ORIGINAL ARTICLE

Collapse hazard assessment in evaporitic materials from groundpenetrating radar: a case study

Tomas Martın-Crespo Æ David Gomez-Ortiz

Received: 26 June 2006 / Accepted: 12 December 2006 / Published online: 10 January 2007� Springer-Verlag 2007

Abstract Evaporitic materials have been studied by

means of ground penetrating radar (GPR) in order to

evaluate the collapse hazard. The obtained 200 MHz

GPR profiles show a low signal-noise ratio over the

first 3 m depth, where well-defined and continuous

reflectors can be observed. Between 3 and 4.5 m depth,

the signal to noise ratio decreases due to attenuation of

the electromagnetic (EM) waves. As a result, reflectors

located deeper than 3 m become more discontinuous

and poorly defined. GPR profiles show trails of con-

tinuous and subhorizontal reflectors, with a slightly

undulated and irregular geometry. Although some of

these reflectors laterally vanish or seem to disappear,

sudden interruptions or hyperbolic reflections that

could be originated by the occurrence of cavities have

not been detected. These reflectors have been inter-

preted as corresponding to several evaporitic layers

(gypsum) that constitute the main lithology in the area.

Clear interruptions of reflectors have only been ob-

served in some GPR profiles, and they could be

attributed to small (1–2 m long) subvertical faults, with

only a few centimetres offset. These faults may be

generated by the accommodation of the evaporitic

layers to local collapses affecting deeper materials.

Keywords Risk assessment � Geohazards �Ground penetrating radar � Spain

Introduction

Dissolution processes are common in soluble sedimen-

tary materials such as limestone and gypsum, where the

presence and circulation of liquid water can generate

caves and cavities by dissolution. If this process is

intensive enough, collapses are generated by the sinking

of large cavities in the ground. If dissolution occurs on a

small scale, only little cavities are generated, and the

upper materials deform and adapt to the new mor-

phology, filling up the generated cavity. Deformed and

folded materials, on occasion affected by small faults

and fractures, are usually defining these structures just

above the dissolved zone. Such features are not always

indicative of a cavity beneath, although dissolution

processes producing adaptation of the surface materials

can be expected in depth. Dissolution cavities are often

a major hazard occurring in soluble sedimentary mate-

rials. Location and size determination of these cavities

and associated collapse structures are essential in order

to avoid engineering hazards like road subsidence or

building collapse (e.g. Batayneh et al. 2002; Zhou et al.

2002; Gutierrez-Santolalla et al. 2005; Soupios et al., in

press). Benito et al. (1995) studied the environmental

and engineering problems associated with natural and

human-induced sinkholes in gypsum-rich terrains. In

that study, gravimetric and ground penetrating radar

methods were used in order to determine both the

location and depth of anomalies and cavities along the

Zaragoza–Barcelona railway, NE Spain, in evaporitic

facies similar to those studied in the present work.

Ground penetrating radar (GPR) is a geophysical

technique that is used to characterize the physical

properties, thickness, spatial distribution, internal

structures and discontinuities of subsurface materials. A

T. Martın-Crespo (&) � D. Gomez-OrtizArea de Geologıa, Dpto. de CC. de la Naturalezay Fısica Aplicada, ESCET, Universidad Rey Juan Carlos,C/Tulipan s/n, 28933 Mostoles (Madrid), Spaine-mail: tomas.martin@urjc.es

123

Environ Geol (2007) 53:57–66

DOI 10.1007/s00254-006-0618-1

recent revision of the principles, problems and progress

of the use of GPR technique, mainly in sedimentology,

is given in Neal (2004). It has also been applied to a wide

variety of problems including location and characteri-

sation of geological structures such as fractures (Basson

et al. 2002; Porsani et al 2006), to estimate water-table

depth (Doolittle et al. 2006), to locate hydrocarbon

contaminated sites (Atekwana et al. 2000), to cha-

racterise landfill areas (Splajt et al. 2003), to perform

archaeological studies in ancient human civilizations

(Tsokas et al. 1999) and to restore historic buildings

(Ranalli et al. 2004). In volcanic areas GPR has been

applied as a useful methodology to complete field

observations when information obtained from outcrops

is limited. Thus, this technique has been used to improve

the mapping of volcanic areas and to identify hidden

structures such as lava tubes inside lava flows in order to

determine and minimize volcanic hazard (e.g. Russell

and Stasiuk 1997; Cagnoli and Ulrych 2001; Miyamoto

et al. 2003). Recently, Gomez-Ortiz et al. (2006) have

characterised the GPR signature and mean electro-

magnetic waves velocity of different volcanic rocks and

deposits. Some pitfalls in GPR data interpretations

have also been described (e.g. Radzevicius et al. 2000).

