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Application of ground penetrating radar (GPR) to delineate clay layers in wetlands.A case study in the Soto Grande and Soto Chico watercourses, Doñana (SW Spain)

D. Gómez-Ortiz a,⁎, T. Martín-Crespo a, S. Martín-Velázquez a, P. Martínez-Pagán b,H. Higueras b, M. Manzano b

a ESCET, Universidad Rey Juan Carlos, C/Tulipán s/n, 28933, Móstoles, Madrid, Spainb Escuela Universitaria de Ingeniería Técnica Civil, U. Politécnica de Cartagena, P Alfonso XIII 52, 30203 Cartagena, Murcia, Spain

a b s t r a c ta r t i c l e i n f o

Article history:Received 31 July 2009Accepted 26 July 2010

Keywords:Ground penetrating radarHydrogeological modelDoñana aquiferWetlands

The potential utility of ground penetrating radar (GPR) in delineating the geometry of clay layers under twowatercourses of the Doñana National Park (SW Spain) has been checked. The interpretation of the GPRprofiles, calibrated with piezometers, shows the presence of two clay layer roughly continuous along some2 km. The work allowed to gain insight about the watercourses relationship with groundwater. Up to now,watercourses in the area have been associated as a matter of fact to the intersection of the water table withthe topography. But this work shows the existence of shallow, roughly continuous clay layers under thewatercourses that may generate perched phreatic levels even if the regional water table is too deep to cropout. Thus, the GPR proved to be very helpful and suitable in supporting hydrogeological studies in wetlands.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Ground penetrating radar (GPR) is used in geological investiga-tions to define lithological contacts (Jol and Smith, 1991; Pratt andMiall, 1993), locate faults (Rashed et al., 2003; Slater and Niemi, 2003;Gross et al., 2004), in soil studies (Doolitle and Collins, 1995), inglaciology (Overgaard and Jakobsen, 2001) and to estimate the depthto groundwater (Beres and Haeni, 1991; Menezes Travassos and LuizMenezes, 2004). This technique is being increasingly used inhydrogeological studies in general, and in wetlands in particular. Itsapplications range from determining the stratigraphy of peat inwetlands (Theimer et al., 1994), defining the boundaries betweenpeat and organic-rich lake sediment (Hanninen, 1992; Slater andReeve, 2002), estimating peat reserves (Warner et al., 1990), andevaluating the gas accumulation (Comas et al., 2005).

Shallow clay layers can play a foremost role on the hydrology ofsomewetlands as their presence influences the groundwater flow net.Knowing the geometry of those layers is basic to design hydrody-namic studies andmodels. The potential utility of GPR to delineate thegeometry of clay layers under two watercourses of the DoñanaNational Park (SW Spain), that behave as wetlands, has been checked.

Near surface geophysics, specifically GPR, combined with tradi-tional hydrologic tools such as piezometers, allows a more completedescription of hydrogeological parameters that control groundwaterpattern. In this paper, the detection of these clay layers by means of

GPR has allowed us to obtain the relationship between the water-courses and the groundwater in the area during the dry season. Thestudy also highlights the potential application of GPR techniques inwetland hydrology.

2. Hydrogeological setting

The Doñana area is situated in the SW Atlantic coast of Spain,between the Guadalquivir river and the Tinto river, not far away fromPortuguese border. The surface area is around 3000 km2 and hosts theGuadalquivir river marshes and a protected natural area of interna-tional relevance accounting some 1100 km2. It was declared Bio-sphere Reserve in 1981, RAMSAR site in 1982 and Natural WorldHeritage Site in 1994. Doñana is characterised by an extraordinaryabundance of wetlands. The geo-morphological origin and hydrologyof these wetlands vary greatly, but most of them are dependent ongroundwater (Custodio, 2000; Manzano, 2001): fluvial marshes, tidalmarshes, springs, watercourses, small ponds, lagoons, riverine forests,meadows and patches of phreatophyte vegetation.

