Engineering Geology 115 (2010) 105–121

Contents lists available at ScienceDirect

Engineering Geology

j ourna l homepage: www.e lsev ie r.com/ locate /enggeo

Study of the land subsidence in Orihuela City (SE Spain) using PSI data: Distribution,evolution and correlation with conditioning and triggering factors

R. Tomas a,⁎, G. Herrera b, J.M. Lopez-Sanchez c, F. Vicente c, A. Cuenca a, J.J. Mallorquí d

a Departamento de Ingeniería de la Construcción, Obras Públicas e Infraestructura Urbana, Escuela Politécnica Superior, Universidad de Alicante, P.O. Box 99, E-03080 Alicante, Spainb Área de Peligrosidad y Riesgos Geológicos, Departamento de Investigación y Prospectiva Geocientífica, Instituto Geológico y Minero de España (IGME), Ministerio de Ciencia y Tecnología,c/Ríos Rosas 23, E-28003 Madrid, Spainc Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal (DFISTS), Escuela Politécnica Superior, Universidad de Alicante, P.O. Box 99, E-03080 Alicante, Spaind Remote Sensing Lab. (RSLab), Dept. of Signal Theory and Communications, Universitat Politècnica de Catalunya (UPC), C/Jordi Girona, 1-3, Ed. D-3, E-08034, Barcelona, Spain

⁎ Corresponding author.E-mail addresses: roberto.tomas@ua.es (R. Tomas), g

juanma.lopez@ua.es (J.M. Lopez-Sanchez), fernando.viceartemio.cuenca@ua.es (A. Cuenca), mallorqui@tsc.upc.ed

0013-7952/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.enggeo.2010.06.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 November 2009Received in revised form 3 June 2010Accepted 19 June 2010Available online 1 July 2010

Keywords:SubsidenceGround water extractionDInSARPSISpatial analysis

This paper presents a study about the subsidence affecting the city of Orihuela, placed on the Western sectorof the Vega Baja of the Segura River Basin (SE Spain). The study is based on deformation data obtained byDifferential SAR Interferometry (DInSAR), and more precisely by the coherent pixels technique (CPT), analgorithm of the Persistent Scatterers Interferometry (PSI) family. Images acquired between June 1993 andApril 2009 by ERS and ENVISAT satellite sensors have been processed by the CPT technique in order toretrieve the ground subsidence affecting this area. Accumulated deformation maps and displacement timeseries are analyzed. Moreover, these data have been contrasted with subsidence triggering and conditioningfactors by means of a Geographical Information System (GIS) interface. The performed analysis shows thatthere exists a relationship between subsidence and piezometric level evolution in time. Moreover, thecorrelations among subsidence and location of pumping wells, distance to the river, geology, and soilthickness have been derived from this analysis.

.herrera@igme.es (G. Herrera),nte@ua.es (F. Vicente),u (J.J. Mallorquí).

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Subsidence is an important hazard closely related with thedevelopment of urban areas. In general, a complete study of theground surface deformation requires a wide and continuous spatialand temporal monitoring. With this information, a better compre-hension of the mechanisms that govern ground consolidation can beachieved, hence contributing to prevent or reduce the negative effectsof such risks on buildings and infrastructures. In this work we focus ona case of subsidence caused by aquifer overexploitation, which usuallyentails the settlement of ground surface over wide areas. Actuallythere exist hundreds of cities in the world with problems ofsubsidence due to excessive groundwater extraction. In Spain, morethan 51 overexploited hydrogeological units are reported (Custodio,2002), and some of them (depending of the type of aquifer system)might potentially cause ground subsidence. Murcia city, located in theVega Media of the Segura River, was the first case in Spain wheresubsidence due to aquifer overexploitation was manifested as a resultof a severe drought period in 1992–95, causing important damagesascending to 50 million euros (Rodríguez Ortiz and Mulas, 2002;

Mulas et al., 2003). During that period the near Vega Baja of the SeguraRiver also suffered the same process. The maximum piezometricdepletion reached 15m in this area, although neither complaint reportsnor damages in buildings were reported, except a few structuralincidents in historical buildings located at W of the Orihuela City.

In this work we analyze the subsidence occurred in the 1993–2009period in the Western sector of the Vega Baja of the Segura River (SESpain) where the historical city of Orihuela is placed. Firstly weexamine the temporal evolution of ground deformation (provided byDInSAR), which is closely linked to the pumping activity on this areaduring drought periods. Then ground surface deformation is alsoanalyzed in terms of piezometric measurements, geological units, softsoil thickness and well point distribution. Moreover, a simple spatialanalysis using GIS tools of the conditioning and triggering factors thatcontrol subsidence mechanisms due to ground water extraction isperformed.

2. Description of the study area. Geological and geotechnicalsetting

The Vega Baja of the Segura River (VBSR) is located in the orientalsector of the Betic Cordillera and is connected to the MediterraneanSea. The study area constitutes a monoclinal structure essentiallycontrolled by the strike–slip Crevillente Fault Zone at the N. This faultzone represents the convergence of two main structures of the Betic

106 R. Tomas et al. / Engineering Geology 115 (2010) 105–121

Cordillera: the Cádiz–Alicante Fault System (Sanz de Galdeano, 1990)and the Trans-Alborán Shear Zone (De Larouziére et al., 1988).

The Mesozoic basement of the basin consists of carbonate andevaporitic rocks from the Betic External Zones (External Subbetic andInternal Prebetic) crop out at the N and E area of the study zone. TheBajo Segura Basin is filled by Neogene–Quaternary sediments (Fig. 1).

The materials found in the valley are Holocene sediments at theground surface beyond Pleistocene sediments deposited by RiverSegura, whereas the eastern zones towards the Mediterranean Sea areoccupied by littoral and lagoonal sediments. Anthropic deposits canbe also found at certain points in the valley generally related withurban areas. Recent sediments are the most compressible ones in thezone and the most problematic from a geotechnical point of view.Delgado et al. (2002) made a geotechnical characterization of all thesematerials for the VBSR. This model shows that the same sedimentaryrocks outcropping at the valley borders are also found at some depthin the valley, varying between 0 to 30 m towards the W of the valleyand 0 to N60 m close to the river end (Delgado et al., 2000), andconstitute the geotechnical substratum of the area (Fig. 2). Thesesedimentary rocks exhibit much better geotechnical properties thanthe recent sediments (Fig. 2). The Standard Penetration Tests (SPT) of

Fig. 1. Location and geological scheme of the

these soils provide values higher than 30, and there is an importantnumber of tests with refusal.