One of the most common ones is to identify each event

on a radargram as scattering from a discrete horizon,

whereas this can also be due to antenna and target

effects rather than reflections of geological nature.

GPR response of soluble sedimentary materials of

evaporitic origin has been analysed in this study in

order to identify lithological contacts with the adjacent

materials, characterise the internal structure, deter-

mine the occurrence and size of structures associated

to dissolution processes, and calculate EM mean

velocities in evaporitic materials.

The study has been carried out in continental Mio-

cene-Pliocene sedimentary materials of the Jiloca and

Alfambra-Teruel basins originated in an alluvial fan

scenario. They crop out at the western part of the town

of Teruel, eastern Spain (Fig. 1). Limolites and con-

glomerates are the dominant lithologies, laterally

changing to evaporitic materials mainly composed by

gypsum layers and halite beds. These evaporitic units

are commonly 30–50 m thick, and show interbedded

mudstone and limolite beds.

Ground penetrating radar

As ground penetrating radar is a well-established

geophysical method (Yilmaz 1987; Davis and Annan

1989; Telford et al. 1990; Daniels 1996; Reynolds

1997; Claerbout 2004), only a brief overview of it is

presented here. The technique is based on the

measurements of the subsurface response to high fre-

quency (typically 100–1000 MHz) electromagnetic

(EM) waves. A transmitting antenna on the ground

surface emits EM waves in distinct pulses into the

ground that propagate, reflect and/or diffract at inter-

faces where the dielectric permittivity of the subsurface

changes. EM wave velocity data thus allows conversion

of a time record of reflections into an estimated depth.

Reflections of EM waves are usually generated by

the changes in the electrical properties of rocks, vari-

ations in water content, and changes in bulk density at

stratigraphic interfaces. Reflections can also be related

to changes in EM wave velocity, due, for instance, to

the occurrence of voids in the ground. The penetration

depth and resolution of the reflection data are both

function of wavelength and dielectric constant values,

which are in turn mainly controlled by the water

content of the materials.

Data collection and presentation

Data from this study were collected with the subsur-

face interface radar (SIR) 3000 system developed by

the Geophysical Survey Systems, Inc. (GSSI). GPR

measurements were made using a 200 MHz centre

frequency shielded antenna in monostatic mode, which

is considered the best compromise between penetra-

tion depth and event resolution in sedimentary mate-

rials. All the profiles have been collected in continuous

mode with a distance interval between traces of 0.01 m,

keeping elevation constant throughout the whole sur-

vey. In this continuous acquisition mode, each trace of

the radargram is the result of a three times stacking in

order to improve the signal-to-noise ratio. A survey

wheel attachment was used in order to enhance survey

accuracy. During data acquisition, a time window of

100 ns and automatic gain control were employed.

Data processing comprised background noise re-

moval, time-zero corrections and band-pass filtering, as

recommended by Annan (1999). Although published

data for EM waves velocities in evaporitic materials are

available, each specific study area displays particular

dielectric features due to the inherent heterogeneities

of any lithology, mainly in sedimentary rocks. In this

sense, calibration surveys were necessary in order to

obtain a mean EM velocity value applicable to all

profiles so that a representative dielectric constant

could be calculated. A calibration survey was carried

out over a representative zone of the area, where a

metallic bar had been horizontally introduced (Fig. 2).

Once the velocity data were obtained for each profile, a

migration process was applied in order to collapse the

58 Environ Geol (2007) 53:57–66

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diffraction hyperbolae and obtain true geometries and

depths of the subsurface structures along the profiles.

All data were processed, modelled and interpreted

using the software Radan 5.0 (RADAN for Windows

1997). In all the profiles, the position of the antennae is

represented on the horizontal axis, whereas depth is

depicted with no scale exaggeration on the vertical one.

Results

During the field survey, 157 GPR profiles with a total

length of 4,647 m were acquired. The study area was

divided in six work grids to optimise data acquisition

and processing. The location of the profiles were

planned to cover the whole area, and they were carried

Fig. 1 a Location of the city of Teruel in Spain, b geological mapof the surrounding area of Teruel and situation of the work area(modified from Godoy et al. 1983), c schematic plan of the study

area showing the distribution of grids and the location of theselected GPR profiles, d photograph of the study area view fromthe NW, and distribution of grids

Environ Geol (2007) 53:57–66 59

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out with a systematic separation of 1 m (Fig. 1). Four

representative profiles (three from grid 3 and one from

grid 1, Fig. 1) of the study area have been selected in

this work.