The Doñana natural reserve is on top of a large unconfined aquifersystem consisting of detrital, unconsolidated Plio–Quaternary sedi-ments overlapping impervious Miocene marine marls (Fig. 1A). TheQuaternary consists on deltaic and alluvial silts, sands and gravels tothe north, alluvial and aeolian sands to the west and clays to the east.The aquifer extent is around 2700 km2 and the thickness ranges fromsome 20 m inland to N150 m at the coastline.

At regional scale the upper part of the aquifer shows two lithologicdomains: a sandy one to the N andW of themarshes, and a clayey onein the marsh area. The western part of the sandy domain is an aeolian

Journal of Applied Geophysics 72 (2010) 107–113

⁎ Corresponding author.E-mail address: david.gomez@urjc.es (D. Gómez-Ortiz).

0926-9851/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.jappgeo.2010.07.007

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mantle of 400 km2 (Fig. 1A) which behaves as an unconfined aquifer.Rain water infiltrates in this area and flows mostly to the ocean (S), tothe contact between sands and marshy clays (E), and to a local creekcalled La Rocina (N).

In many places of the aeolian mantle, the water table intersectswith the soil surface, originating hundreds of small to mediumphreatic wetlands. The two most common wetland types in this areaare small erosive or aeolian depressions located at the foot of ancientdune fronts, and small watercourses which follow very shallowphreatic flow paths flowing to the marshes (to the east) (Fig. 1B).Close to the marshes these small watercourses behave as marshyareas. This is favoured by the existence of shallow clay layers whichcan generate perched water table levels when the regional phreaticlevel is well under the soil surface.

3. Methodology

Only an overview of ground penetrating radar is presented here asthe method is well established (Davis and Annan, 1989; Reynolds,1997; Neal, 2004; Annan, 2005; Bristow, 2009; among others). Thetechnique is based on themeasurements of the subsurface response tohigh frequency (typically 100–1000 MHz) electromagnetic (EM)waves. A transmitting antenna on the ground surface emits EMwaves in distinct pulses into the ground that transmit, refract, reflect,and/or diffract at interfaces where the dielectric permittivity of thesubsurface changes. EM wave velocity data allow conversion of a timerecord of reflections to an estimated depth. The mean velocity value

may be obtained from: a) standard field procedures (e.g., commonmid-point profiles and hyperbolic velocity analysis), b) representativevalues in the literature or c) calibration with reflectors observed.

Reflections of EM waves are usually generated by changes in theelectrical properties of rocks, variations inwater content at an interface,and changes in bulk density at stratigraphic interfaces. The success of aGPR survey will depend on, mostly, if the surface or interface of interestis associated with a sufficient dielectric contrast and resistive nature toproduce a measurable reflection event. The penetration depth andresolution of the reflection data are function of both thewavelength andthe dielectric constant and conductivity values,mainly controlled by thewater content of materials. Resolution increases with frequency but, inturn, the depth to which we can penetrate decreases.

4. Data collection and data presentation

The subsurface interface radar 3000 system developed by Geophys-ical Survey Systems, Inc. was used in this study. GPR measurementswere made using a 400 MHz centre frequency shielded antenna with aseparation of 0.3 mbetween transmitter and receiver, in considering thebest compromise between the penetration depth and resolution neededfor this study.Distance along the surveyed lineswasaccurately recordedusing a survey wheel. Topography elevation along the profiles wasrecorder using a total station and incorporated for post-processingcorrections.Most of the profiles exhibit a nearflat topographyexcept forthe small watercourses crossing the area. The topography of all thepiezometerswere also recorded using the same levellingmethod. These

Fig. 1. (A) Location of the study area and simplified geological map of the Doñana aquifer (SW Spain). (B) Location of the GPR profiles and piezometers used for calibration.

