MULTI-TEMPORAL L-BAND SAR INTERFEROMETRY CONFIRMS C- BAND SPATIAL PATTERNS OF SUBSIDENCE IN THE ANCIENT WIELICZKA SALT MINE

(UNESCO HERITAGE SITE, POLAND)

D. O. Nitti (1), L. De Vitis (1), F. Bovenga (2), R. Nutricato (3), A. Refice (2) , J. Wasowski (4)

(1) Dipartimento Interateneo di Fisica, Politecnico di Bari, Via Amendola 173, 70126 Bari (Italy), E-mail: davide.nitti@fisica.uniba.it

(2) CNR-ISSIA, Via Amendola 122/D, 70126 Bari (Italy), E-mail: [bovenga,refice]@ba.issia.cnr.it (3) GAP srl, c/o Dipartimento Interateneo di Fisica, Politecnico di Bari, Via Amendola 173, 70126 Bari (Italy),

E-mail: raffaele.nutricato@gapsrl.eu (4) CNR-IRPI , Via Amendola 122/I, 70126 Bari (Italy), E-mail: j.wasowski@ba.irpi.cnr.it

0BABSTRACT This work presents first results of interferometric proc-essing of ALOS PALSAR Single Look Complex SAR images (Fine Beam Mode), concerning ground deforma-tions in the Wieliczka Salt Mine area, a few km from Cracow, Poland. It follows a recent Persistent Scatterers SAR Interferometry (PSI) analysis on this area, obtained by processing several tens of ERS satellite images cov-ering the period 1992-2000. ERS results revealed the presence of a few kilometres long, slowly subsiding zone corresponding very well to the extent of the under-ground salt mine. The present work aims to extend the analysis by exploiting ALOS-PALSAR data especially for the rural areas, which neighbour the salt mine but lack PS in C-band, relying on the lower sensitivity to temporal decorrelation of L-band w.r.t. C-band radar data. This work shows and compares ERS and PALSAR (Fine Beam) ground displacement patterns detected over the Wieliczka Salt Mine area through the application of multi-temporal interferometric techniques. Key words: ALOS PALSAR FBS/FBD; Persistent Scat-terers Interferometry; mining -induced subsidence.

1. INTRODUCTION Wieliczka, a town located 14 km SE of Cracow, is home to a unique salt mine, over 700 years old, one of the best known tourist attractions in Poland, placed by UNESCO on its first International List of the World Cultural and Natural Heritage in 1978. Each year the mine is visited by about 1 million tourists from all over the world and in 1978 UNESCO placed it on its first International List of the World Cultural and Natural Heritage. The salt deposit has been exploited under an area ex-tending 7 km in E-W direction and about 1 km wide, and since the Middle Ages over 7.5 million m3 of un-derground passages have been excavated. There is evi-dence that the mining legacy has influenced the ground and building stability in the town, which is sited directly above the mine. Furthermore, today many buildings in the town show clear signs of distress and ground stabili-ty is an important issue [1]. Ground topographic measurements in this area docu-mented about 1 m of subsidence in the period 1970-2000 [1]. There are also indications of possible linkages between the mine-induced subsidence and the presence of the relatively large landslides occurring on the north slopes facing the Wieliczka area. The Wieliczka area is also one of the test sites selected for the ALOS ADEN 3595 project, whose general goal is to foster an integrated approach (EO and in situ data) to pre-disaster detection and monitoring of landslides, subsidence and related ground instability hazards. In the last several years differential SAR interferometry has become an useful tool for detecting and long-term moni-toring of terrain motion (e.g. [2]). In this context Wie-liczka is of particular interest, because it is affected by both mining-induced subsidence and landslide pheno-mena. It thus offers an opportunity of investigating possible linkages between mine subsidence and slope instability. A first Persistent Scatterers SAR Interferometric (In-SAR) analysis on this area was derived [1] by process-ing several tens of ERS satellite images covering the

Figure 1. Yellow and orange frames refer respectively to ERS and PALSAR ground coverage, green frame encloses

the AOI. Inset shows the location of the study area.

