New Elements on the Hairech Metamorphic Massif Structure ( Northwest Tunisia ) . Geoelectrical Prospecting Contribution

A geoelectrical survey was carried out in order to better understand the subsurface geology of the metamorphic massif of Hairech (Tunisia). Over 50 Schlumberger electrical soundings were performed using a maximum electrode spacing of 3000 m. The carried out qualitative interpretation of resistivity distributions for various depths is based on apparent resistivity maps that were constructed for different AB spacings. Quantitatively, the measured resistivity values were one-dimensionally interpreted. The already drilled boreholes and the position of some electrical soundings on geological outcrops have been helpful for electrical data calibration. The results of the vertical electrical soundings reveal that the metamorphic Hairech massif, correspond to a folded structure which is affected by deep trending east–west to northeast-southwest faults.

Most of the previous works were restricted to surface and little is known about their subsurface geology.
This study, which is based on the electrical resistivity method (Keller & Frischknecht, 1982), aims to infer a more accurate picture of the inland area of the Hairech massif (Figure 1).The used procedure includes the use of conventional vertical electrical sounding through the identification of resistivity layering along specific cross-sections.

Data and methodology
Vertical Electrical Sounding (VES) data acquisition was performed at 52 locations (Figure 2).The Schlumberger configuration was performed, considering a maximum electrode spacing of 3000 m.An electrical field is imposed on the inland area by a pair of electrodes (A and B) at varying spacing, expanding symmetrically from a central point, while measuring the surface expression of the resulting potential field with an additional pair of electrodes (M and N) at appropriate much smaller spacing.Apparent resistivity "a" is subsequently computed according to the following equation: (Dobrin, 1976) Where, I is the current introduced into the earth by A and B electrodes, and ΔV is the potential difference between M and N electrodes.
Apparent resistivity data, obtained at each vertical electrical sounding for different AB spacings, are arranged and contoured in the form of resistivity maps that express a qualitative representation of resistivity distributions for various depths.
For quantitative interpretation, the resistivity data were one-dimensionally evaluated.
The determination of initial thicknesses and resistivities are obtained by manual matching the VES field curves to the theoretical master curves and auxiliary point charts.These parameters are subsequently refined using an inverse technique implemented in WINSEV program (Jenny & Borreguero, 1993).The result is achieved when a good fit between the field data and the theoretically computed curve is obtained.
Interpretation problems, such as equivalence or suppression, show that the same electrical sounding curve a = f (AB/2) may result into different geoelectrical models.Geological data were compulsory in order to constrain the results and to solve the observed ambiguities, so that interpretation ends up to a realistic geological modeling.
The already available boreholes (Figure 2) allowed the calibration of the proximal VES.This information was used to lead the inversion results and to help producing good and accurate pictures of the subsurface geology.

Resistivity maps
The apparent resistivity values in Figure 3a   Figure 3b reveals low resistivity zone (< 50 Ohm.m) in the Kef El Agueb massif.The absence of high resistivity values suggests that the metamorphic layers made of dolomites and marbles do not extend to deeper levels.Whereas high resistivity contours (100 -150 Ohm.m) still persist near the jebel Hairech.This response can be explained by the important thickness of the Hairech series (sandstones, schists and dolomites), which exceed 900m.
Both Figure 3a and Figure 3b show east-west and northeast-southwest trending of the apparent resistivity contours.

Vertical electrical soundings calibration
The electrical identification of the Hairech series was possible thanks to VES 32.Substrata are homogeneous; therefore, apparent resistivity is good approximation of the true resistivity.A value of 155 Ohm.m is typical for the Hairech series (Figure 4).
Compared to the geological data provided by the boreholes (a, b, c and d), the VES (3, 11, 15 and 21) show a resistivity ranging from 100 to 200 Ohm.m for Liassic dolomites and marbles (Figure 5).Resistivity variations can be ascribed to the pore fluid chemistry.Fresh waters tend to be resistive while saline brines are very conductive (Telford et al., 1976).
VES48 calibrated with lithological column of borehole g allows the determination of the specific resistivity of the Mio-Plio-Quaternary series (Figure 6).The obtained values range in a low to moderate interval: 5-32 Ohm.m.
The calibration results, which constitute a guide for the vertical electrical soundings interpretation, are helpful for the construction of the geoelectrical cross-sections.