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values were assigned to their corresponding scans of the radargrams inorder to reproduce the variations of topography and to apply thetopographic correction that would be described later. Radar antennawasmoved along the lines and2-Dprofiles of a large number of periodicreflections were generated, producing a profile of the subsurfacestructure with a fixed gain. Special care was taken in order to avoidartificial high-frequency noise that could deteriorate the radar signal.

During data acquisition, a time window of 50 ns was employed inorder to receive information from sources located as deep as possible.In addition to this, automatic gain control was used, although this gainvalue was removed during the data processing to amplify and clarifysome deep reflections by means of the application of a newexponential gain curve. This process was established individually foreach profile. All the profiles have been collected in continuous modeusing a distance interval between traces of 0.1 m. In this acquisitionmode each trace of the radargram is the result of a 16 times stacking inorder to improve the signal-to-noise ratio.

Once the data were acquired, some further data processing has beenperformed in order to increase the signal-to-noise ratio prior to inter-pretation. Data processing comprised time-zero correction, dewowfiltering, band-pass filtering, background removal and topographiccorrection. The topographic correction has been performed with asimple constant velocity correction that acts in a vertical sense (Yilmaz,2001). So, the two-way travel time of the traces is shifted to positive ornegative times from a given datum reference, using the subsurfacevelocity value obtained in the calibration profile described in thefollowing paragraph.

Due to the fact that the radar velocities and the dielectric propertiesof the studied materials were not known, a comparison of the obtainedreflections with lithological data which were taken from severalpiezometers in the area was used. In this sense, a short preliminarycalibration profile was carried out (Fig. 2) starting and finishing in twopiezometers (UPCT-64 and UPCT-65) of known lithology, and allowingus to measure the depth to the water table at the moment of the GPR

Fig. 2. Calibration profile used to identify the lithological units related to the detected reflections, as well as the location of the water table. (A) Processed unmigrated radargramtogether with the lithological information and depth to the water table obtained from the piezometers; (B) Processed unmigrated radargram interpreted. The velocity valuesobtained from the hyperbolic velocity analysis is also shown.

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survey. Using this information, the obtained reflections were associatedto the location of the water table and the presence of some clay layers.Fig. 2A shows afirst unit definedbyvery subtle andweak sub-horizontalreflectors that correspond to the dry sands present in the upper part ofthe piezometers. The bottom limit of this unit is defined by a continuousstrong reflector, characterised by a slightly undulated pattern andlocated at a mean depth of about 1 m, in strong correlation with thedepth to thewater tablemeasured in thepiezometersduring the survey.Below this reflector, sub-horizontal strong reflectors, laterally discon-tinuous, and located at slightly different depths (ranging from 1 to 2 m)agree well with the occurrence of ~50 cm thick clay layers interbeddedwith the sands as observed in the piezometers. Below the clay layers,water-saturated sand sediments appear. The velocity that produces thebest fit between the electromagnetic propagation times and the realdepth to the previously described lithological boundaries and watertable is 0.15 m·ns-1, in good agreementwith typical values for dry sands(e.g. Davis and Annan, 1989). This mean value was used for time todepth conversion in all the profiles. An independent estimation of theEM velocity value can be obtained using hyperbolic velocity analysis. Inthis sense, we have computed the EM velocity values using differenthyperbolic reflections present in the radargrams and a mean 0.12–0.15 m·ns-1 has been derived, in the range of the 0.15 m·ns-1 valueobtained by correlating the observed reflections with the real depths ofthe lithological boundaries of the piezometers. The geometry of some ofthe hyperbolic refractions and associated velocity values can beobserved in Fig. 2B. All the data were processed, modelled andinterpreted using the software Reflex-Win version 5.0. In all the profilesthe position of the antenna is represented in the horizontal axis,whereas depth is depicted with a 10 times vertical exaggeration.

5. Results

Apart from the calibration profile, three very long sections(ranging from 1400 to 2000 m) were obtained. For the sake ofbrevity, only two of them, the most representative, are described here(Figs. 3 and 4) but similar results were obtained in all of them.