_____________________________________________________ Proc. ‘Fringe 2009 Workshop’, Frascati, Italy, 30 November – 4 December 2009 (ESA SP-677, March 2010)

period 1992-2000 with SPINUA processing chain [3]. Recently [4], the interferometric analysis has been ex-tended by exploiting ALOS-PALSAR Fine Beam Single Polarization (FBS) data in order to cover especially the rural or vegetated areas in the peripheral parts of the mine, where C-band decorrelate and PS-like targets are very few, because of the L-band higher penetration co-efficient on the ground and the consequent lower volu-metric and temporal decorrelation in vegetated areas w.r.t. C-band radar data. Moreover, episodic occurrences of fast subsidence (tens of cm per year or even more) in the recent past cannot be a priori ruled out [1], but they could not be unambi-guously detected with C-band data, thus further motivat-ing this L-band SAR application effort. L-band InSAR monitoring of the salt mine subsidence is feasible in theory, since a periodic revisit of the area is guaranteed by JAXA. Indeed, at least two FBS acquisi-tions per year are expected according to the actual PALSAR acquisition plan. However, due to both the longer wavelength wrt C-band and the limited acquisi-tion rate, the monitoring of low rate displacements (few cm/yr) is hard to accomplish when processing only the few PALSAR FBS images actually available. In the present work, the interferometric analysis has been fur-ther extended to all the PALSAR Fine Beam images (Single and Double Polarization) acquired by JAXA un-til January 2009 (Off-Nadir look angle: 34.3°). Full frame ground coverage is shown in Fig.1 for both the processed ERS and PALSAR datasets. The yellow and orange frames refer respectively to the ERS and ALOS PALSAR acquisitions, while green frame encloses the AOI. Inset in Fig.1 shows the location of the study area.

2. ERS DATASET AND PSI PROCESSING The selected dataset consists of 44 ERS-1/2 descending acquisitions (track 179, frame 2601) and covers a period from 1992 to 2000. Before selecting the radar images we examined historical precipitation records of the Wieliczka meteorological station. This was done to avoid ordering images acquired on days with significant snow cover or heavy snow/rain precipitations. The SAR data were analyzed using the SPINUA (Stable Point In-terferometry over Unurbanised Areas) algorithm (see [3] for more details), developed by the Department of Physics of Bari (Italy) in collaboration with the CNR-ISSIA institute of Bari, which belongs to the family of Persistent Scatterers Interferometry (PSI) processing tools [5]. A patch-wise processing scheme is adopted that relies on processing small zones (usually a few km2) within a larger area of interest. The patches are selected with the aim to optimise the density and the distribution of po-tential PS. Their small size allows to use locally linear approximations of the atmospheric phase signal, which in turn ensures high processing robustness. Atmospheric phase residuals can then be interpolated over larger ar-eas through a kriging procedure. Final data consist of a precisely georeferenced PS database, including several pieces of information such as point average velocity, DEM estimated error, as well as the complete displace-

ment time series measured at the acquisition dates. The data can be imported into several geographic visu-alization environments such as Google Earth. Differential interferograms of the Wieliczka test site were generated by using an external SRTM-derived DEM with spatial resolution of 90x90 m2 [6].

3. ALOS DATASET AND PSI PROCESSING All the ALOS PALSAR FBS & FBD Single-Look-Complex acquisitions planned by JAXA until January 2009 for the area of interest (AOI) have been processed with the SPINUA processing chain. Relevant parame-ters about resolution and side looking geometry as well as the processed PALSAR stack are listed respectively in Tab. 1 & 2. For the multi-temporal interferometric processing, only HH (FBS or FBD) images have been considered. The yaw steering seems to be very accurate. The mean Doppler centroid (values given in Tab. 2) of the images is indeed 37 Hz, which is a small fraction of the PRF of 2155 Hz. The maximum difference from the mean is less than 30 Hz, so there is nearly complete overlap of the Doppler spectra. Ad hoc algorithmic solutions are necessary for a proper interferogram generation with ALOS data, as summa-rized here below. � Range Oversampling of FBD acquisitions All the HH images of FBD acquisitions have to be over-sampled by a factor of 2 in range in order to have the same pixel spacing of HH FBS acquisitions, thus allow-ing FBS/FBD coregistration. � Hermite Interpolation of orbital state vectors Time interval between annotated ephemerides is 60s (more than 6 times wider than the full frame acquisition time extent), much longer than for C-band (ERS, EN-VISAT) or X-band (TerraSAR, COSMO-SkyMED) SAR products. Hermitian approaches are therefore ne-cessary for a proper orbital interpolation. Orbital resi-duals may become indeed not negligible at all if dis-carding the ephemerides velocity information, as shown in Figure 2. In order to better appreciate the higher accuracy of the Hermite interpolation wrt standard approaches, the fol-lowing test has been carried out for all the processed PALSAR images listed in Table 1.Orbital interpolation has been repeated by considering or neglecting state vector velocities. Then, the errors between the two in-terpolated orbits along XYZ directions have been esti-mated since the start until the end of each full frame ac-quisition. Their amount may be negligible or reach even few tens of centimeters, depending on the relative temporal posi-tion of the nearest state vector wrt the acquisition inter-val. Finally, the variations of the errors in XYZ position have been computed along azimuth direction: their amount is comparable with the wavelength (~24 cm), thus explaining the likely occurrence of even more resi-dual orbital fringes along azimuth direction in the ALOS differential interferograms when Hermite inter-polation is not adopted, as shown in Figure 2 on the left.