Above this “basement”, recent sediments are characterized bymoderate to high compressibility, with compression indexes (Cc)varying from 0.07 to 0.29 and with an average value of 0.18. Thesesediments are less compressible at the first 5 m depth due to theirhigh overconsolidation (average OCR of 5.3) due to knownwater levelfluctuations (Tomás et al., 2007a). This overconsolidation is lowerfrom 5 to 10 m, with average OCR values of 2.1, and 1.3 for largerdepths (Tomás et al., 2007a). The SPT values of these soils vary from 0to 31 with an average value of 10.

From a hydrogeological point of view, the study area is part of theso-called “Guadalentín–Segura Quaternary aquifer System N° 47”(IGME, 1986). This aquifer is characterized by two units. The first one,a surface unit, is an unconfined aquifer, of fine sand and silts depositedby the recent activity of the Segura River and coastal processes(towards the E of the zone). The water table of this aquifer is found afew meters below the ground surface. Due to the high fine content ofsediments this aquifer is characterized by low hydraulic conductivity.The second unit is formed by gravels, usually stratified interbeddedwith marls, and constitute a confined aquifer with greater hydraulic

Vega Baja and Media of the Segura River.

Fig.

2.Geo

tech

nicala

ndhy

drog

eologicalb

oreh

oles.S

eebo

reho

leslocation

inFig.

3.

107R. Tomas et al. / Engineering Geology 115 (2010) 105–121

Fig. 3. Location of available hydrological and geotechnical data in the study area.

108 R. Tomas et al. / Engineering Geology 115 (2010) 105–121

conductivity than the superficial aquifer, with values obtained frompumping tests varying from 34 to 264 m/day (DPA-ITGE, 1996).

3. Previous researches and available data

During the 1994–97 period, an excessive water withdrawal derivedfrom the necessity of alternative water sources, due to the highmeteorological draught existing at that moment, caused an importantpiezometric level drop that entailed an important consolidation of soildue to pore water pressure reduction in the whole valley (Mulas et al.,2003; Martínez et al., 2004; Tomás et al., 2005; 2008; Herrera et al.,2009a,b). This consolidation is manifested at ground surface as asettlement that can affect buildings and infrastructures.

A small area located at the W of Orihuela City was damaged bysubsidence during that period. This fact was published in local pressduring several months, but did not cause the same local alarm than, forinstance, inMurcia City (located in the same basin). Nowadays buildingdifferential settlements, dilatation joints apertures, sidewalks settle-ment, cracks on buildings and other pathologies can be frequently foundin Orihuela City. Whereas in Murcia City the local authorities haveinvested large quantities of technical resources to study andmonitoringsubsidence, in Orihuela City there not exists any information directlyrelated with subsidence over the whole study period. Consequently, noquantitative data about ground vertical movements are available, withthe exception of a DInSAR processing conducted for the period 1993–2001 with a previous version of CPT and only using ERS images (Tomáset al., 2007b). In thatwork, a qualitative comparisonwith a fewavailablegeotechnical data was also presented.

DPA-ITGE (1996) carried out a hydrological study of the wholeVBSR area in order to analyze the influence of the construction ofdesalation plants in the aquifer system. More recently, DPA-IGME(2002) have elaborated a free software tool with basic hydrologicalinformation of the whole Alicante province, including the VBSR area.

The hydrographical authority of the Segura river basin conducted ahydrogeological study (CHS, 2007) of the Vega Baja and Media of theSegura River in order to improve the knowledge of the deep aquifer.

From 1993 to present, hundreds of kilometers of roads and newbuildings have been built in the Vega Baja area providing an importantamount of geological–geotechnical data. The available data inOrihuela City and its surroundings have been used in this paper.

On the other hand, Delgado et al. (2000) published soft soil mapsof the whole Vega Baja of the Segura River obtained by means ofgeophysical techniques. Delgado et al. (2002) also proposed ageological–geotechnical model of the Vega Baja of the Segura Riverbasin with average values of principal properties of soil. Tomás et al.(2007a) studied the overconsolidation ratio of the Vega Media andBaja of the Segura River, which demonstrated the high overconsolida-tion of the more superficial layers of soil.

As a consequence, available information is very dispersed andheterogeneous, especially the hydrological information related withthe exploitation of the aquifer (water flow rates, exploited levels,piezometric levels, etc.), due to the sensible policy of the local andnational governments with the water resources management of thisarea. Fig. 3 shows the spatial distribution of the geotechnical/hydrological boreholes and the piezometers used in this study. Thelithological description of the mentioned boreholes and the maingeotechnical properties of soils are graphically shown in Fig. 2.

This work studies a new area affected by water withdrawalinduced subsidence in Spain, providing deformation data for morethan a decade. In fact, these results constitute the longest subsidenceobservation time span analyzed bymeans of CPT so far. Moreover, thisarea is characterized, in comparison with similar subsidence areasstudied with CPT as Murcia, by the proximity of the stable relieves, alarger depth of soft soils, larger extreme subsidence values, and by alimited number of hydrogeological information and the inexistence ofin situ subsidencemeasurements. In addition, a simple spatial analysis

Table 1ERS and ENVISAT SAR SLC data used for the analyzed period 1993–2009.

