International Journal of Geosciences, 2011, 2, 373-387
doi:10.4236/ijg.2011.23040 Published Online August 2011 (http://www.SciRP.org/journal/ijg)
Lineaments Extraction from Gravity Data by Automatic
Lineament Tracing Method in Sidi Bouzid Basin (Central
Tunisia): Structural Framework Inference and
Hydrogeological Implication
Hajer Azaiez1*, Hakim Gabtni1, Imen Bouyahya1, Dorra Tanfous2, Soumaya Haji3, Mourad Bedir1
1Laboratoire de Géoressources, Centre de Recherches et des Technologies des Eaux,
Technopôle Borj-Cedria, Soliman, Tunisia
2Institut Préparatoire aux Etudes d’Ingénieur de Bizerte, Zarzouna, Bizerte, Tunisia
3Laboratoire 3E, Ecole nationale des ingénieurs de Sfax, Route de S oukra Km 4, Sfax , Tunisi a
Received Feburary 2, 2011; revised May 11, 2011; accepted June 24, 2011
Abstract
The gravity method may be used in the exploration of deep sedimentary basins. It allows the structuring and
the lateral and vertical extent of sedimentary fill to be determined. This study has concerned a qualitative and
quantitative gravity analysis of Sidi Bouzid Basin in Central Tunisia. Bouguer anomaly analysis and Gravity
data filtering allow us to emphasize the structures affecting the basin. The Automatic Lineament Tracing
method helps to quantify the different gravity responses of faults located in the shallow and deep sedimen-
tary sections and in the basement. The elaborated structural map of the study area constitutes a useful docu-
ment for rationalizing the future groundwater exploration in the arid area of central Tunisia since it shows
faults dipping and deep hydrogeologic sub-basin delineation.
Keywords: Gravity, Lineaments, Extraction, Tunisia, Hydrogeology
1. Introduction
The Sidi Bouzid Basin, situated in central Tunisia (Fig-
ure 1), is characterized by a Mediterranean semi-arid to
arid climate with irregular annual rainfalls and long dry
periods. Use of groundwater is increasing in order to
meet the demand for domestic, agricultural, and indus-
trial needs. Therefore deep aquifer exploration and ex-
ploitation become a n ecessity in this area.
Gravity data have been traditionally thought of as re-
gional screening tools capable of providing basin defini-
tions and basement mapping. However, in recent years,
the application of potential field data has been greatly
expanded to include global paleotectonic modelling
through to modelling of prospect-level targets. One of the
most important phases of any exploration screening pro-
gram, particularly, in areas that lack seismic a n d well data,
is the integration of potential field data with various geo-
logical datasets to define structural elements, continental
block outlines, and crustal types, with the aim of produc-
ing a detailed, digi t a l st ruct ural and geological coverage.
The gravity survey method was selected as the geo-
physical method that would give a regional picture of the
subsurface geology before making extensive surveys by
the seismic reflexion method. Basically, the gravity sur-
vey method detects and measures variations in the earth’s
gravitational force. These variations are associated with
changes in rock and alluvium density near the surface.
Many geologic structures of interest in watershed ground-
water hydrology cause disturbances in the normal density
distribution which give rise to anomalies.
2. Geological Setting
Central Tunisia is a part of the Atlassic chain. This com-
partment is composed of NE-SW trending structures as-
sociated with some reverse faults and thrusts, particularly
within the northern and central portions, and composed
of Mesozoic and Cenozoic rocks (Figure 2). The struc-
tures correspond to folded and thrusted Cretaceous, Pa-
leogene and Neogene rocks, forming asymmetric anti-
clines. The Tunisian Atlas is also transacted by mid-

H. AZAIEZ ET AL. 378
Figure 1. (a): Tectonic pattern of the western Mediterranean domain (Bouaziz et al., 2002) and location of the study area. (b):
Shaded relief map of the study area and locations of gravity data and seismic profile.1: Sidi Bouzid basin; 2: N-S Axis; 3:
Sahel basin.