From the calibration survey and given that the

depth of the point source (a metallic bar) was well

known (0.4 m) and the reflectors were perfectly rec-

ognizable in the obtained radargram, a mean velocity

of 0.097 m ns–1 was estimated. In addition, an inde-

pendent velocity estimation was performed by deter-

mining the velocity value that better fitted the

geometry of the hyperbolic reflection due to the

metallic bar (Fig. 2). In this case, a 0.11 m ns–1 mean

velocity was obtained. Therefore, we can conclude

that a velocity interval of 0.097–0.11 m ns–1 was taken

as representative of the materials in this area. These

values are on the range of those reported by Holub

and Dumitrescu (1994) for gypsum (0.13 m ns–1) and

altered gypsum (0.086 m ns–1).

Taking into account the mean EM velocity obtained

from the calibration survey, a maximum depth of 4.5 m

is reached employing a time window of 100 ns for data

acquisition.

As a general statement, GPR profiles exhibit a

good signal-to-noise ratio up to a time window of

65 ns, corresponding to a depth of 3 m. Thus, well-

defined and continuous reflections can be observed in

the first 3 m depth of the profiles. Between 3 and

4.5 m depth, the noise level increases markedly due to

the attenuation of the EM waves. This effect makes

much more complicated the identification and inter-

pretation of the reflections and structures located

below a depth of 3 m.

Field photographs showing the lithological materials

in exposed cross-sections were taken in selected areas.

These photographs were superimposed to the corre-

sponding radargrams for comparison, allowing us to

estimate the degree of correlation between the geom-

etry and depth of the lithological contacts and the

observed reflectors (Fig. 3). It was observed that usu-

ally a good correlation between them exists, both in

depth and dip angle of the geological formations. The

fact that the reflectors identified in the radargrams as

lithological boundaries fit well with the position and

dip of the lithological contacts observed in the field

confirms that the mean velocity interval of 0.097–

0.11 m ns–1 determined by means of the calibration

survey was correct. As a result, we can affirm that the

mean EM wave velocity value is valid for these mate-

rials, at least for the study area.

The obtained radargrams (Figs. 4, 5) show sub-hor-

izontal reflectors or slightly dipping towards the

southwest. They exhibit an irregular or undulated

geometry, but are laterally continuous for at least

several meters horizontally. Locally, some reflectors

laterally vanish and are difficult to identify. These

reflectors correspond to the different gypsum beds that

constitute the main lithology in the study area. No

sudden interruptions or hyperbolic diffractions are

present, indicating that large cavities are absent.

However, some sections of the radargrams show

reflectors with higher undulations and stronger dip,

defining geometry of very gentle anticline and syncline

fold geometries (Figs. 4, 5). In the innermost part of

these gentle folds, it is common to find 1–2 m long

narrow subvertical fractures, identified by a vertical

Fig. 2 Radargram of thecalibration survey andcorrelation with the situationof the metallic bar into theterrain. Antenna (200 MHz)and survey wheel can beappreciated

60 Environ Geol (2007) 53:57–66

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Fig. 3 Profile 1: a Photographof the lateral slope from grid3, b radargram of the profilejust above the lateral slopefrom grid 3, c previousphotograph and radargramsuperimposed

Environ Geol (2007) 53:57–66 61

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Fig. 4 Profile 2: Radargram (a) and interpretation (b) of aselected profile where undulated reflectors and accomodationfaults in evaporitic materials can be observed

Fig. 5 Profile 1: Radargram (a) and interpretation (b) of aselected profile where small faults and the contact betweenevaporitic beds and clay layers (dark grey) can be observed

62 Environ Geol (2007) 53:57–66

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displacement of some cm in the reflectors. Basson et al.

(2002) shows that discontinuities in GPR profiles could

be interpreted as fractures and faults with variety of

depths of penetration and spatial orientations.

Following those authors, three main criteria can be

used for this interpretation:

1. Minor discontinuities of reflectors that indicate

fractures

2. Abrupt unconformities and sudden variation of

lateral reflectors, which is indicative of fractures

with a strike slip component

3. Vertical displacements of reflectors, which indicate

either normal or reverse faults with a dominant

dip-slip motion, and sub-horizontal bedding.

As the third criterium is clearly present in the ra-

dargrams obtained in this work (see Figs. 4, 5, 6), we

can conclude that some minor subvertical faults can be

identified. This agrees well with field observations

made in exposed cross-sections (such as the one in

Fig. 3) where several subvertical faults with displace-

ments of only a few centimetres were observed.