Nr Year Month Day Sensor Nr Year Month Day Sensor Nr Year Month Day Sensor

1 1993 06 23 ers1 32 2000 05 06 ers2 63 2005 02 19 envi2 1993 11 10 ers1 33 2000 06 10 ers2 64 2005 03 26 envi3 1995 07 21 ers1 34 2000 07 15 ers2 65 2005 04 30 envi4 1995 08 26 ers2 35 2000 08 19 ers2 66 2005 06 04 envi5 1995 09 30 ers2 36 2000 09 23 ers2 67 2005 06 04 ers26 1995 11 04 ers2 37 2000 10 28 ers2 68 2005 08 13 envi7 1996 07 06 ers2 38 2000 12 02 ers2 69 2005 09 17 envi8 1996 10 19 ers2 39 2001 01 06 ers2 70 2005 11 26 envi9 1996 11 23 ers2 40 2001 08 04 ers2 71 2005 11 26 ers210 1997 04 12 ers2 41 2001 09 08 ers2 72 2005 12 31 envi11 1997 05 17 ers2 42 2001 10 13 ers2 73 2006 04 15 envi12 1997 07 26 ers2 43 2001 11 17 ers2 74 2006 06 24 envi13 1997 08 30 ers2 44 2002 03 02 ers2 75 2006 07 29 ers214 1997 10 04 ers2 45 2002 08 24 ers2 76 2007 03 31 envi15 1997 11 08 ers2 46 2002 11 02 ers2 77 2007 10 27 ers216 1998 01 17 ers2 47 2003 01 11 ers2 78 2008 01 05 envi17 1998 03 28 ers2 48 2003 03 22 envi 79 2008 01 05 ers218 1998 07 11 ers2 49 2003 10 18 envi 80 2008 02 09 envi19 1998 09 19 ers2 50 2003 11 22 ers2 81 2008 03 15 envi20 1998 11 28 ers2 51 2003 12 27 envi 82 2008 03 15 ers221 1999 03 13 ers2 52 2004 01 31 envi 83 2008 04 19 envi22 1999 04 17 ers2 53 2004 03 06 envi 84 2008 05 24 envi23 1999 05 22 ers2 54 2004 04 10 envi 85 2008 06 28 envi24 1999 07 31 ers2 55 2004 05 15 envi 86 2008 08 02 envi25 1999 09 04 ers2 56 2004 06 19 envi 87 2008 09 06 envi26 1999 10 09 ers2 57 2004 10 02 ers2 88 2008 10 11 envi27 1999 11 13 ers2 58 2004 11 06 envi 89 2008 11 15 envi28 1999 12 18 ers2 59 2004 11 06 ers2 90 2008 12 20 envi29 2000 01 22 ers2 60 2004 12 11 envi 91 2009 02 28 envi30 2000 02 26 ers2 61 2004 12 11 ers2 92 2009 04 04 envi31 2000 04 01 ers2 62 2005 01 15 envi

109R. Tomas et al. / Engineering Geology 115 (2010) 105–121

of the causative and conditioning subsidence factors has beenperformed for the first time in this area.

4. PSI applied to Vega Baja of the Segura River subsidence study

4.1. General methodology

The differential interferometric phase variation (Δψint) betweentwo neighboring pixels, obtained by combining two complex SARimages, can be expressed as an addition of several terms (Hanssen,2001):

Δψint = Δψflat + Δψtopo + Δψmov + Δψatmos + Δψnoise ð1Þ

where Δψflat is the flat-earth component related with range distancedifferences in absence of topography, Δψtopo is the topographic phase,Δψmov is the phase contribution due to ground displacementsoccurred between the two SAR acquisitions, measured along theline of sight (LOS), Δψatmos is the phase component due toatmospheric disturbances or artifacts, and, finally, Δψnoise includesthe rest of noise sources. The first two terms in Eq. (1) can beexpressed analytically. More specifically, Δψflat is assumed to beknown, and Δψtopo can be extracted from an external DEM. So, if weassume thatΔψnoise andΔψatmos are negligible we can determinate thephase contribution due to ground deformation. This is the basis ofconventional DInSAR techniques. Nevertheless, atmospheric andnoise phase contributions are unknown, and the atmospheric termcan be of the same order of the phase term produced by deformation.A significant improvement in the quality of the DInSAR results is givenby a family of algorithms called Persistent Scatterer Interferometry(PSI) which are based on the simultaneous processing of multipleinterferograms derived from a large set of SAR images and only over asubset of quality pixels selected from the whole images (Ferretti et al.,2000; 2001; Berardino et al., 2002; Arnaud et al., 2003; Mora et al.,2003; Werner et al., 2003; Hooper et al., 2004).

In this study, we have made use of the Coherent Pixels Technique(CPT) algorithm (Mora, 2004), a PSI technique that allows to isolatethe contribution of atmospheric artifacts and to minimize the noiseterm. It assumes that the phase component linked to deformation(Δψmov) can be decomposed, as shown in Eq. (2), into two new phaseterms, one due to linear deformation (Δψlinear), that is, with a constantvelocity or rate, and other due to non linear deformation (Δψnon-linear).

Δψmov = Δψlinear + Δψnon�linear =4πλ

⋅Δv⋅T +4πλ

⋅Δρnon�linear ð2Þ

where λ is the radar wavelength, Δν is the velocity incrementbetween pixels, T is the temporal separation between both SARacquisitions, and Δρnon-linear is the non linear term of the surfacedeformation.

The CPT processing is divided into two main steps, correspondingto the extraction of both linear and non-linear components. Firstly,the retrieval of the linear term includes the estimation of both themean velocity deformation and the DEM error. They are calculated bymeans of the adjustment of a model function applied only over therelations of those pixels of the scene, selected with an iterative multi-layer process (Blanco et al., 2006), that show a good interferometriccoherence along time. The coherence uses spatial averages ofneighboring pixels to estimate the phase quality of an interferogram.Coherences close to one indicates excellent phase quality whilecoherences near to zero total decorrelation and useless phases. That is,those pixels whose coherencewas above a certain thresholds for morethan an established percentage of the interferograms. An integrationprocess is needed to obtain absolute value of velocity and DEM errorfor each pixel. This integration starts from a predetermined number ofseeds that are stable points with zero velocity. In this case, seeds havebeen placed attending to geological criteria over non deformable areascorresponding to the Permo-Triassic and the Miocene/Pliocene reliefsof the study area.

Fig. 4. Cumulative displacement along LOS for the period 1993–2009 in the lower part of Segura River basin.

110 R. Tomas et al. / Engineering Geology 115 (2010) 105–121

Then, the non-linear term is estimated applying a spatio-temporalfiltering to extract the contribution of atmospheric artifacts and thelow and high resolution components of the non-linear deformation.Atmospheric isolation is possible because of the different frequencycharacteristics of the atmospheric artifacts in time and space withrespect to the non linear movement. Atmospheric perturbations showa correlation distance of approximately 1 km (Hanssen, 2001), hencewith a low spatial frequency signal in each independent interfero-gram. However, for a single pixel, atmospheric contribution can beconsidered a random variable in time, i.e. changing randomly fromone date to another. In contrast, non-linear movements show asmaller correlation window in space and a lowpass behavior alongtime. Therefore, a low spatial frequency non-linear component isderived by removing the atmospheric artifacts with a filter in time,whereas a high resolution non-linear term can be estimated from theremaining part of the spatial spectrum (see Mora et al. (2003), andBlanco et al. (2008), for details).