5.1. Profile 1

A 1450-m-long GPR profile running SSE–NNWwas carried out in atrail that runs along the central part of the study area (Figs. 1B and 3).This trail crosses several of the watercourses in the area, being themore important Soto Grande and Soto Chico. The topography isroughly sub-horizontal, and only small (b1 m) topographic depres-sions are related to the watercourses.

The profile shows a first shallow unit with very weak sub-horizontal reflectors, being its lower limit defined by a moderatelystrong continuous reflector, slightly undulating in geometry, locatedat a mean depth ranging from 1 to 1.5 m below the surface. Locally,this reflector can approach or even intersect the topography, as in theSoto Grande watercourse (section A, about 100 m from the beginningof the profile). Below this, non-continuous undulating strongreflectors appear. They are located at two different depth levels: theshallower one appears at a mean depth of about 2 m, and ischaracterised by a variable thickness (ranging from 20 to about80 cm) and roughly present in the whole profile; the deeper one islocated at a mean depth of about 4 m, it is more discontinuous thanthe shallower one and its thickness varies from 20 to 50 cm.

Taking into account the data from the calibration profiles and thepiezometers, the reflector located at 1–1.5 m depth corresponds to thelocation of the water table, that seems to reproduce the geometry ofthe shallower strong reflector, associated to the presence of a claylayer. The close spacing between the water table and the clay layer(where present), as well as the strong correlation between thegeometry of the water table and the clay layer suggest that the lattercontrol the location of the water table, and thus influences the

groundwater flows. The strong reflector located at a mean depth of4 m corresponds to a clay layer that represents the bottom of theaeolian deposits and is related to the progradation of marsh deposits.This layer has been detected in one piezometer located near theposition of both profiles, by the marsh border (UPCT-34, Fig. 1B).

5.2. Profile 2

A 1550-m-long GPR profile running SSE–NNWwas carried out in atrail running along the eastern central part of the study area (Figs. 1Band 4). This trail ends against the southern part of the Soto Grandewetland. As in the previous profile, topography is roughly sub-horizontal, and only small (b0.5 m) depressions appear.

The profile is similar to the previous one, with a shallowmoderately strong reflector that corresponds to the water table andtwo different levels of strong reflectors corresponding to twodistinctive clay layers located at 2 and 4 m mean depth. In thisprofile, the shallower clay layer seems to be more continuous that inthe previous one, and locally appears as a double layer interbeddedwith the aeolian sands (e.g. between 250 and 350 m of section A).Opposite to this, the deeper clay layer, associated to marshprogradation deposits, is more scarce and discontinuous than inprofile 1, and only appears almost continuously in the last 300 m ofthe profile (section D), near the location of the Soto Grande wetland.Again, the water table location and geometry seem to be conditionedby the presence of the shallower clay layer.

6. Discussion

In hydrogeological studies, GPR constitutes a useful tool in order todetermine the depth to water table by reducing the need ofconstructing a dense and expensive network of piezometers. In theexamples shown in this study, as well as in many others (e.g. Doolittleet al., 2006; Costas et al., 2006; Chapman et al., 2009), the water tableprovides a continuous, medium amplitude reflection that is easilyidentified and traced across the radar record. It has been demon-strated that, under favourable conditions, GPR could estimate thedepths to shallow water tables with an accuracy of about 20 cm(Bentley and Trenholm, 2002; Jacob and Hermance, 2004). However,in case of perched water tables, the presence of several reflections canlimit the ability of GPR to distinguish closely spaced interfaces(Doolittle et al., 2006), and so it can be difficult to distinguish thereflection corresponding to the water table from underlying, denserand less permeable clayey layers. The high vertical resolution of the400 Mhz antenna used in this study has allowed us to distinguish thereflections from the water table and the clay levels situated somecentimeters below it, but if antenna of lower frequencies (e.g. 100 or200 Mhz) were used, it would have been impossible to separate botheffects, and the interpretation would be different too.