� Resampling along azimuth direction Significant differences have been observed between the PRF values of the PALSAR FBS acquisitions, as well as between FBS and FBD acquisitions (see Table 2). They could be explained in terms of the relatively small an-tenna size (3.1 by 8.9 m), orbit altitude and large Dopp-ler bandwidth [7]. In our case, for instance, PRF changes of more than 20 Hz are noticeable even for the same frame/track (Tab. 2). Therefore, for a proper image alignment, all the HH ALOS acquisitions have to be first interpolated along azimuth direction in order to uniform their PRF values. � Zero-Doppler Timing Correction SPINUA is actually capable to process only focused im-ages, thus requiring in input a stack of SLCs. ALOS Level 1.1 products may be provided by JAXA, as in our case, or ERSDAC facilities. Single Look Complex data provided by ERSDAC are generated with Zero-Doppler annotation. Also the ground range Level 1.5 JAXA products are processed to zero Doppler but the Level 1.1 products are not. The ALOS yaw axis is aligned with the centre of the Earth rather than being aligned to maintain local ortho-gonality. A consequence of this type of yaw steering is that the Doppler frequency is not set to zero and changes as a function of latitude and beam number [8]. InSAR co-registration of JAXA L1.1 products with dif-ferent Doppler centroid frequencies may be therefore an issue [9], as they are not in zero-Doppler coordinates. In this sense, azimuth shift correction could help when coregistering Level 1.1 products. The expression for the azimuth shift, δy, in m, at a given range pixel is [9]:

S

DC

V

fRy

2

λδ ⋅⋅−= (1)

where R is the slant range (m), fDC is the Doppler fre-quency (Hz) at the corresponding range, λ is the radar wavelength (m) and Vs is the spacecraft velocity (m/s). Since Doppler centroids are really a very small fraction of the PRF, a mean timing correction could be estimated by simulating the SLC amplitude thanks to the availabil-ity of an external DEM [11] and by coregistering the simulated SLC amplitude with the real one. This should be repeated for every PALSAR acquisition. � DEM-Assisted coregistration DEM-assisted approach [12] for SAR image coregistra-tion is necessary in order to deal with normal baselines up to 5.5 km between master and slave SLCs (Table 1). Kilometric baselines require more care in image align-ment even in case of gentle topography. These, together with the 2 times better slant range resolution of ALOS FBS images w.r.t. ERS, lead to local misregistrations, in case of uncompensated topography, that may become unacceptable (i.e., more than a 1/8 pixel). The benefits of a DEM-assisted approach for the coregistration of ALOS FBS images may be not negligible at all, as shown in [12]. DEM-assisted coregistration has been performed with DORIS software [13].

� Flat-earth and topographic phase simulation Flat-earth and topographic phase have to be simulated for each interferogram by taking into account the non-zero Doppler geometry of the JAXA Level 1.1 data. In the present work, we selected as supermaster the image acquired on January 5, 2008, whose Doppler centroid frequency ranges from +5.5 Hz @ near range to -2.5 Hz @ far range, thus being very close to zero. SPINUA is however capable to perform proper geocod-ing step as well as topographic phase removal regardless the actual supermaster SAR geometry, i.e. regardless the zero-Doppler assumption. Next step will be therefore to repeat the generation of the stack of differential interferograms by choosing the supermaster as a closer acquisition to the "mass centre" of the point cloud representing the various images in the (Bt; B┴) space.

4. RESULTS ANALYSIS The application of the PSI SPINUA technique to the ERS dataset has led to the identification of numerous radar targets (over 100 PS/km2), suitable for ground motion monitoring in the Wieliczka area. The results show the presence of continuous subsidence with aver-age movements ranging from about 1 to 2 cm/yr in the period 1992-2000 (Fig. 3), while no appreciable dis-placement rates have been detected over all Cracow. The detected subsiding zone well corresponds to the ex-tent of the underground salt mine. The moving PS indicate that a zone of marked subsi-dence extends for at least 4.5 km in roughly E-W direc-tion. Nevertheless, the average LOS velocities show significant spatial variations, with maximum downward movements reaching 27 mm/yr along the ERS Line of Sight (off-nadir angle: θ=20.4°) in the western periphery of the town.