Geoelectrical cross-sections
Six geoelectrical cross-sections are established along northeast-southwest and northwest-southeast directions.The most representative cross-section is exposed in Figure7.Through its vertical and horizontal resistivity variations, this section clearly reflects an irregular subsurface structure.
The resistive 140m thick zone (130 Ohm.m) beneath VES33, rooted near the kef El Agueb massif, may express the continuity of the Liassic metamorphic formations.This is underlain by a conductive 13Ohm.mlayer which can be correlated with the Triassic rocks made of clays, dolomites and gypsum.Nearby, VES32 characterizes the resistivity Hairech series which read 155 Ohm.m.
According to the inferred geoelectric models, a Permo-Triassic age would be more logical for the highly resistive Hairech series.This result corroborates the works made by Burollet, 1973 andAlouani et al., 1991.Relying on the observed sudden resistivity change between neighbor locations VES 33 and VES 32 and the tectonic contact between the Liassic outcrops (Chemtou, Sidi Assem, Koudiat Nouamia and Sidi Abdallah) and Hairech jebel revealed by the geological map (Figure1a), a fault Fa (Figure7) that separates the kef El Agueb Liassic layers from the Hairech series is strongly suspected.
The observed 152 and 148 Ohm.m resistivity values at VES31 and VES30 are close to the characteristic Hairech series resistivity.This allows the extension of the metamorphic formations towards the north, beyond jebel Hairech.
The thickness decrease of the conductive zones located at VES33, VES31 and VES30 translates the thinning of the Mio-Plio-Quaternary cover over the metamorphic formations.It is interesting to note that beneath VES33, this cover points up a small depression which is individualized between Hairech and Kef El Agueb raised structures.This suggests that the metamorphic formations are folded into anticline and syncline shape structures.
It is clearly visible that the northern and southern parts of cross-section 4 (Figure7) reflect lateral resistivity discontinuities.The high resistivity layers consisting of the Hairech and the Liassic series disappear abruptly at VES 26 and VES 29 locations which resistivity (lower than 35 Ohm.m) fits the electric response of the Mio-Plio-Quaternary deposits.The geoelectric discontinuities match with the extension limit of the Hairech metamorphic massif (Liassic and Hairech series); it may indicate also Fb and Fc faults (Figure7).

Subsurface geological maps
As a synthesis of the vertical electrical soundings' results, subsurface geological maps are constructed (Figure 8).The maps, drawn for 50, 150 and 250m depth levels, reflect the lithological variations and characterize the structuring of the studied area.
The 50m depth map level (Figure 8a) shows a succession of northeast-southwest folds: Kef El Agueb massif and the eastern part of the jebel Hairech correspond to anticlinal structures separated by a small syncline, filled with Mio-Plio-Quaternary sediments.Similar structural features were described in the Medjerda molassic border and attributed to the Neogene compressive phases (Rouvier, 1977;Ben Ayed, 1986;Ould Bagga, 2003).This suggests that Hairech metamorphic massif was folded during these phases.
150m deeper (Figure 8b) a quite different pattern is outlined.The Mio-Plio-Quaternary cover response is not observed.Kef El Agueb Liassic layers and Hairech series are in contact through a northeast-southwest directed fault.The limits between Hairech metamorphic massif and Medjerda basin are drawn by east-west to northeast-southwest directed faults.Following these directions, geometrical contours of jebel Hairech and Kef El Agueb massif reflect the signature of fault activity.
The ENE-WSW trending faults are the predominating features is the Mediterranean region where the ongoing convergence between Africa and Europe is accommodated by subduction (Morel et al., 1996;Domzig et al., 2006).
Figure 8c, representing the deeper 250m map, expresses a quite similar structural geometry to that one exposed in Figure 8b, thereby indicating a deep faulting.However, a vertical lithological change is noticed at Kef El Agueb massif.So, Liassic layers disappear in favor of Triassic deposits.
The east-west to northeast-southwest fault activities and the Neogene folding constitute the two mechanisms contributing to the structuration of the Hairech metamorphic massif.These mechanisms are well expressed by the elaborated subsurface geological maps.

Conclusion
In this paper, we have presented new results based upon resistivity mapping and one dimensional geoelectrical modeling of the inland area of the Hairech metamorphic massif.

Figure 1 .Figure 6 .Figure 8 .
Figure 1.Structural setting and geology of the Hairech massif.(a) Structural elements of northern Tunisia (modified from Rouvier, 1977), (b) Geological map of the Hairech massif (modified fromGottis & Sainfeld, 1955) The geoelectrical data have shown the following: Hairech series, consisting of sandstones, schists and clays, are characterized by a resistivity as high as 155 Ohm.m.They extend towards the north, beyond jebel Hairech location.Their thickness exceeds 900 m.  The resistivity range 100-200 Ohm.m is typical of the Liassic dolomites and marbles.These metamorphic formations, which average thickness is 160m, are underlain by the Triassic conductive layers (10-18 Ohm.m) consisting of clays, dolomites and gypsum. The Hairech metamorphic massif is a folded northeast-southwest directed structure.Neogene compressive phases may be responsible of this folding.The small syncline, separating Kef El Agueb and Hairech jebel anticlines, constitutes the receptacle of a thin Mio-Plio-Quaternary sedimentation. The structural pattern is affected by deep faults expressing the abrupt lateral resistivity variations.A northeast-southwest fault insures the contact between Kef El Agueb Liassic formations and Hairech series.Additionally, the conductive Mio-Plio-Quaternary deposits (2-35 Ohm.m) filling the Medjerda basin, are separated from the metamorphic formations by east-west to northeast-southwest trending faults. The geometry of Hairech metamorphic massif is the result of the east-west to northeast-southwest faults activity associated to the Neogene folding.