The retrieval of the deformation maps from SAR imagery is acomplex job involving several steps: image focusing from the rawdata, image co-registration and filtering for the generation of theinterferograms, proper calculation of satellite orbits, and the DInSAR

processing itself previously described. The complexity of the differentsteps involved in the whole chain and the different processingconditions (number of images available, baseline distribution,Doppler differences, etc.) make it almost impossible to find a closeexpression for evaluating the error budget of this technology. Fromthe results obtained using this technique with different datasets(Mora et al. 2003; Lanari et al., 2004; Tomás et al., 2005; Herrera et al.,2009b), comparing them with available ground-truth, and looking atthe oscillations in the deformation pattern for stable zones, it can beassumed that the error in estimating accumulated non-lineardeformation is around ±5 mm in the worst case. In contrast, thelinear estimate is more robust, as it comes from amodel adjustment toall available data, and can achieve a millimetric precision. In all cases,the larger the number of images, the more precise the results are forboth linear and non-linear estimates. However, the error may exhibita strong dependence on the conditions of the acquired images, andthemost perturbing elements are the atmospheric artifacts, which canbe coupled with the non-linear deformation.

A detailed description of the whole algorithm can be consulted inMora et al. (2003) and Blanco et al. (2008). This summary wasincluded here for the sake of completeness.

Fig. 5. Subsidence projected along LOS in the city of Orihuela from 1993 to 2009.

111R. Tomas et al. / Engineering Geology 115 (2010) 105–121

Fig.

6.Re

lation

ship

betw

eensu

bsiden

ceprojectedalon

gLO

San

dpiez

ometricleve

latseve

ralp

ointsof

theVBS

R.Se

ewellloc

ations

inFigs.1

and3.

112 R. Tomas et al. / Engineering Geology 115 (2010) 105–121

Fig. 7. Cumulative subsidence projected along LOS from 1993 to 2009 superimposed to soft soil thickness.

113R. Tomas et al. / Engineering Geology 115 (2010) 105–121

In general, DInSAR techniques have been demonstrated to be veryuseful for the study of aquifer compaction due to water withdrawal,but they exhibit several well known limitations for these applicationsas the rapid loss of coherence between two acquisitions for manykinds of terrains that causes temporal decorrelations (specially inagricultural and vegetated areas), the atmospheric effects that canaffect the deformation estimation, the availability of the images thatdepends on the satellites repeat-orbit cycle (35 days for ERS andENVISAT sensors), and the low sensitivity to horizontal displacementsin comparison to vertical ones (Galloway and Hoffmann, 2007).

4.2. Data set and processing details

A set of 92 SLC SAR images acquired by European Spatial Agencyremote sensing satellites ERS-1 (3 images), ERS-2 (54 images) andEnvisat (35 images) between June 1993 and April 2009 has been usedin this study (Table 1). Note that no valid images are available in 1994and, as a consequence, the non linear deformations occurred duringthis period cannot be retrieved. However, the linear trend estimatesare not importantly affected by this gap, as demonstrated in previousworks in the near area of Murcia city (Mora et al., 2003; Tomás et al.,

2005, Herrera et al., 2009b). An area of about 16×16 km centered inOrihuela City was selected from the original images.

Among all the possible interferograms formed by combining theavailable SAR images (but not combining ENVISAT with ERS due totheir slightly different center frequencies), only 217 interferogramshave been selected for the CPT processing by means of a Delaunaytriangulation in the 3-D space formed by the spatial and temporalbaselines and Doppler differences. This choice has been limited tothose interferogramswith spatial and temporal baselines smaller than300 m and 1000 days, respectively, and Doppler centroid differencesbelow 500 Hz. The external DEM used to cancel the topographiccomponent of the interferometric phase has a resolution of 25×25 mand belongs to the cartographic numeric database E20 from IGN(National Cartographical Service of Spain).

The estimation of the deformation was computed in those coherentpixels (CPs) selected with an iterativemulti-layer process (Blanco et al.,2006) whose coherence is above a certain threshold for most of theinterferograms (in this study case0.40 and 0.35 formore than 45%of theinterferograms set). Among other advantages, this multilayer approachenables the computation of subsidence at low quality pixels becausethey are helped by those with higher quality previously employed

Fig. 8. Relationship between soft soil thickness and subsidence.

114 R. Tomas et al. / Engineering Geology 115 (2010) 105–121

(Blanco et al., 2006; 2008). Coherence computation implies a spatialaveraging of the interferograms, also known as multi-looking, thatdegrades the original resolution of the SAR images. Due to the multi-look factor of 20×4 pixels (azimuth×range), the final resolution of thedeformation results has been degraded to 80×80 m.

5. Spatiotemporal evolution of subsidence

This section is dedicated to present and analyze the results obtainedwith this PSI technique. The CPT processing of orbital SAR images haspermitted to measure ground surface deformation in 7433 coherentpixels (CPs) in a 16.4 km×16.3 km area (28 CPs/km2). Fig. 4 shows that

Fig. 9. Left: Geological map of the city of Orihuela. Right: subsidence pr

the greatest concentration of CPs is found on urban areas and rockoutcrops of the surrounding mountains, whereas a low density iscommon of irrigated agricultural areas defined by Gumiel et al. (1999).

Fig. 4 presents the cumulative deformation at ground surface from1993 to 2009 projected along LOS. During this period the maximumand average values of subsidence for the whole study area have been17.6 and 1.2 cm, respectively. Ground surface deformations arenegligible in the reliefs of Sierra de Orihuela and Sierra de Pujálvarez.In contrast, the maximum values of accumulated subsidence arefound in the flood plain area, where deformable lithologies arepresent. Within this area the towns of El Raal, Beniel (W of the studyarea) and the city of Orihuela exhibit the greatest values of deformation.

ojected along LOS at the city of Orihuela for the period 1993–2009.

Fig. 10. a) Geological cross section of the Orihuela City and b) accumulated subsidence isochrones obtained by interpolation of CPT data (see cross section location and boreholeslithology in Figs. 2 and 3, respectively).

115R. Tomas et al. / Engineering Geology 115 (2010) 105–121

Significant deformations have been also obtained towards the South-East, in La Pedrera Dam. This is an earth damwith a clay core 61 m highand 700 m longbuilt in 1983, and founded overMiocene lithologies. Thedetected deformations on the dam are probably due to the compactionof the earth dam material and are currently object of further research.

Fig. 5 shows the temporal–spatial evolution of subsidence retrievedfrom 1993 to 2009, superimposed over an orthoimage centered in thecity of Orihuela. It can be appreciated that theNorth-Western part of thecity, founded on the Triassic relief of Sierra de Orihuela, is stable alongthe whole monitoring period. The North-Eastern part of the urban areadefines the contact between the Triassic relief and the Holocene floodplain deposits, providing a moderate accumulated deformation be-tween 2 and 4 cm. On the other hand the greatest deformation values,up to 8 cm in 2009, are found towards the South. In fact following theNorth–South direction from theTriassic reliefswe observe a progressiveincrease of deformation towards the South where the maximumthickness of deformable deposits is located.

