Journal of Applied Geophysics 51 (2002) 97 – 106
www.elsevier.com/locate/jappgeo
Analysis of the karst aquifer structure of the Lamalou area
(Hérault, France) with ground penetrating radar
Walid Al-fares a,b, Michel Bakalowicz a, Roger Guérin c,*, Michel Dukhan d
a
Université Montpellier II, CNRS Hydrosciences, c.c. MSE, 2 place Eugène Bataillon, 34095 Montpellier Cedex 5, France
b
AECS, B.O.Box 6091, Damascus, Syria
c
UMR 7619 Sisyphe, Département de Géophysique Appliquée, Université Pierre et Marie Curie (Paris 6),
case 105, 4 place Jussieu, 75252 Paris Cedex 05, France
d
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 50MHz 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-
* Corresponding author. Tel.: +33-1-44-27-45-91; fax: +33-144-27-45-88.
E-mail address: [email protected] (R. Guérin).
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: S 0 9 2 6 - 9 8 5 1 ( 0 2 ) 0 0 2 1 5 - X
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W. Al-fares et al. / Journal of Applied Geophysics 51 (2002) 97–106
proceedings of several conferences as the ‘‘Multidisciplinary Conference on Sinkholes and the Engineering
and Environmental Impacts of Karst’’ (Stangland and
Kuo, 1987; Roark and Lambert, 2001), the ‘‘International Conference on Ground Penetrating Radar: GPR’’
(Geraads and Omnes, 2002), the ‘‘European Association of Geoscientists and Engineers (EAGE) conference’’ (Yelf and Creswell, 1988), the ‘‘Environmental
and Engineering Geophysical Society-European Section (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; Elbehiry 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, hydrogeological, 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 (Thérond,
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 (Hérault, France).
W. Al-fares et al. / Journal of Applied Geophysics 51 (2002) 97–106
99
Fig. 2. Geological cross-section of the Hortus karst aquifer. Karst site of Lamalou spring (Durand, 1992).
France (Fig. 1). This limestone plateau, which covers
an area of 50 to 70 km2, is limited at the west by SaintMartin 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 Mediterranean climate characterized by irregular rainfall:
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.
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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 cultivated. 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 karstic 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 computer for the direct recording of raw data. The
console itself is connected to the radiating – receiving
antennae via optical fibers. The measurement parameters are summarized in Table 1. With this equipment, it is necessary to move the console and the
Table 1
GPR measurement parameters
Impulse power
Center frequency of antennae
Length of antennae
Measurement step
Recording time window
Sampling interval
Number of stacks
Battery power supply
400 V
50 MHz
2m
0.5 m
400 to 600 ns
1600 ps
32
12 V
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 electromagnetic 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 geological 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
W. Al-fares et al. / Journal of Applied Geophysics 51 (2002) 97–106
101
Fig. 4. Topographic representation in 2D of the zone of GPR prospecting on the Lamalou karst site.
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 (Bakalowicz, 1995) that plays a very important part with
regard to the processes of water storage close to the
surface and vertical infiltration towards the unsaturated 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. This
dip 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
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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.
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
W. Al-fares et al. / Journal of Applied Geophysics 51 (2002) 97–106
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 extension 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 dimensions (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
103
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 porosimeter 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 negligible. 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.
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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 dimensions 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 bedding 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 interpreted 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.
W. Al-fares et al. / Journal of Applied Geophysics 51 (2002) 97–106
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 bedding 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.
105
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, Université Montpellier II), UMR 7619
Sisyphe (Université Paris 6, CNRS) and the Geophysics 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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