. S-W Kim was supported by Global Surveillance Research Center (GSRC) program funded by the Defense Acquisition Program Administration (DAPA) and Agency for Defense Development (ADD). We thank the reviewers for their remarks which greatly improved the manuscript. Author details 1 2 IPGS-UMR 7516, CNRS-Université de Strasbourg, 5 rue René Descartes, Strasbourg 67084, France. 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Geothermal Energy – Springer Journals
Published: Jan 30, 2015
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