analysis of the karst aquifer structure of the lamalou area

Analysis of the karst aquifer structure of the Lamalou area

(Herault, France) with ground penetrating radar

Walid Al-faresa,b, Michel Bakalowicza, Roger Guerinc,*, Michel Dukhand

aUniversite Montpellier II, CNRS Hydrosciences, c.c. MSE, 2 place Eugene Bataillon, 34095 Montpellier Cedex 5, FrancebAECS, B.O.Box 6091, Damascus, Syria

cUMR 7619 Sisyphe, Departement de Geophysique Appliquee, Universite Pierre et Marie Curie (Paris 6),

case 105, 4 place Jussieu, 75252 Paris Cedex 05, Franced IRD, 911 avenue Agropolis, 34000 Montpellier, France

Received 4 October 2001; accepted 23 August 2002

Abstract

The study site at Lamalou karst spring (Hortus karst plateau) is situated 40 km north of Montpellier in France. It consists of a

limestone plateau, drained by a karst conduit discharging as a spring. This conduit extends for a few dozen meters in fractured

and karstified limestone rocks, 15 to 70 m below the surface. The conduit is accessible from the surface. The main goal of this

study is to analyze the surface part of the karst and to highlight the karstic features and among them the conduit, and to test the

performances of ground penetrating radar (GPR) in a karstic environment. This method thus appears particularly well adapted to

the analysis of the near-surface ( < 30 m in depth) structure of a karst, especially when clayey coating or soil that absorbs and

attenuates the radar is rare and discontinuous. A GPR pulseEKKO 100 (Sensors and Software) was used on the site with a 50-

MHz antenna frequency. The results highlight structures characterizing the karstic environment: the epikarst, bedding planes,

fractured and karstified zones, compact and massive rock and karrens, a typical karst landform. One of the sections revealed in

detail the main conduit located at a depth of 20 m, and made it possible to determine its geometry. This site offers possibilities of

validation of the GPR data by giving direct access to the karstic conduits and through two cored boreholes. These direct

observations confirm the interpretation of all the GPR sections.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Ground penetrating radar (GPR); Karst; Cave; Epikarst; Karst plateau; South of France

1. Introduction

In hydrogeology, ground penetrating radar (GPR) is

applied to locate fractured or karstified zones, faults

and cavities (Beres and Haeni, 1991; Holub and Dumi-

trescu, 1994; Robert and de Bosset, 1994; McMechan

et al., 1998; Beres et al., 2001), in aquifers (Sellmann et

al., 1983; Arcone et al., 1998) as well in the study of the

water contamination (Benson, 1995; Atekwana et al.,

2000). Several studies also showed that this method of

prospecting becomes, in certain cases, a more effective

means in the study of karst than other geophysical

methods like microgravity and electrical resistivity

(Yelf and Creswell, 1988; Chamberlain et al., 2000).

For a review of GPR investigations for karst, see also

0926-9851/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0926 -9851 (02 )00215 -X

* Corresponding author. Tel.: +33-1-44-27-45-91; fax: +33-1-

44-27-45-88.

E-mail address: [email protected] (R. Guerin).

www.elsevier.com/locate/jappgeo

Journal of Applied Geophysics 51 (2002) 97–106

proceedings of several conferences as the ‘‘Multidisci-

plinary Conference on Sinkholes and the Engineering

and Environmental Impacts of Karst’’ (Stangland and

Kuo, 1987; Roark and Lambert, 2001), the ‘‘Interna-

tional Conference onGround Penetrating Radar: GPR’’

(Geraads and Omnes, 2002), the ‘‘European Associa-

tion of Geoscientists and Engineers (EAGE) confer-

ence’’ (Yelf and Creswell, 1988), the ‘‘Environmental

and Engineering Geophysical Society-European Sec-

tion (EEGS-ES) meeting’’ (Finetti et al., 1995), and the

‘‘Symposium on the Application of Geophysics to

Engineering and Environmental Problems: SAGEEP’’

(Carpenter et al., 1995; Valle and Zanzi, 1996; El-

behiry and Hanafy, 2000). We will show that GPR not

only makes it possible to describe in detail the epikarst

(i.e. the shallow part of the karst) and the infiltration

zone of a karst aquifer, but also to locate and describe a

natural cavity located at 20 m below the surface.

2. Site

From 1976 to 1996, many geological, hydrogeo-

logical, hydrodynamical, geochemical and geophysical

studies were carried out on the experimental site of

Lamalou with the aim of studying the structure and the

general functioning of the karstic aquifer (Therond,

1976; Bonin, 1980; Chevalier, 1988; Durand, 1992;

Turberg, 1993; Climent, 1996). The site is situated on

Hortus karst plateau, 40 km north of Montpellier, south

Fig. 1. Geographical and geological situation of the Hortus karst plateau and of the experimental site of Lamalou (Herault, France).