6. Analysis of subsidence conditioning and triggering factors

Ground subsidence is determinedbygeometrical factors (i.e. soft soilthickness), geological factors (i.e. geological age and lithologies),hydrogeological factors (i.e. piezometric levels of different aquiferlayers), and geotechnical factors (i.e. soil deformability). These factorscan be divided into two main groups: conditioning or passive factors,which are inherent to thematerials of the aquifer system, and triggeringor active factors that cause subsidence once certain conditions concur.

In the next subsections, the obtained values of subsidence havebeen spatially and/or punctually compared with several conditioningfactors (geological units, distribution of water extraction points,distance to the Segura River, and soft soil thickness) and a triggeringfactor (piezometric level evolution).

6.1. Piezometric level changes

In the study area, drought periods entail an increase of groundwater extraction in authorized drought-wells and illegal wells whileground infiltrations are reduced due to low precipitations, triggeringthe depletion of the ground water level. The 1994–97 piezometriccrisis caused a generalized lowering of 8–10 m, withmaximum valuesexceeding 15 m (Fig. 2). More recently, since 2005, greater piezo-metric falls have been measured (Fig. 2).

The time series of DInSAR measured ground deformations show agood correlation with piezometric level changes. Fig. 6 shows therelationship between measured surface deformation and the onlyavailable long piezometric level time-series, corresponding to fourrepresentative sites located over deformable sediments.

Ground deformation was fast in sites a, c and d during thepiezometric level lowering occurred between 1993 and 1995, and didnot stop until the piezometric level of the aquifer completelyrecovered its previous level in 1997. This can be interpreted asdeformation occurring while ground water conditions remain altered.During 2000 and 2001, a remarkable piezometric level fluctuation

Fig. 11. Temporal evolution of subsidence projected along LOS at several points of the city of Orihuela. See borehole locations in Fig. 3, and lithological description and geotechnicalproperties in Fig. 2.

116 R. Tomas et al. / Engineering Geology 115 (2010) 105–121

lower than 6 m was measured, causing a smaller subsidence than inthe previous mentioned period.

The last piezometric level depletion (started in 2005) reachedpiezometric falls higher than 10 m and caused an important consolida-tion of unconsolidated sediments of the aquifer system (Fig. 6).

6.2. Soft soil thickness

The thickness of late Pleistocene to Holocene soft sediments thatfill the Segura River flood plain was established by Delgado et al.

(2000) using microtremors as a geophysical exploration tool. Theobtained results show that soft sediments thickness increases towardsthe centre of the valley reaching 50 m thick in some points. In the caseof the city of Orihuela, soft soil thickness reaches 40 m in theproximities to the Segura River. On the other hand, in the proximitiesof the existing relieves no or thin soft soils thickness are found. In thiszone Triassic dolomite rock outcrops are being used as the foundationof several buildings. Fig. 7 shows the DInSAR subsidence computed forthe 1993–2009 period superimposed to the soft soil thickness map.The latter has been derived from Delgado et al. (2000) maps including

Fig. 12. Pumping wells of the study area superimposed over cumulative subsidence projected along LOS.

117R. Tomas et al. / Engineering Geology 115 (2010) 105–121

additional geotechnical and hydrological information from newavailable boreholes drilled in the study area.

In order to analyze the relationship between accumulateddeformation and soft soil thickness, the mean and the standarddeviation of the subsidence of all the pixels (CPs) located within thedifferent compressible thickness areas have been calculated. Fig. 8shows, as it was expected, that larger subsidence is present in areaswhere soft soil is thicker. One can observe that accumulatedsubsidence varies in the range 1.1±2.0 cm and 4.0±1.3 cm for softsoil thickness varying from 0 to 10 m and N50 m, respectively.

6.3. The geological units

The city of Orihuela is located at the toe of a mountain. This relief iscomposed of carbonatic rocks (mainly dolomites) of the Permo-Triassic age where subsidence is not expected. In this formation noground surface deformation has been retrieved from CPT analysis(Fig. 9). Historically, the city has developed around a part of this relief,occupying the soft soil flood plain Holocene sediments whosethickness increases from the topographical relief towards the centerof the valley. However, several buildings are totally founded or even

excavated partially on the dolomites that constitute the relief, such asSantiago's church. Hence, the pixels detected over non deformablePermo-Trassic rocks exhibit a stable behavior along time. Associatedwith the Segura River there are channel deposits that are highlyaltered due to anthropogenic actuations, as well as an abandonedmeander found at the West part of the city. Even though the aerialextent of these geological units is reduced, a few pixels have beendetected providing high deformation values. Finally, the greatestamount of pixels has been detected on the flood plain zone of theSegura River. These points show accumulated deformations from1993 to 2009 that vary from 3 to 9 cm. Concerning the spatialdistribution of the measurements, one can appreciate in Fig. 6 that thehighest density of CPs is found on the urban area, whereas no CPs havebeen detected on agricultural fields. This is because the response ofthe buildings to the satellite radar signal is stable in time, whereas theseasonal variability of the ground surface of agricultural fieldsprovides an unstable radar response in time, and hence no coherentpixels are detected.

In this sense, the spatial analysis performed for the two maingeological units existing in Orihuela City shows that the ave-rage subsidence in the Permo-Triassic lithologies is very low (0.4±

Fig. 13. Subsidence projected along LOS variation with respect to the (a) drought pumping wells distance and (b) other pumping wells. Maximum (Max), minimum (Min), average(Mean) and standard deviation (±σ) of subsidence. See spatial distribution in Fig. 12.

118 R. Tomas et al. / Engineering Geology 115 (2010) 105–121

0.8 cm) in comparison with the one corresponding to flood plain areas(−4.9±2.7 cm).

Fig. 10a shows a geological cross section from north to south ofOrihuela City (see location in Fig. 3). Holocene sediments of the basinhave been delimited using several available boreholes (see boreholeslithological description on Fig. 2). The bottom figure (Fig. 10b) showsthe isochrones of the interpolated ground subsidence along the crosssection obtained by means of DInSAR. As it can be observed,subsidence increases towards the center of the basin, being almostnil in the Permo-Triassic relieves, and shows a gradual increaseproportional to Holocene sediments thickness.

Fig. 11 presents the temporal evolution of subsidence at thelocation of several available boreholes. Associated piezometric levelfalls and soft soil thicknesses (considering it as the soil existing overthe gravels) have been represented in this figure too.