Figure 2. Differential Interferograms generated by com-bining the acquisitions of 05/01/2008 and 20/02/2008, and simulating (and removing) flatearth & topographic phase contributions by interpolating the orbits with a cu-bic spline (on the left) or with an Hermitian approach (on the right). Orbital residuals become unacceptable when discarding the ephemerides velocity information in orbit-al interpolation.

The ground topographic measurements in this area do-cumented about 1 m of vertical subsidence in the period 1970-2000 (i.e. 3 cm/yr along up/down direction, cor-responding to ~27 mm/yr along the LOS), thus in good agreement with ERS data. Significantly lower velocities are present to the east of the Wieliczka centre (generally below 10 mm/yr), as clearly visible in the inset in Fig.3. It is possible that the spatial variations in the subsidence rates in the Wielicz-ka area are linked to the past and recent salt mine histo-ry. Although the number of PALSAR acquisitions is quite limited (6 FBS, 6 FBD), spanning a period of only 2 years (January 2007 – January 2009), SPINUA was ca-pable to retrieve preliminary ground displacement pat-terns that confirms the C- band spatial extent of the sub-siding area, as shown in Fig. 4. A reliable cross-comparison of the results obtained in C- and L-band could be hindered in general by the different pass directions and look angles of the two dataset, with-out a 3D geophysical model for the subsiding area. The predominance of the vertical displacement component in the area mostly affected by the mining-induced sub-sidence could effectively help for a quantitative compar-ison of the results at least in that specific sub-area. However, this is inevitably complicated by the limited number of PALSAR FBS/FBD images so far acquired

that still leads to a poor accuracy of the millimetric dis-placement trends so far estimated. Anyway, this preliminary analysis confirms that the subsidence is still active with displacement rates that are comparable to those detected with ERS imagery for the last decade of the last century. When an higher number of PALSAR images will be available for the AOI, a deeper investigation will be feasible, in order to cover especially the rural areas in the peripheral parts of the mine, because of the L-band higher penetration coefficient on the ground and the consequent lower volumetric and temporal decorrelation in vegetated areas w.r.t. C-band radar data. Furthermore, another advantage of L-band SAR data (wavelength λ = 23.6 cm for ALOS) w.r.t. C-band (λ = 5.6 cm for ERS) is the intrinsic capability to detect faster ground movements without ambiguities, such as those related to sudden subsidence events occasionally occurred in Wieliczka during the recent past (early 1990’s) [1]. Assuming a repeat-pass interval δt coinci-dent with the ALOS orbital repeat cycle (46 days), we have a maximum detectable displacement rate along the line of sight (LOS) vmax = λ/(4δt) = 46.8 cm/yr, com-pared to the value of 14.6 cm/yr valid for the C-band ERS/ENVISAT missions, where the orbital repeat cycle is 35 days.

Table 1. Parameters relevant to the stacks of ERS and ALOS processed images. A/D: ascending/descending pass di-rection; fc: carrier frequency; λ: wavelength; BW: chirp bandwidth; Rgr: ground range resolution; Raz: azimuth resolu-tion;θ,θinc: scene centre look and incidence angle; Bc: critical baseline, estimated for the specific beam & image mode

Sensor Track Frame A/D Pol. fc (GHz) λ

(cm) Rgr (m)

Raz (m) θ (°) θinc (°) Bc (km)

Day/Night acquisition

Swath extent

ERS 179 2601 D VV 5.3 (C band) 5.6 24.5 5 20.4 23.0 1.1 Day 100km

PALSAR FBS

622 990 A HH 1.27 (L band) 23.6 8.6 5 34.3 38.7 15.4 Night 70 km

PALSAR FBD

622 990 A HH+HV 1.27 (L band) 23.6 17.2 5 34.3 38.7 15.4 Night 70 km

Table 2. Dataset of FBS/FBD ALOS PALSAR processed acquisitions (in bold the master acquisition)

Acquisition Date [dd/mm/yyyy]

Cycle Acquisition

Mode Range BW

[MHz] RSR

[MHz] Azimuth BW [Hz]

PRF [Hz]