Another limitation of the technique corresponds to the signalattenuation produced when water saturated clayey material appears,making very difficult the interpretation of the radargrams below theselevels. In our study, the clay levels detected in both the GPR profilesand the piezometers are not very thick (b~30 cm in general) and thus,attenuation is not very high and reflectors located at depths of about4 m can be detected. However, the presence of water on the surface inrelation to some local watercourses, together with the presence ofvegetal remains and clay material by decantation, constitutes shallowlevels where the signal is attenuated in a more strongly way. This maybe the case (Fig. 3) of two small local watercourses (profile 1, sectionC, metres 160 and 225) where the water table is clearly located about1 m below the surface, but a stronger reflector situated near theground surface suggest the presence of a small clay level that could beresponsible of the formation of a perched water table and accumu-lation of water on surface as well as the increasing attenuationobserved in these areas. This effect is more clearly seen in the Soto

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Chico watercourse (profile 1, section D, meter 140), where a strongattenuation of the signal is observed making the location of the watertable virtually impossible to delineate. However, taking into accountthe depth and geometry of the reflector associated to the water tablein the surrounding areas of the Soto Chico watercourse, a subhor-izontal geometry at a mean depth of 1 m can be deduced below thewatercourse, suggesting again the occurrence of a local clay layerclose to the ground surface that retains surface water, although this isnot the only possible explanation. In all cases, it can be deduced that,at least during the dry season, when the GPR survey was carried out,the surface water present at the watercourses are not associated tothe intersection of the regional water table with the topography (thatseems to be the case for Soto Grande water course, profile 1, section 1,Fig. 3), but it corresponds to local water table induced by a lowpermeability clay layer that retain water infiltration. This is importantbecause it provides a new hydrological model for the watercourses ofthe area.

7. Conclusions

In this study, the local underground stratigraphy of Soto Grandeand Soto Chico watercourses area (Doñana, SW Spain) has beenobtained by means of GPR technique. From the analysis of theradargrams and comparison with the lithological information derivedfrom several piezometers present in the area, two different clay levelsat mean depths of about 2 and 4 m depth have been imaged, togetherwith the location and geometry of the water table, ranging from 1 to1.5 m depth.

The shallowest clay level, roughly continuous, seems to have afluvialorigin. It controls the location and geometry of a local, perched watertable. The deepest clay level is more discontinuous and irregularlydistributed in the study area, and probably corresponds to a marsh claylayer which is the lower boundary of the aeolian sand cover.

The GPR data have provided insight about the relationshipbetween the watercourses and the groundwater. Although somewatercourses seem to be controlled by the intersection of the regionalwater table with the topography, in others this relationship is notclear at all as the course bed elevation is higher than the water table.In fact, out of the rainfall season, the studied watercourses acttemporarily as excess-irrigation evacuation channels, carrying to the Eexcess water from cultivation areas placed about 1 km to the west.Thus, the prevalent hypothesis is that the courses behave temporarilyas infiltration areas, and the identified clay layers could maintainperched water tables whose duration may last for some weeks.

GPR has proven to be a useful tool to identify the existence anddelineate geometry of clay layers under wetlands. This informationcan be easily incorporated to the conceptual (and numerical) modelsof wetlands areas, thus providing soundness to the simulations andforecasts related to wetlands management.

Acknowledgements

The work has been financed by Spanish Ministry of Environmentthrough the Project Doñana 2005, whose main objective is to restorethe hydrology of the fluvial basins flowing to the Doñanamarshes andof the marshes themselves. The authors are indebted to the personnelof the Doñana Natural Space (Andalusian Government) and of theDoñana Biological Station (Spanish Council for Scientific Research).We greatly appreciate the comments and suggestions of both the

editor and two anonymous reviewers that have considerablyimproved the original manuscript.

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