The association of syncline folds and subvertical

fractures commonly occurs during the development of

collapse areas in soluble materials, such as gypsum (e.g.

Gutierrez-Santolalla et al. 2005). According to this, it is

possible to interpret the structures observed in some

sections of the radargrams as a result of the accom-

modation of materials above a cavity at depth. Thus,

the overlaying materials fill the void adapting their

geometry to the previously created cavity. It is during

this adaptation when the small fractures develop.

Given that the occurrence of cavities has not been

identified in the radargrams, we can conclude that the

observed structures correspond to the adaptation of

gypsum and clay layers to dissolution voids located at a

depth greater than 3 m and, probably 4–5 m (i.e. the

maximum depth reached in the GPR survey).

Local outcropping of greyish clay materials at sur-

face can be related to greater amplitudes of the

reflectors in that area (Fig. 5). This has been inter-

preted as a lower attenuation of the EM waves when

clay materials, instead of gypsum, are present in the

ground. This causes the reflectors to be better defined

in the clay materials than in the evaporitic ones.

Several reflectors, showing an undulated but later-

ally continuous geometry, are present (Fig. 6). These

reflectors show a slight dip towards the SW during the

first 20 m of the profile, whereas from that point to the

SW and they exhibit a subhorizontal geometry. Only a

few interruptions with a vertical offset of a few centi-

metres affect these reflectors (e.g. at 8, 16 and 34 m

from the beginning of the profile). These interruptions

Fig. 6 Profile 3: Radargram (a) and interpretation (b) of aselected profile where slightly undulated reflectors defining a verygentle syncline fold and vertical faults showing displacement inthe reflectors of some cm in evaporitic materials can be observed

Environ Geol (2007) 53:57–66 63

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can be interpreted as small subvertical fractures, pre-

dominantly showing a normal dip-slip motion that does

not reach the surface.

Finally, a profile from grid 1 has been selected as it

displays the contact between evaporitic facies and

detritical material, filling a palaeo depression (Fig. 7).

Two different units can be distinguished: an upper

one, located between 0 and 9 m reaching a maximum

depth of about 2 m, and a lower one, defined by

reflectors slightly dipping towards the SW, exhibiting

Fig. 7 Profile 4: Photograph(a), radargram (b) andinterpretation (c) of theselected profile from grid 1where the contact betweenred clays and gravel depositand evaporitic beds can beobserved

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an irregular or undulated but laterally continuous

geometry. The upper unit corresponds to red clays

and gravel filling a pre-existing topographic depres-

sion, as can be observed in the field. Reflectors

belonging to the lower unit are interrupted by the

upper unit (e.g. at 5 m in the horizontal distance and

a depth of about 2 m) confirming that the latter unit

corresponds to a more recent deposit infilling a pre-

existing depression.

Conclusions

The present work demonstrates the possibilities of

GPR technique in identifying lithological contacts of

sedimentary materials and for characterizing their

internal structure in order to determine the presence

and size of the structures associated with dissolution

processes from the determination of the mean veloc-

ity of EM waves. This information is very useful in

order to properly assess the collapse hazard for a

specific area. Different radar signatures, correspond-

ing to the aforementioned evaporitic facies and

structures have been described in this work, and how

they can be used to identify them when no subsurface

information is available. Evaporitic materials usually

display continuous reflectors that are horizontal or

slightly undulated due to accommodation to possible

underlying collapse structures. The associated clay

materials show better defined reflectors due to the

lower attenuation of the electromagnetic waves

crossing these materials. Sharp interruptions or dif-

fraction hyperboles indicating the presence of voids or

cavities have not been identified in the profiles. Short-

slip faults have been recognised in the radargrams.

Moreover, the EM mean velocities for evaporitic

materials have been obtained, and they can be used to

determine real depths. Thus, once the material has

been well characterised in a studied area, the GPR

technique will provide more confident results in

imaging shallow structures.

In order to obtain a more constrained analysis in

future studies, the comparison of the GPR data with

those obtained from complementary geophysical

techniques—such as electrical resistivity imaging or

shallow reflection seismic—is recommended.

Acknowledgments The authors would like to thank HenriqueLorenzo and Javier Lillo for their critical revision of the originalmanuscript. We wish to thank Antonio Mas Atienza and DavidOlmedilla, for their technical support during the data acquisition.The useful comments from an anonymous reviewer also helpedto improve the manuscript.

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