Analyzing Figs. 2, 3 and 11 we can retrieve several interestingconclusions. Northern boreholes (O1, Palm3 and A7S2) show thelower compression indexes (Cc) with values lower than 0.20 givingaccumulated settlements lower than 3 cm. The rest of boreholeslocated on the flood plain (IGMES4, SNAg, O4, O3 and ElVeg) presentusually a higher compressibility (Cc higher than 0.20) and soft soilthickness (up to 40 m), and as it is expected settlements are largerthan at northern boreholes, with values up to 5 cm. Unfortunatelypiezometric information is only available for a few points in this area,which show piezometric level falls for the 1994–97 and the 2005–09periods up to 6 and 10 m, respectively. One borehole (Big2) is locatedon Bigastro town, over marly Pliocene–Miocene rocks. This material is

characterized by its better deformational behavior in comparisonwiththe Quaternary sediments and, consequently, lower deformations areexpected even for higher piezometric level falls.

6.4. Well points distribution

Wells represent an alternative water source during droughtperiods. Hundreds of these wells have been drilled along the VBSRbasin since the 80 s (Fig. 12). A field inventory of these wells wasperformed by local authorities (DPA-IGME, 2002). Unfortunately, thisinventory only contains basic information: code, location, depths, anduse. Therefore, the inventory has been employed in this section inorder to perform a very simple spatial analysis about the influence ofthe presence of these wells on subsidence. 81.4% of these wells havedepths lower that 40 m, thus pumping water from the first unit of theaquifer. The rest of the wells withdraw water from larger depths,hence affecting the deep aquifer unit.

The last legal “drought” wells were drilled in 2004 by the WaterHydrographic Authority (CHS, 2007) in order to maintain water supplyduring drought periods. A total of 17 drought wells (Figs. 12 and 14a)with depths varying from 55 to 216 m and an average value of 170 m(CHS, 2007) were drilled in the proximities of the Segura River (88% ofthem are closer than 500 m). These wells currently withdrawimportant water flows from depths higher than 42 m, that vary from40 to 250 m3/s with an average value of 98 m3/s (CHS, 2007).

When groundwater is pumped from an aquifer in a homogeneoussoil, a groundwater cone of depression is formed around the well. This

Fig. 14. (a) Number of inventoried wells with respect to the river axis distance. (b) Maximum (Max), minimum (Min), average (Mean) and standard deviation (±σ) of subsidenceprojected along LOS variation with respect to the river axis distance. See wells location in Fig. 12.

119R. Tomas et al. / Engineering Geology 115 (2010) 105–121

cone of depression leads to differential settlements that increase withthe proximities to thewell. As a result, similar values of subsidence areexpected at similar distances from a well with the same water headdrop. Therefore, a ring buffer analysis seems to match this problemand can be used to study the influence of wells on subsidence.However, we have to take into account that the variability of soilsdistribution can distort the shape of the cone of depletion, causingdeviations on the results of the spatial analysis. Moreover, theinterference of two cones of depression causes a water head dropequals to the sum of the water head drops caused by each one, henceincreasing the magnitude of the settlements on these points.

Therefore, although this analysis presents several limitations, aring buffer spatial analysis has been carried out to analyze therelationship between the distance to the drought wells recentlydrilled in the study area and the measured subsidence. Results arerepresented in Fig. 13, showing that the influence of these waterextraction points on subsidence is evident. Subsidence is larger from 0to 1000 m and decreases gradually from 3000 m (Fig. 13a) for draughtpumping wells. The other wells, that have a shorter depth thandraught wells, show a similar trend but with smaller subsidencevalues than draught wells (Fig. 13b). It is observed that for both typesof wells the shape of the deformation curve with respect to thedistance to the wells is similar to the depletion curve of a pumpingtest as described above.

If we analyze the proximity of the wells to the Segura River axis,we observe that the greatest concentration of wells is closer than500 m to the river (Fig. 14a). In order to analyze the relationshipbetween ground surface deformation and the distance to the Segurariver, we have computed the average and standard deviation of thedeformation measured in those coherent pixels (CPs) included withinthe different buffer ring areas defined by the distance to the SeguraRiver.

Fig. 14b shows that the maximum subsidence, 4.3±2.5 cm, isfound in those CPs included within the area defined by a 500 m bufferfrom the river course. This spatial analysis evidences how accumu-lated subsidence for the period 1993–2009 decreases proportionally

to river distance. In fact, both Figs. 13 and 14 evidence the presence ofa subsidence bowl from 0 to 1500 m from the river axis, that seems tobe related with maximum well density. Notice that from 1500 to3000 m away from the river axis, the average subsidence is almostconstant with values below 1 cm, coinciding with a lower density ofpumping wells. A similar relationship was noticed by Hu et al. (2004)and Manunta et al. (2008) showing that subsidence is higher near ariver than far from it, due to the existence of soft and unconsolidatedsediments related with the activity of a river.

7. Results discussion

The evaluation of subsidence or ground settlement, δ, is a commontask in geotechnical engineering. Settlement is generally evaluated byconsidering three kinds of parameters: (i) thickness of potentiallydeformable soil, D, (ii) variation of stress state, which can beexpressed as a piezometric level fall, Δh, and (iii) a soil modulus, S,that represents the deformability of the aquitard and varies with thestress state. This relationship can be written as:

δ = S × Δh × D ð3Þ

Compressible soil thickness, called soft soil thickness in theprevious sections, is information at our disposal for the whole studyarea. The direct relationship between ground subsidence and soft soilthickness, in agreement with expression (3), has been verified bymeans of the simple spatial analysis conducted in Section 6.4 whichshows that the thicker the accumulated soft soil the larger thesubsidence.

Another variable considered in the expression (3) for subsidencecomputation is piezometric level fall, which is the responsible ofeffective stress increase. Unfortunately, complete time series ofpiezometric level evolution are only available at a few locations on thestudy area where a direct relationship between these two variables hasbeen demonstrated. This lack of data is common in aquifers sufferingsubsidence, hence complicating the implementation of reliable

120 R. Tomas et al. / Engineering Geology 115 (2010) 105–121

predictive tools or allowing only applying the simplest models(UNESCO, 1984; Budhu and Adiyaman, 2009). Complementarily topiezometric level information, pumping well spatial distribution hasbeen used to relate subsidence with the piezometric level fall becausethese are the responsible of water head drop that cause subsidence. Theanalysis demonstrates the higher rate of subsidence near thewells thanfar from them according with the expected theorical shape of thedepletion curves around a pumping well. Ground subsidence has beendemonstrated too to be larger near the Segura River than far from it due,mainly, to the existence of an important amount of wells.