H. AZAIEZ ET AL.379
Figure 2. Geologic map of Sidi Bouzid basin and surrounding area (adapted after Smida, 2008).

H. AZAIEZ ET AL. 380
Miocene NW-SE transverse grabens. The Atlassic chain
in central Tunisia was marked during the Mesozoic by a
complex mosaic of basins and highs (Kasserine Island)
separated by major faults. One particular major struc-
ture of this domain is the N-S axis (NOSA) [1] (Figure
2). It is a 100 km long N-S trending tight fold crossing
the whole Atlasic domain of central Tunisia. This struc-
ture resulted from the polyphase reactivation of an inher-
ited Pan-African or Paleozoic lineament. During several
Mesozoic periods, the N-S axis acted as a basin bound-
ary, separating zones with low and high subsidence rates
[2].
Central Tunisia, including the study area, was part of
the southern Tethyan Platform which underwent Trias-
sic-Jurassic extension. During Tertiary compression, the
Atlasic domain was highly deformed. The end of the
Eocene was marked by the onset of strong compressional
tectonism, causing the destruction of the basin and the
end of marine conditions in southern Tunisia [3] and [4].
An example of seismic profile (Figure 3, location on
Figure 1) inside Bouzid basin shows the variety of struc-
tural styles including horst, graben, half-graben, uplifted
fault blocks.
3. Hydrostratigraphic Setting
The geological cover of the Sidi Bouzid basin is a thick
sedimentary stack from Triassic to Quaternary. Here
down, a brief and synthetic description according to the
geological maps of Sidi Bouzid basin (Figure 4):
- Composed of gypsum, anhydrites, clays or dolomites
(Rheouis formation), the discordant Triassic extrusions
are behind the structural complexity in Central Tunisia,
and contribute to the mineralization and moderation of
the ground water quality [5].
- The Jurassic is represented by Nara Formation [1]
with two carbonate members separated by an irregular
marly and oolitic middle member [6]. The outcrops are
along the N-S Axis.
- The Cretaceous outcrops are wide-spread and com-
mon in Central Tunisia and form the body structure of
the main anticlines. The Neocomian series consist of
three formations representing a deltaic progra- dation
towards the North: Sidi Khalif, Meloussi, and Boudinar
Formations. Lower Cretaceous deposits are characterized
by competition between terrigenous progradation from
the Saharan Craton and marine carbonate deposits that
predominate in the North [7].
The Zeabbag (Albo-Cenomanian) formation includes
two carbonate members separated by a clay and gypsum
middle member. The Aleg (Turonian to early Campa-
nian) is a thick series of gray marl and shale interbedded
between the top of Zebbag or Fahdene formations and
Figure 3. Seismic profile (location on Figure 1) inside Bouzid basin shows the variety of structural styles.

H. AZAIEZ ET AL.381
Figure 4. Lithostratigraphic column of Sidi Bouzid basin and surrounding area.

H. AZAIEZ ET AL. 382
the Abiod formation [8]. The Abiod (Campanian-Maestri-
chtian) is essentially made up of carbonates, generally
chalky limestones.
4. Gravity Data
The Land gravity data used for this study were obtained
from the “Entreprise Tunisienne d’Activités Pétrolières”
(ETAP) (Figure 1). All the data were merged and re-
duced using the 1967 International Gravity formula [9].
Free Air and Bouguer gravity corrections were made
using sea level as a datum and 2.67 g/cm3 as a reduction
density. The Bouguer gr avity anomaly data were gridded
at 2 km spacing and contoured to produce a Bouguer
gravity anomaly map (Figure 4).
The Satellite Bouguer Grav ity data were obtained from
the Bureau Gravimétrique International (BGI). The re-
gional Free-air and Bouguer gravity anomaly grids (av-
eraged over 2.5 arc-minute by 2.5 arc-minute) are com-
puted at BGI from the EGM2008 spherical harmonic
coefficients [10]. The Bouguer corrections computed at
regional scales are obtained using the FA2BOUG code
developed by [11]. The topographic correction is applied
up to a distance of 167 km using the 1 arc-minute by 1
arc-minute ETOPO1 Digital Elevation Model. Density
reduction for Bouguer anomaly: 2.67 g/cm3.
The Satellite Bouguer gravity an omaly data were gridded
and contoured to produce a Satellite Bouguer gravity
anomaly map (Figure 5).
5. Bouguer Gravity Analysis
5.1. Land Bouguer Gravity Map
The Figure 5 represents the Land Bouguer gravity map
of the study area. The anomaly values vary from –80
mGal to 5 mGal. It shows gravity highs and lows of
variable dimensions and amplitudes. Bouguer gravity
lows represent potential areas for hydrogeological ex-
ploration associated with the filling of this area by light
sediments.
5.2. Satellite Bouguer Gravity Map
The Bouguer gravity anomaly values (Figure 6) in the
study area vary from –85 mGal to 0 mGal and are gener-
Figure 5. Land Bouguer gravity map of Sidi Bouzid basin and sur r ounding area.

H. AZAIEZ ET AL.383
Figure 6. Satellite Bouguer gravity map of Sidi Bouzid basin and surrounding ar e a.
ally low in its western parts. The highest anomaly values
are observed in eastern part of the map. The Bouguer
gravity map shows a regional variation from East (posi-
tive anomalies) to West (negative ano malies). This varia-
tion represents the regional gravity field that is deter-
mined from crustal thickness variations [12] and [13].
6. Automatic Lineament Detection Map
Analysis
The automatic lineament detection algorithm required
the data to have been processed (or transformed) such
that the edge of a causative body is located beneath a
maximum in the grid. Several transforms satisfy this re-
quirement e.g. horizontal derivative of gravity data [14]
and also analytic signal.
The results help to quantify the different gravity re-
sponses of structures located in the shallow and deep
sedimentary sections and in the basement. A significance
factor N, ranging in value from 0 to 4, is assigned to each
grid cell depending on the relation to its neighbours. N =
1 might represent a point on a spur, N = 2 and N = 3 a
point on a ridge and N = 4 a point on a peak. The values
of N are colour coded and displayed as a grid [15]. These
lineament grids can then be displayed on top of any other
grid.
Maxima and horizontal derivative map from the satel-
lite Bouguer anomaly are shown in Figure 7. They show
alignments outlining the contacts. The resulting struc-
tural map explains some hydrogeological problems: 1)
the change of direction of groundwater flow; 2) change
of quality of groundwater (like salinity).
The overlay of maxima and horizontal derivative map
from the satellite Bouguer anomaly and vector direction
derived from satellite Bouguer gravity and Digital earth
Model (DEM) of the study area shows alignments and
contacts (Figure 8). Generally, the area may be dissected
by major faults striking in N120-140, N0, N45 and N90
with a clear prevalence of the first family direction. The
network NW-SE crosses the area transversely, and cor-
responds to kilometric faults parallel to the major axis of
the gravimetric anomalies. Other directions N-S, NE-SW
and E-W have a rather homogeneous distribution in the
area study and are observed on various scales. The vector