Doppler Centroid Frequency [Hz] @ near range

Bt [days]

|Bn| [m]

ha [m]

02/01/2007 8 FBS 28 32 1518 2155.172 44.7 -368 3670.1 17.6 17/02/2007 9 FBS 28 32 1518 2132.196 18.7 -322 1974.4 32.6 05/07/2007 12 FBD 14 16 1518 2145.922 35.4 -184 1044.5 61.7 20/08/2007 13 FBD 14 16 1518 2132.196 50.5 -138 905.4 71.2 05/01/2008 16 FBS 28 32 1518 2155.172 5.7 0 0 ∞ 20/02/2008 17 FBS 28 32 1518 2132.196 30.3 46 897.6 71.7 06/04/2008 18 FBS 28 32 1518 2155.172 23.3 92 1460.2 44 22/05/2008 19 FBD 14 16 1518 2145.922 48.2 138 1412.6 45.5 07/07/2008 20 FBD 14 16 1518 2150.538 44.6 184 2314.6 27.8 22/08/2008 21 FBD 14 16 1518 2132.196 30.0 230 5447.8 11.8 22/11/2008 22 FBD 14 16 1518 2145.922 43.8 322 4285.3 15 07/01/2009 24 FBS 28 32 1518 2136.752 64.8 368 3782.7 17

Figure 3. Geo-coded ERS Persistent Scatterers (PS) distribution in the Cracow-Wieliczka area. Background optical

image is from Google Earth. The average Line of Sight (LOS) velocity (for the period 1992-2000) has been satu-rated at ±20 mm/yr for visualisation purposes. Note the predominance of motionless PS in the city of Cracow (up-per-left part of the figure) and the presence of downward moving PS in the town of Wieliczka (lower right part of the figure). The white inset shows the only Wieliczka area with a saturation of ±30 mm/yr for the average LOS ve-locity in order to better appreciate the velocity spatial gradient from the eastern to the western part of the town.

Figure 4. Preliminary ALOS PALSAR Persistent Scatterers (PS) distribution in the Cracow-Wieliczka area. Back-

ground optical image is from Google Earth. The average Line of Sight (LOS) velocity (for the period January 2007-January 2009) has been saturated at ±20 mm/yr for visualisation purposes. ALOS confirms C- band spatial pat-

terns of subsidence in the ancient Wieliczka salt mine. The white inset shows instead the only Wieliczka area.

5. FINAL COMMENTS The application of the PSI SPINUA technique to long historical C-band data records (a dataset of 44 ERS-1/2 images acquired between 1992 and 2000) has led to the identification of a consistent number of radar targets (over 100 PS/km2), suitable for high precision ground motion monitoring in the Wieliczka area. The results show the presence of continuous subsidence with aver-age movements ranging from about 1 to 2 cm/yr in the period 1992-2000. The detected width of the subsiding zone corresponds very well to the extent of the under-ground salt mine, whereas its length (around 4.5 km) is somewhat shorter with respect to that of the mining works and of the known salt deposit. This discrepancy results in part from the lack of suitable radar targets in the rural areas east and west of the town of Wieliczka. The interferometric analysis has been therefore ex-tended to PALSAR imagery, with the aim to extend the PS coverage to larger areas with few man-made fea-tures, where C-band PSI methods fail, thanks to the re-duced temporal and volumetric decorrelation in L band. The present work provides an example of using PAL-SAR data for monitoring slow ground movements, like subsidence, through SPINUA multi-temporal InSAR technique. Although the number of PALSAR acquisitions is quite limited (6 FBS, 6 FBD), spanning a period of only 2 years (January 2007 – January 2009), SPINUA was ca-pable to retrieve preliminary ground displacement pat-terns that confirms the C- band spatial extent of the sub-siding area. From a more general point of view, it can be concluded that L-band and C-band InSAR can play a complemen-tary role in deformation monitoring, since C-band data allow to detect precisely small displacement rates, while L-band data are more sensitive to strong deformation episodes, for which C band is beyond the aliasing limit. Future work will concentrate on the following aspects: � further investigations on the coherence aspects; � investigations about the availability of other data

such as X-band for further comparisons (definition of a “super site” for L-C-X band InSAR application studies).

In order to improve the reliability and the accuracy of the L-band displacement rate map so far derived by SPINUA, the analysis will be extended to a higher number of PALSAR acquisitions, when available.

ACKNOWLEDGEMENTS ERS and PALSAR satellite images were provided by ESA under the ALOS ADEN AO 3595 project.

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