The third variable that affects subsidence (see Eq. (3)) is theparameter related with the deformability of the soil. This parameterdepends principally on the nature of materials and its geologicalhistory. For example, in general under a certain stress a largerdeformation is expected over a soil than over a rock, and a normallyconsolidated soil is potentially more deformable than an over-consolidated soil. In previous sections we have shown that thelower values of subsidence correspond to the border area and thereliefs, where generally older rocky lithologies are present, incomparison with the younger Holocene sediments that fill the valleyand concentrate the greatest values of subsidence.

It is important to point out that the differences in water-expulsionrates of the fine layers can cause different subsidence rates for thestudied period.

In summary, DInSAR derived ground deformation data haveallowed studying the spatial and temporal evolution of subsidenceaffecting the VBSR and they have permitted to perform a simplespatial analysis of the most common conditioning and triggeringfactors that control subsidence mechanisms due to ground waterextraction.

In the future, the improvement of both hydrological andgeotechnical data over this area will allow to analyze and quantifymore precisely the role of different causative and/or triggering factorsof subsidence.

8. Conclusions

A spatiotemporal study of ground subsidence in the Westernsector of the Vega Baja of the Segura River Basin using PSI-DInSARdata has been presented. This technique has revealed to be very usefulfor this purpose, providing a dense amount of ground deformationmeasurements in a wide area along the period 1993–2009 at a lowcost.

Subsidence has been widespread in the study zone during theperiod 1993–2009, with settlements up to 17 cm. Only the borders ofthe valley, constituted by old materials (Tertiary and Permo-Triassic)did not suffer any significant deformation.

Several relationships among subsidence and related factors (softsoil thickness, piezometric level changes, distance to the river, andwell distribution) have been demonstrated. As it was expected,deformations are larger at the areas where soft soil thickness is high.The relationship between distance to the river and subsidence hasbeen also demonstrated, showing larger settlements in the proximi-ties of the river probably due to the existence of low consolidatedyoung deformable soils and the more intensive water extractionactivities fromwells. In fact, the main concentration of wells is locatednear the Segura River. Similarly, the existing relationship betweendistance to wells, mainly located near the river, and subsidence hasbeen evidenced.

In this case, PSI-DInSAR results have allowed to improve thetemporal and spatial characterization of a case of ground surfacedeformation due to ground water exploitation and to analyze itsrelationship with the conditioning and triggering factors. Theconclusions obtained by combining radar-derived deformation dataand pre-existing geo-information improve the knowledge of themechanisms governing this phenomenon, which could be used in the

future for developing adequate ground water exploitation policiesthat prevent or reduce its negative effects on urban areas. However,the acquisition of a larger set of geotechnical and hydrological dataand the improvement of the available ones is also required in the nearfuture for improving this analysis and even perform more sophisti-cated methodologies.

Acknowledgements

The SAR images used in this work were provided by EuropeanSpace Agency (ESA) in the framework of the EO Cat.1-2494 project.This studywas partially funded by the SpanishMinistry of Science andTechnology and EU FEDER (Project TEC-2008-06764), by the General-itat Valenciana (Project ACOMP/2010/082), and by the University ofAlicante (Projects VIGROB-157 and VIGROB-184). The InstitutoGeológico y Minero de España (IGME) and Diputación Provincial deAlicante kindly provided piezometric and hydrological data. Finally,authors want to thank Dr. José Delgado (University of Alicante) for theimportant contributions to this work and the provided soft soil data.

References

Arnaud, A., Adam, N., Hanssen, R., Inglada, J., Duro, J., Closa, J., Eineder, M., 2003. ASARERS interferometric phase continuity. Proc. IEEE International Geoscience andRemote Sensing Symposium (IGARSS-03), 2. Toulouse, France, pp. 1133–1135. 21–25 July.

Berardino, P., Fornaro, G., Lanari, R., Sansosti, E., 2002. A new algorithm for surfacedeformation monitoring based on small baseline differential SAR interferograms.IEEE Trans. Geosci. Remote Sens. 40, 2375–2383.

Blanco, P., Mallorquí, J.J., Duque, S., Navarrete, D., 2006. Advances on DInSAR with ERSand ENVISAT data using the coherent pixels technique. Proc. IEEE InternationalGeoscience and Remote Sensing Symposium (IGARSS-06). Denver, U.S.A. July.

Blanco, P., Mallorquí, J.J., Duque, S., Monells, D., 2008. The coherent pixels technique(CPT): an advanced DInSAR technique for non-linear deformationmonitoring. PureAppl. Geophys. 165, 1167–1193.

Budhu, M., Adiyaman, I.B., 2009. Mechanisms of land subsidence due to groundwaterpumping. Int. J. Numer. Anal. Meth. Geomech. doi:10.1002/nag.863.

CHS, 2007. Nuevas aportaciones al conocimiento hidrogeológico del entorno urbano deMurcia. 363 pp. Technical report. Available in: www.chs.es.

Custodio, E., 2002. Aquifer overexploitation: what does it means? Hydrogeol. J. 10,254–277.

De Larouziére, F.D., Bolze, J., Bordet, P., Hernández, J., Montenat, C., Ott D'Estevou, P.,1988. The Betic segment of the lithospheric trans-Alboran shear zone during theLate Miocene. Tectonophysics 152, 41–52.

Delgado, J., López-Casado, C., Estévez, A., Giner, J., Cuenca, A., Molina, S., 2000. Mappingsoft soils in the Segura River Valley (SE SPain): a case study of microtremors as anexploration tool. J. Appl. Geophys. 45, 19–32.

Delgado, J., Alfaro, P., Andréu, J.M., Cuenca, A., Doménech, C., Estévez, A., Soria, J.M.,Tomás, R., Yébenes, A., 2002. Engineering–geological model of the Segura riverflood plain (SE Spain): a case study for engineering planning. Eng. Geol. 68,171–187.

DPA-IGME, 2002. Sistema de información hidrológica GESPLAN v 6.54.00. DiputaciónProvincial de Alicante & IGME. CD ROM.

DPA-ITGE, 1996. Estudio de los recursos de agua salobre en la zona Sur de la provincialde Alicante. Previabilidad de la gestión de la salmuera de rechazo mediante ISP.Diputación Provincial de Alicante & ITGE. Technical Report, 340 pp.

Ferretti, A., Prati, C., Rocca, F., 2000. Nonlinear subsidence rate estimation usingpermanent scatterers in differential SAR interferometry. IEEE Trans. Geosci. RemoteSens. 38, 2202–2212.

