Monitoring of infrastructural sites by means of advancedmulti-temporal DInSAR methods

Andreas Vollratha, Francesco Zuccaa and Salvatore Stramondob

aUniversita degli studi di Pavia, Via Ferrata 1, 27100 Pavia, Italy;bIstituto Nazionale per Geofisica e Vulcanologia (INGV), Via Vigna Murata 605, 00143 Roma, Italy

ABSTRACTWith the launch of Sentinel-1, advanced interferometric measurements will become more applicable then ever. The fore-seen standard Wide Area Product (WAP), with its higher spatial and temporal resolution than comparable SAR missions,will provide the basement for the use of new wide scale and multitemporal analysis. By now the use of SAR interferometrymethods with respect to risk assessment are mainly conducted for active tectonic zones, plate boundaries, volcanoes aswell as urban areas, where local surface movement rates exceed the expected error and enough pixels per area contain arelatively stable phase. This study, in contrast, aims to focus on infrastructural sites that are located outside cities and aretherefore surrounded by rural landscapes. The stumbling bock was given by the communication letter by the EuropeanCommission with regard to the stress tests of nuclear power plants in Europe in 2012. It is mentioned that continuouslyre-evaluated risk and safety assessments are necessary to guarantee highest possible security to the European citizens andenvironment. This is also true for other infrastructural sites, that are prone to diverse geophysical hazards. In combina-tion with GPS and broadband seismology, multitemporal Differential Interferometric SAR approaches demonstrated greatpotential in contributing valuable information to surface movement phenomenas. At this stage of the project, first resultsof the Stamps-MTI approach (combined PSInSAR and SBAS) will be presented for the industrial area around Priolo Gar-gallo in South East Sicily by using ENVISAT ASAR IM mode data from 2003-2010. This area is located between theMalta Escarpment fault system and the Hyblean plateau and is prone to earthquake and tsunami risk. It features a highdensity of oil refineries that are directly located at the coast. The general potential of these techniques with respect to theSENTINEL-1 mission will be shown for this area and a road-map for further improvements is given in order to overcomelimitations that refer to the influence of the atmosphere, orbit or DEM errors. Further steps will also include validation andtectonic modeling for risk assessment.

Keywords: Differential InSAR, SENTINEL-1, surface deformation

1. INTRODUCTIONThe 2011 Tohoku earthquake in Japan and its catastrophic aftermath that triggered the disastrous nuclear accident ofFukushima demonstrated the necessity of continuously re-evaluated risk and safety assessments that consider even acci-dents that are categorized as highly improbable.1 This includes also the consideration of geophysical hazards, especiallyearthquakes.Seismic hazard models represent one meaningful way of assessing potential risk originating from geological subsurfaceprocesses, albeit lead times are usually in the order of years. Nevertheless, their output relies heavily on a priori informationof the local geology. Hence, imperfect knowledge about the boundaries of crustal blocks can lead to an underestimationof hazard from unknown seismogenic sources.2 The 2010/2011 Darfield/Christchurch events in New Zealand exemplar-ily showed that even in well-known regions strong shocks can occur not only along major faults, but also in previouslyunidentified areas of hazard.3

Further author information: (Send correspondence to Andreas Vollrath)Andreas Vollrath: E-mail: andreas.vollrath01@universitadipavia.itFrancesco Zucca: E-mail: francesco.zucca@unipv.itSalvatore Stramondo: E-mail: salvatore.stramondo@ingv.it

Beside GPS-measurements and broadband seismology, space-borne Differential SAR Interferometry (DInSAR) demon-strated its potential to operationally monitor ground displacement. While the accuracy in terms of velocity is comparableto GPS-measurements, the spatial sampling of information is much more dense.4–6 Therefore it can provide a spatiallymore detailed insight into the ongoing seismic processes and geological structures.During the last decades several advanced approaches have been developed in order to tackle the main limitations7–11 suchas decorrelation of the phase signal and atmospheric, orbital and topographic error contributions within the signal by ex-ploiting long time-series of SAR data. These techniques are especially interesting for the estimation of interseismic platemotion, where rates of surface displacement are usually small, while the signal is distributed over large areas.12

In this study the Stamps-MTI (Stanford Method for Persistent Scatterers - Multi Temporal SAR) method is applied10 overan area southeast Sicily.

2. STUDY AREAThe study area, located in south-east Sicily, extends along the Ionian coast from Augusta down to Siracusa and includesits hinterland up to Francofonte. The coastel zone around Priolo-Gargallo features a lot of petro-chemical industry. Inthe north and south there are the urban areas of Augusta and Siracusa. The hinterland is characterised by plantations andagriculture as well as grasslands ( Fig. 1).