H. AZAIEZ ET AL. 384
Figure 7. Overlay of maxima and horizontal derivative map from the satellite Bouguer anomaly and vector dire ction derived
from satellite Bouguer gravity of Sidi Bouzid basin and surrounding area.
Figure 8. A: Digital earth Model (DEM) of Sidi Bouzid basin and surrounding area. B: Overlay of maxima and horizontal
derivative map from the satellite Bouguer anomaly and vector direction derived from satellite Bouguer gravity and 3D Digi-
tal earth Model (DEM) of Sidi Bouzid basin and sur rounding are a .1: Hajeb El Ayoun graben, 2: Jelma basin, 3: Oule d Asker
basin, 4: Oued El Hajal basin, 5: Sidi Bouzid basin, 6: Horchane-Braga basin, 7: Meknassy basin, 8: Bled Regueb basin.

H. AZAIEZ ET AL.385
Figure 9. The structural lineaments after the Horizontal Gravity Gradient map (A) and their surimposition on the Sidi
Bouzid hydrogeological map (B).

H. AZAIEZ ET AL. 386
direction derived from Satellite Bouguer gravity give an
aspect of the preferred directions of lateral water accu-
mulation. This original prediction allowed as better rec-
ognizing and evaluating the regional groundwater poten-
tial of Sidi Bouzid sedimentary sub- basins (1: Hajeb El
Ayoun graben, 2: Jelma basin, 3: Ouled Asker basin, 4:
Oued El Hajal basin, 5: Sidi Bouzid basin, 6: Hor-
chane-Braga basin, 7: Meknassy basin, 8: Bled Regueb
basin) in central Tunisia.
7. Hydrogeological Implication
The surimposition of lineaments after the Horizontal
Gravity Gradient map (Figure 9(A)) and the hydro-
geological map of the Sidi Bouzid basin (Figure 9(B))
shows the importance of the determined regional gravity
trends and their influence related to the groundwater
flow directions and the groundwater systems relations.
The lineaments: L1, L2 L3, L4, L5 and L6 (Figure 9(B))
correspond to deep faults bordering the Hajeb El Ayoun
groundwater system (1, Figure 9(B)). Lineaments L4
and L5 are, also, associated with hot springs (Figure
9(B)). L7, L8, L9, L10 and L11 lineaments embody ma-
jor limits between different groundwater systems: Oued
El Hajel groundwater system (2, Figure 9(B)); Jelma
groundwater system (3, Figure 9(B)) and Sidi Bouzid
groundwater system (4, Figure 9(B)). We can note also
the influence of faults on the hydrodynamism and
groundwater flow directions in the Bahira groundwater
system (5, Figure 9(B)).
8. Conclusions
The structural map produced, according to the gravity
data analysis and processing, shows the N-S, NE-SW
and NW-SW fault system bordering th e sub-basins in the
survey area. These faults may have significant implica-
tions for groundwater quality and qu antity in Sid i Bouzid
basin. Indeed, they may exh ibit enh anced permeability o r
serve as barriers to subsurface fluid flow, depending
upon a number of variables related to host rock/sediment
lithology, fault zone diagenesis, and faulting mecha-
nisms.
This map forms the basis for planning future hydro-
geological research in this region. Further investigation,
is necessary to verify the presence of the lineaments
identified during this study, and their relationship to hy-
drogeologic features. Some of the types of investigation
that could be initiated are:
• Seismic reflection-refraction geophysical surveys.
Two-dimensional surveys across the study area will pro-
vide valuable data to further evaluate precisely fracture
zones. Specific areas of interest can be further defined
with 3-dimensional sei s mic surveys.
• Hydrogeologic investigations such as aquifer per-
formance with multiple monitoring zones in the aquifers
and confining units will help to validate potential im-
pacts of fractures on the hydrogeologic system. Tracer
tests or tomography could also be employed at specific
locations to evaluate the presence of fractures and
groundwater movement.
• Detailed geologic analyses incorporating available
geophysical, hydrogeologic, and geochemical data will
provide further analyses of variances in hydraulic char-
acteristics and water quality.
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