Ferretti, A., Prati, C., Rocca, F., 2001. Permanent scatterers in SAR interferometry. IEEETrans. Geosci. Remote Sens. 39, 8–20.

Galloway, D.L., Hoffmann, J., 2007. The application of satellite differential SARinterferometry-derived ground displacements in hydrogeology. Hydrogeol. J. 15,133–154.

Gumiel, J.C., Fabregat, V., Antón-Pacheco, C., Ramos, G., Sánchez, E., 1999. Clasificaciónde imágenes Landsat TM para la estimación de superficies de regadío en la VegaBaja y Campo de Elche, unidad Hidrogeológica 07.24, (Alicante). Revista deteledetección 12, 1–5.

Hanssen, R.F., 2001. Radar interferometry. Data Interpretation and Error Analysis.Kluwer Academic publisher, The Netherlands. 308 pp.

Herrera, G., Tomás, R., Lopez-Sanchez, J.M., Delgado, J., Vicente, F., Mulas, J., Cooksley, G.,Sánchez, M., Duro, J., Arnaud, A., Blanco, P., Duque, S., Mallorquí, J.J., Vega-Panizo, R.,Monserrat, O., 2009a. Validation and comparison of Advanced DifferentialInterferometry Techniques: Murcia metropolitan area case study. ISPRS J.Photogramm. Remote Sens. 64, 501–512.

Herrera, G., Fernández, J.A., Tomás, R., Cooksley, G., Mulas, J., 2009b. Advancedinterpretation of subsidence inMurcia (SE Spain) using A-DInSAR data—modellingand validation. Natural Hazard. Earth Syst. Sci. 9, 647–661.

121R. Tomas et al. / Engineering Geology 115 (2010) 105–121

Hooper, A., Zebker, H., Segall, P., Kampes, B., 2004. A new method for measuringdeformation on volcanoes and other natural terrains using InSAR PersistentScatterers. Geophys. Res. Lett. 31. doi:10.1029/2004GL021737.21.

Hu, R.L., Yue, Z.Q., Wang, L.C., Wang, S.J., 2004. Review on current status and challengingissues of land subsidence in China. Eng. Geol. 76, 65–77.

IGME, 1986. Calidad de las aguas subterráneas en la Cuenca Baja del Segura y Costerasde Alicante. Instituto Geológico y Minero de España, Madrid.

Lanari, R., Mora, O., Manunta, M., Mallorquí, J.J., Berardino, P., Sansosti, E., 2004. A smallbaseline DIFSAR approach for investigating deformations on full resolution SARinterferograms. IEEE Trans. Geosci. Remote Sens. 7, 1377–1386.

Manunta, M., Marsella, M., Zeni, G., Sciotti, M., Alzori, S., Lanari, R., 2008. Two-scalesurface deformation analysis using the SBAS-DInSAR technique: a case study of thecity of Rome, Italy. Remote Sens. Environ. 29, 1665–1684.

Martínez, M., Mulas, J., Herrera, G., Aragón, R., 2004. Efectos de una subsidenciamoderada por extracción de agua subterránea en Murcia, España. Proc. XXXIIICongress of IAH-ALHSUD, Zacatecas, Mexico. Conference on Groundwater FlowUnderstanding from local to regional scales. CD ROM.

Mora, O. (2004). Advanced differential SAR techniques for detection of terrain andbuilding displacements. Ph. D. Thesis, Polytechnic University of Catalonia, 182 pp.

Mora, O., Mallorquí, J.J., Broquetas, A., 2003. Linear and nonlinear terrain deformationmaps from a reduced set of interferometric SAR images. IEEE T. Geosci. Remote 41,2243–2253.

Mulas, J., Aragón, R., Martínez, M., Lambán, J., García-Arostegui, J.L., Fernández-Grillo, A.I., Hornero, J., Rodríguez, J., Rodríguez, J.M., 2003. Geotechnical and hydrologicalanalysis of land subsidence in Murcia (Spain). Proc. 1ª International Conference onGroundwater in Geological Engineering, 50. Bled, Slovenia, pp. 249–252. 22–26September.

Rodríguez Ortiz, J.M., Mulas, J. (2002). Subsidencia generalizada en la ciudad de Murcia(España). In Ayala Carcedo, J. & Olcina Cantos Coords., Riesgos Naturales (pp. 459–463). Barcelona: Editorial Ariel.

Sanz de Galdeano, C., 1990. Geologic evolution of the Betic Cordilleras in WesternMediterranean, Miocene to the present. Tectonophysics 172, 107–119.

Tomás, R., Márquez, Y., Lopez-Sanchez, J.M., Delgado, J., Blanco, P., Mallorqui, J.J.,Martinez, M., Herrera, G., Mulas, J., 2005. Mapping ground subsidence induced byaquifer overexploitation using advanced Differential SAR Interferometry: VegaMedia of the Segura River (SE Spain) case study. Remote Sens. Environ. 98,269–283.

Tomás, R., Domenech, Mira, A., Cuenca, A., Delgado, J., 2007a. Preconsolidation stress inthe Vega Baja and Media areas of the River Segura (SE Spain): causes andrelationship with piezometric level changes. Eng. Geol. 91, 135–151.

Tomás, R., Lopez-Sanchez, J.M., Delgado, J., Vicente, F., Cuenca, A., Mallorquí, J.J., Blanco,P., Duque, S., 2007b. DInSAR monitoring of land subsidence in Orihuela City, Spain:comparison with geotechnical data. IEEE International Conference on Geoscienceand Remote Sensing Symposium, 2007 (IGARSS 2006), pp. 3027–3030. July 23–27.

Tomás, R., Lopez-Sanchez, J.M., Delgado, J., Mallorquí, J.J., Herrera, G., 2008. DInSARmonitoring of aquifer compaction due to water withdrawal: Vegas Baja and Mediaof the Segura river (SE, Spain) case study. In: Sánchez, J.M. (Ed.), Droughts: Causes,Effects and Predictions. Nova Publishers, New York, pp. 253–276.

UNESCO, 1984. Guidebook to studies of Land Subsidence Due to Ground-WaterWithdrawal. J.F. Poland Ed, United Nations Educational, Scientific and CulturalOrganization, Michigan, USA.

Werner, C., Wegmüller, U., Strozzi, T., Wiesmann, A., 2003. Interferometric point targetanalysis for deformation mapping. Proc. IEEE International Geoscience and RemoteSensing Symposium (IGARSS-03), 7. Toulouse, France, pp. 4362–4364. 21–25 July.