Figure 1. Study Area: top left: Overview of the area with its recent earthquake record,13 the PGA hazard classification14 and mainfaults.15 lower left: Corine Land Cover 200616 of the area. lower right: main geological units of the area17

For several aspects the area is of particular interest with regard to seismic ground displacement assessments. Besideanthropogenic deformation sources like water pumping, heavy construction along the coast and local stone quarries it fea-tures a couple of natural phenomena that overlay the seismic signal. A general observed uplift along the coast is overlaidby seasonal vertical displacement caused from sea wedging.Furthermore, the investigated zone is located between major geological fault systems. As part of the Hyblean foreland itacts as an indenter into the Maghrebian thrust belt, which causes the arc-shaped deflection of the belt.18 In the north-west,it adjoins the Scordia-Lentini-Agnone Graben, which is part of the Gela-Catania foredeep zone. In the western and south-western zone, the Hyblean Plateau forms an elevated bulge-structure dissected by several normal and strike-slip faults aspart of the Tertiary orogenic foreland.18 Both structures are bordered in the east by the prominent physiographic MaltaEscarpement fault system. This system’s secondary faults control the development of minor basins at the Ionian coast.There are 3 main sedimentary depositions in the area (Fig. 1). Along the coastline plio-quaternic sediments can be foundin the grabens. The horst structures along the coast and the NNW-SE Ionic sector are characterized by oligocenic-mioceniccarbonat deposits. In the south-east shelf carbonates are dominating the upper crust. In the north plio-pleistocene basaltsof volcanic origin align in a SWW-NEE direction.

3. METHODOLOGYFor the undertaken study we used 96 ENVISAT ASAR Image mode acquisitions between 2003 and 2010 (50 ascending and46 descending scenes, see Tab.1). Data processing was performed by the StaMPS software package following the StaMPS-MTI processing chain.19 This includes the focusing of the raw data with ROI-PAC20 and the interferogram generation bythe DORIS software21 .Considering a quite high number of available scenes, the dataset was divided into 2 periods, 2003-2006 and 2007-2010respectively. Due to the integration of the single-master PS approach, a higher density of slowly-decorrelating phase-filtered (SDPF) pixels was expected, by guaranteeing a still sufficient number of scenes for reliable results simultaneously.Subsequently, the StaMPS methods for PS and the SBAS algorithm were conducted for ascending and descending orbitsfor each period.10 Except for the amplitude dispersion of the PS approach, which was set to 0.42, standard parameters havebeen used.Afterwards, the results were combined using the StaMPS-MTI approach.10 This produces a higher number of pointsoverall, whereas more confidence is given for overlapping pixels.22

Table 1. Overview of the datasets

Asc. (2003-2006) Asc. (2007-2010) Desc. (2003-2006) Desc. (2007-2010)Nr. of Images: 25 25 22 24Master Date: 8.9.04 4.6.08 24.11.04 7.5.08Max. BLperp: -1005m 437m 656m 371mSBAS IFGs: 131 161 100 136

In the post-processing the different data-sets were combined to retrieve the mean velocity fields. First of all the meanvelocities in line-of-sight from the 2 periods of the ascending and descending orbits were combined. Only overlappingpixels have been considered. Subsequently the mean values were calculated.In order to distinguish between vertical and horizontal movement (mv and mh) a simple triangulation, compromising bothline-of-sight vectors (i.e. ascending (a) and descending (d)), was applied:

mh =d− a

2 ∗ sinθ(1)

and

mv =d+ a

2 ∗ cosθ(2)

where θ is the mean incidence angle of the sensor. For simplicity of interpretation we defined the mh as the east meanvelocity field, although the orbit inclination and the north-south movement were disregarded.As a final step the vertical component (i.e. up mean velocity field) and the east mean velocity field have been interpolatedby an ordinary kriging algorithm in order to obtain the displacement rates for the whole area.

4. RESULTSThe processing output of the different methods revealed some general observations that are not in line with the expecta-tions. Even though the major part of the area is non urban, the PS approach clearly outperforms SBAS in terms of detectedSDPF pixels except for 2003-2007 period of the ascending track (Tab. 2). Our assumption is that the altered amplitudedispersion for the PS processing had a major influence, since this led to an increase in the number of PS-candidates. More-over, Fig. 2 shows that the higher density is not attributed to certain land cover classes. Except for the 2003-2007 periodof the ascending track, the PS results exhibits a higher density for the whole area.However, by combining both approaches an increase in terms of SDPF pixels up to almost 200 % with respect to the PSapproach, and up to 350 % for the SBAS approach, could be retrieved. This underlines the complimentary character ofboth approaches.Another feature is the higher density of SDPF pixels for the 2007-2010 period for both orbits in all methods. The smallerbaselines for this period result in a higher coherence so that more stable scatterers could be detected.The aspects of most hillsides from the Hyblean Plateau down to the coast facing to the east are considered as the majorinfluence for the higher point density of the data retrieved by the descending orbit. In addition, smaller baselines of thedescending orbits favored a better coherence.