W. Al-fares et al. / Journal of Applied Geophysics 51 (2002) 97–10698

France (Fig. 1). This limestone plateau, which covers

an area of 50 to 70 km2, is limited at the west by Saint-

Martin de Londres basin and the dense Monnier

Woods, at the northeast by the plains of Pompignan

and Claret, at the south by the Pic Saint-Loup. Altitude

varies between 195 m at the southwest and 512 m at the

southeast. The Hortus plateau is subject to a Mediter-

ranean climate characterized by irregular rainfall:

Fig. 2. Geological cross-section of the Hortus karst aquifer. Karst site of Lamalou spring (Durand, 1992).

Fig. 3. Location of the study area and of the GPR profiles superimposed to the map of the karstic conduit of Lamalou. F are the boreholes, S1

and S2 are the cored boreholes crossing the main cave. All the profiles were leveled compared to the origin of profile 1.

W. Al-fares et al. / Journal of Applied Geophysics 51 (2002) 97–106 99

heavy rainfalls during October and moderate rainfalls

during spring followed by summer dryness from

May until August. To the east, the plateau is covered

by Mediterranean shrubby vegetation, holm oaks,

durmastoaks and kermes oaks. Generally, soil exists

only in fractures. In certain parts, the surface consists

of a karren, sometimes covered by scree resulting

from its weathering. Some closed depressions with a

clayey infilling, developed in marly facies, are culti-

vated. The Hortus plateau is formed by the structural

surface of the top of Valanginian limestone. This

limestone lies in concordance on lower Valanginian

marls that in turn lie upon the Berriasian and upper

Jurassic limestone (Fig. 2). The average thickness of

this limestone ranges from 80 to 100 m (Durand,

1992). The aquifer is composed of strongly fractured

and karstified Valanginian limestone. Dozen of kars-

tic caves are known. This limestone has a very low

porosity (1.84%) and is almost impermeable (Bonin,

1980). The water pathflows are completely directed

by cracks and the more or less karstified fractures of

the rock. Groundwater is collected by a partly

flooded conduit that develops near the top of the

saturated zone and discharges at the Lamalou spring.

This conduit, known to extend for several dozen

meters, widens to form a cave accessible in the

vicinity of the spring (Fig. 3). The zone close to

the spring has been the object of many studies and

experiments; it is equipped with 10 boreholes, of

varying depths, between 32 and 80.5 m. Boreholes

F1 (32 m) and F7 (80.5 m) reach the karstic conduit.

Other boreholes are established in the fissured rock.

Two boreholes S1 and S2 of a small diameter, 18.5

and 18.2 m deep, cross the cavity. The average

thickness of the unsaturated zone is 20 m and that

of the saturated zone is estimated at 50 m.

3. Characteristics of GPR used

The GPR pulseEKKO 100 (Sensors and Software)

was used for the measurements. It is composed of a

control unit (console), connected to a portable com-

puter for the direct recording of raw data. The

console itself is connected to the radiating–receiving

antennae via optical fibers. The measurement param-

eters are summarized in Table 1. With this equip-

ment, it is necessary to move the console and the

computer away from the antennae, in order to

eliminate all parasitic sources from interferences. A

common mid-point profile 20 m long was carried out

to evaluate the propagation velocity of the electro-

magnetic wave in the ground. In the studied case, i.e.

in limestone, the calculated average speed is 0.1 m

ns� 1. It is the value used for all the profiles carried

out in reflection mode on the site. According to the

average speed and the recording time window, the

depth of investigation is between 20 and 30 m.

Seven parallel profiles were carried out on the top

of the major part of the karstic conduit, in the

vicinity of the spring (Fig. 3). The 120 m profile

lines were spaced at 15 m.

4. Interpretation

To obtain the real positions of the various geo-

logical structures, all profile values were leveled

relative to values at the origin of profile 1 (x= 15 m,

y = 60 m in the general grid of the experimental site

presented in Fig. 3). A topographic chart of the

studied zone (Fig. 4) was then composed; it shows

that there is a general slope of 12j of direction

perpendicular to the profiles. A thalweg, 3 to 4 m

deep and 5 to 10 m wide, crosses through the whole of

the profiles; it is directed towards the permanent

spring. This depression could be associated with a

fault of weak throw or an important fracture. Only one

representative profile (5) is shown here (Fig. 5). The

radargrammes clearly show several structures that

characterize the karstic aquifer near the source:

– A shallow zone (noted A in Fig. 5), marked by

multiple reflections, is limited at its base by a well

contrasted interface (noted P1). Its thickness varies

Table 1

GPR measurement parameters

Impulse power 400 V

Center frequency of antennae 50 MHz

Length of antennae 2 m

Measurement step 0.5 m

Recording time window 400 to 600 ns

Sampling interval 1600 ps

Number of stacks 32

Battery power supply 12 V


between 8 and 12 m. This zone is characterized by

strong fracturing, cracks and faults (noted F and

P3) of varied sizes distributed on the whole of the

zone. This zone constitutes the epikarst (Bakalo-

wicz, 1995) that plays a very important part with

regard to the processes of water storage close to the

surface and vertical infiltration towards the unsa-

turated and saturated zones. The general slope of

the clear oblique reflector (noted P1) represents the

dip of the layers; checked by direct measurements

on the ground, it varies between 12j and 18j. Thisdip cuts the surface of the ground in the last parts

of the studied zone. At the surface and by direct

observation, the trace of this bedding plane

separates a part where the ground is composed of

stone debris with some rock elements in place from

another part where massive limestone appears in

the form of a well developed karren, or lapiaz

(noted L) in small limestone peaks separated by

fractures widened by solution. This bedding plane

is sometimes crossed through by faults or great

fractures that disturb its continuity locally. A fault

with weak throw or a large fracture seems to

correspond to the karstic conduit.

– A deeper zone, with an average thickness varying

between about 8 and 10 m, is made up of dark gray

compact limestone (noted B), limited at the bottom

by a bedding plane (noted P2) parallel to the upper

one (noted P1); the distance between the two planes

is 13 m. The weak registration of the radar signals

in this zone is due to the absence of reflectors and

the low heterogeneity of the physical and structural

Fig. 4. Topographic representation in 2D of the zone of GPR prospecting on the Lamalou karst site.


properties of this layer. The intersection of this zone

with the topographic surface is illustrated by the

presence of fractured massive limestone in which

the karren develops.

Profile 5 is located directly above the cave (noted

C), accessible by a vertical shaft 18.5 m deep (noted

D). This profile reveals the position of the cavity and

its geometry with precision. Moreover, the reflections

Fig. 5. Interpretation of profile 5. A: fractured limestones in the epikarst; B: massive and compact limestones; C: karstic cave of Lamalou; D:

pothole, inlet of the cave; F: fault; L: karren; P1, P2, P3: bedding planes; X: unknown cave.


near the cave also show that it is prolonged laterally

more or less horizontally along the bedding plane

which corresponds to smaller unknown cavities

(noted X). This disposition is in agreement with the

direct observations in the cavity; but the side exten-

sion is much broader on the profile than exploration

enables us to see, because of the narrowness of the

passage. In addition, the vertical shaft that develops in

line with the main fracture is quite visible on the

profile; furthermore, the cave has the largest dimen-

sions (height: 1 to 3 m, width: 3 to 8 m) at the

intersection of the fracture and the main bedding

plane. All the elements and the various structures

previously interpreted are also observed on the other

profiles: epikarst, dip of the layers, massive limestone

deposit at the base and underground cavities, some of

which are known.

In the vicinity of the shaft, profile 5 was compared

with the sections provided by boreholes and the direct

observations made in the shaft and the cave (Fig. 6).

Two cored boreholes (S1 and S2) provide a detailed

section above the cave. The description of cores

shows the following lithological column:

– surficial stony layer from 0 to 0.6 m,

– yellow limestone sometimes compact, sometimes

weathered and strewn with open fractures, from

approximately 0.6 to 11 m,

– gray compact limestone from 11 to 16.5 m,

– yellow limestone, weathered and fractured, from

16.5 m up to the ceiling of the cave.

The total porosity, measured with a mercury poros-

imeter on the cored samples is 1.84% (Bonin, 1980).

This very low value shows that water infiltrates from

the surface towards the saturated zone mainly along

the open fractures, cracks and karst conduits. The role

of the rock matrix can be regarded as almost negli-

gible. Between 11 and 16.5 m, the limestone is gray

because it was not weathered by water circulation.

The yellow color observed between 0.6 and 11 m

corresponds to a weathering by water circulating in

Fig. 6. Location of the karstic cave of the Lamalou experimental site showing the radargramme of profile 5 and the lithological column of

boreholes S2 carried out above the cavity. A: fractured and karstified yellow limestone of the epikarst; B: massive and compact gray limestone;

P: bedding plane.


the more fractured zone near to the surface, the

epikarst. At depth, limestone presents the same yellow

weathering related to water circulation in fractures in

the vicinity of the karstic conduit.

After having interpreted the profiles, we visited the

cave by the vertical shaft to compare the cave dimen-

sions with the obtained results. It appears that the

position and the real geometry of the cave accurately

correspond to the GPR profile. As shown by the

radargramme, the cave has developed along the bed-

ding plane and at the intersection of a vertical fault.

These features therefore explain the origins of the

room in the cave located 18.5 m below the surface.