Table 2. Overview of the datasets

Asc. (2003-2006) Asc. (2007-2010) Desc. (2003-2006) Desc. (2007-2010)PS points (tot / per km2): 34521 / 25 89138 / 65 95270 / 69 109863 / 80

SBAS points (tot / per km2): 36727 / 27 40391 / 29 34510 / 25 41552 / 30MTI points (tot / per km2): 69315 / 50 123282 / 89 119750 / 86 142767 / 103

By comparing the detected stable pixels (Fig. 2) to the CLC 2006 (Fig. 1) it is notable that apart from an almost completecoverage of the urban and industrial areas, a high density of SDPF pixels can be retrieved over grasslands. For agriculturalareas and plantations there is a high divergence between different zones. This is attributable to various plant cultures, butcan not be apportioned without further knowledge of ground truth. As expected, forested areas showed almost no detectedstable scatterers.It is known that the main displacements appear in the north-south direction, due to the northwards drifting of the Nubian

plate. Therefore, they are difficult to detect by INSAR, since it is more sensitive to latitudinal motions. Nevertheless, thepost-processed east-mean velocity field reveals distinguishable patterns that can be attributed to documented processes inthe area.CATALANO ET AL. 2010 reported the occurrence of an NE-SW oriented mobile crustal block for the investigated area.Along the Mt.Climiti Fault, on the eastern side of the Melilli, he identified an extension zone with rates of 0,4 mm/y.23

This feature becomes also visible in our data (Fig. 3), whereby displacement rates are in the order of 2-3 mm/y and aremainly concentrated on top and the western side of the horst. Reoccurring small magnitude earthquakes along local NW-SE oriented faults give additional prove for ongoing seismic activity in this zone. The detected east movement affects boththe carbonatic succession as well as the plio-quaternic sediments of the Floridia basin.In contrast, the north-eastern part moves towards the sea and gives therefore evidence of the extensional process. Here therates are even higher according to our data. However, due to the rough estimate of the vertical movement we consider it asan overestimate.The displacements in the western direction of the Scordia-Lentini-Agnini Graben in the northern part of the covered areais attributed to the general NW movement of the Nubian plate, where main thrust mechanisms take place.An interesting feature represents the eastward movement of the south-eastern and upper-central part which overlays with

Figure 2. The mean velocity fields in line-of-sight for all the processed data and algorithms

the plio-pleistocenic basalts and is to our knowledge not documented in the literature so far. Due to the complicated thrustmechanisms between the Eurasian and Nubian Plate influenced by the Malta Escarpment Fault System and other localphenomena, we believe that it is not a general error in the data, although the rates suffer from inaccuracy introduced byrough post-processing.Fig. 3 also shows the vertical component. The most striking feature is the subsidence of Augusta. Heavy water-pumpingalready reported in other publications24 leads to a distinctive subsidence. Again, our estimates of 7 mm/y are not validatedand are assumed to be biased due to the rough post-processing. Nevertheless, it is notable how the borders of the adjacentcarbonatic horst and the sediments can be distinguished by the data. The other area of subsidence in the upper-center isnear to the town of Villasmundo, an area where water-pumping for agricultural purposes is common as well.24

5. CONCLUSIONSThis study demonstrated the feasibility of capturing small velocity, locally apparent, tectonic movements by means ofmulti-temporal DInSAR for the area of south-east Sicily. A combination of the PS and SBAS methods produced thehighest density of SDPF pixels.Our approach in splitting the data for different periods enabled a successful unwrapping. However, the integration of allscenes would lead to a smaller error introduced by atmosphere. Most recently, DInSAR methods, that exploit not onlypoints that are coherent over all the period give promising results.25 Hereby the pixels, which are coherent over all theperiod would give a smaller error, whereby intermittent scatterers contribute to a higher density of information.We are aware that our results are biased by a considerable high error with respect to the retrieved strain rates due to thesplitting in time and the rough post-processing. We are however optimistic to optimize our results by integrating GPSmeasurements and implementing atmospheric weather records. In addition, we assume to obtain even more SDPF pixelsfor the PS approach in rural areas by oversampling the data. Similar studies already demonstrated the positive effect.26

Figure 3. Interpolated east and up mean velocity field seen from SEE direction

As soon as we have the GPS data available, a re-estimation of the 3d deformation processes in the post-processing stepwill be conducted following the approach of WRIGHT ET AL. 2004.5 Since the orbit inclination, local incidence anglesand the contribution of the north-south movement constrained by the GPS are considered, resultant mean velocity fieldsshould exhibit an error of not more than 1 mm/y according to other studies.4, 5

ACKNOWLEDGMENTSThis research was funded by the MIUR, University of Pavia. The work was conducted at the INGV in Rome. Fruitfuldiscussions and valuable input was given by Carlo-Alberto Brunori (INGV Roma), Guido Ventura (INGV Roma), FabianoCostantini (Univ. di Tor Vergata/ESA) & Christian Bignami (INGV Roma). Additionally the authors want to acknowledgethe European Space Agency for the kind provision of ENVISAT data under the project ID 13948.

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