The height of the cave roof varies from 1 to 3 m. In

places, it reaches 4 to 5 m or even more, in relation to

main vertical fractures going up sometimes close to

the surface and enlarged by solution. In the same way,

the characteristics of the various levels of rocks fit the

GPR data well.

5. Model suggested

A geological model was built up from the inter-

preted radargrammes, cored boreholes and the direct

observations of the surface and inside the cave (Fig. 7).

The model aims to translate the geophysical data into a

geological model in 3D representative of the various

prospected elements. It describes the whole of the

various structures supporting the shallow part of the

karstic aquifer of Lamalou in the vicinity of the spring.

We can distinguish, on the surface of the study zone,

three different facies related to the three limestone

layers of a different nature. On the first 40 m along

the profiles, the surface consists of a clayey soil thick of

a few centimeters, stone debris and of bedrock. This

part is limited by a thalweg located in the middle of the

model. This thalweg is directed towards the permanent

spring and crosses perpendicularly through the studied

zone. It seems that this thalweg is related to a regional

fault. Straight below this fault, the Lamalou cave

develops along a bedding plane. Then, in the right part

of the model, the surface becomes less argillaceous and

made up of much more abundant stone debris and

bedrock. This facies finishes 20 m away at the end of

the profiles by a quite visible bedding plane on the GPR

profiles and crossing the ground surface. This bedding

plane separates the layers made up of marly limestone

and a massive limestone that is densely fractured and

karstified in a karren.

Fig. 7. Synthetic model in 3D showing the general structure of the shallow part of the Lamalou karstic aquifer from the interpretations of the

whole set of GPR profiles, in particular the profile 5 radargramme and the cored drillings S1 and S2. (1) epikarst (fractured and karstified yellow

limestone); (2) infiltration zone (gray massive and compact limestone); (3) main room of the cave; (4) bedding plane; (5) pothole; (6) karren.

The arrows indicate the direction of the horizontal and vertical flows.


Vertically, the model consists of two main zones:

– A shallow zone, representing the epikarst, is made

up of strongly fractured and karstified yellow

limestone. Its average thickness varies from 8 to

12 m, according on the one hand to the state and the

nature of the surface, and on the other hand to the

distribution and the direction of the fractures. The

yellow coloring of the rocks is due to the processes

of abundant seepage in all the epikarst.

– Below the epikarst, the limestone becomes gray,

massive and compact and less fractured. This part

represents the infiltration zone of the karstic

aquifer. It is marked by the non-existence of

horizontal reflectors, therefore by the absence of

strong contrasts, except for the strong reflections of

the bedding plane. The infiltration of water towards

the conduit and the saturated zone is controlled by

fast flows in rare open vertical fractures and by

flows through microscopic cracks of limestone.

At the interface between the infiltration zone and

the saturated zone, the cave develops along the bed-

ding plane 20 m below the surface, probably in

relation to the fracture on which the thalweg is

established. This model is typical of karsts that are

not covered by thick soil or non-carbonate sediments.

It can be regarded as being representative of all the

Mediterranean karsts.

6. Conclusion

The absence of electrical conducting sediments

such as clays and the use of low frequencies (50

MHz in this study) render the application of the GPR

on the limestone formations efficient and useful

because of the weak attenuation of the radar waves.

The topographic corrections carried out on all profiles

contributed to reconstructing the various structures

obtained geometrically and to placing them in their

real position. That processing resulted in revealing

discontinuities of the rock, bedding planes, faults and

fractures. The interpretation of the radargrammes

underlined the structures that characterize the shallow

part of the karstic aquifer (epikarst, fractured and

karstified zones, bedding planes, massive limestone

beds and karren) as well as the conduit in the vicinity.

It also made it possible to locate the main cave 20 m

below the surface, even when its size is small.

The results obtained by the GPR are confirmed by

the boreholes carried out on the site. Thus, the direct

observations made on the surface, in the pothole and

inside the cavity also made it possible to compare and

validate all the prospected structures. The results of

this study can be generalized to karstic aquifers of

Mediterranean type. In that way, GPR prospecting is

very efficient for describing in detail the shallow part

of karst aquifers, when limestone outcrops at the

surface. It seems particularly useful for determining

vulnerability characteristics as well as geotechnical

properties or for positioning boreholes.

Acknowledgements

This work was realized in the frame of a scientific

cooperation between UMR 5569 Hydrosciences

(CNRS, Universite Montpellier II), UMR 7619

Sisyphe (Universite Paris 6, CNRS) and the Geo-

physics Department of IRD. It was supported by the

Atomic Energy Commission of Syria (AECS). The

manuscript benefited from the critical comments of T.

Horscroft, J.E. Nyquist and G. Buselli.

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analysis of the karst aquifer structure of the lamalou area

Documents

karst conduit

study of karst

karst aquifer structure

gpr sections

typical karst landform

gpr data

gpr pulseekko

main conduit