(ERT) Applied to Karst Carbonate Aquifers: Case Study from Amdoun, Northwestern Tunisia
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Pure Appl. Geophys. Ó 2015 Springer Basel DOI 10.1007/s00024-015-1173-z Pure and Applied Geophysics Electrical Resistivity Tomography (ERT) Applied to Karst Carbonate Aquifers: Case Study from Amdoun, Northwestern Tunisia 1 2 1 1,3 1 BELGACEM REDHAOUNIA, BATOBO OUNTSCHE ILONDO, HAKIM GABTNI, KHOMSI SAMI, and MOURAD BE´ DIR Abstract—The Amdoun region is characterized by a high Key words: Electrical resistivity tomography (ERT), Karst, degree of karstification due to the climate impact Cavities, Perched aquifer, Amdoun (NW Tunisia). (±1500 mm year-1) and the development of fracture network. Survey using electrical resistivity tomography (ERT) is deployed to provide a cost-effective characterization of the subsurface karst environments. A total of seven ERT profiles with lengths of 315 m were evaluated at the Be´ja governorate (NW Tunisia). The area 1. Introduction represents a small syncline of Boudabbous limestone rocks (Lower Eocene), which is covered by a thin layer of clay. In this study, an ERT survey was conducted to examine the spatial distribution and Karst aquifers are known to be highly heteroge- shape of underground cavities in the karst area in Jebel Sabah neous, formed by a complex conduit system that is anticline and Aı¨n Sallem-Zahret Medien syncline. In this study, generally impossible to locate. Moreover, the geological, hydro-geological and electrical resistivity tomography (ERT) methods were applied to determine the geometry of the resources are very hard to exploit, with permeability perched aquifer in the Amdoun region (NW Tunisia). The area is coefficients ranging from 10-9 to 10-1 ms-1, and characterized by fractured and karstic limestone aquifer of Late flow velocities ranging from a few millimeters a day Cretaceous (Abiod Fm.) and Lower Eocene (Boudabbous Fm.). The aquifers have a karstic functioning and drain aquifers of eco- to hundreds of meters an hour (BAKALOWICZ 2005). nomical interest, despite some wells exploiting them. Seven Carbonate rock is soluble in the presence of acid resistivity profiles were conducted along the survey area at three dissolved in water, most commonly resulting from sites. The orientation, extension and the degree of inclination of CO solution. The process that combines solution of those profiles are shown in the location map. The correct resistivity 2 data were interpreted using Earth Imager 2D software. The results carbonate rock and flow in openings is named of the interpreted geo-electrical sections showed that the resistivity ‘‘karstification’’ (FORD and WILLIAMS 1989). Karst of the carbonate aquifer varied between 2.5 to over 5794 Xm. The aquifers constitute 12 % of the global land surface thickness of the perched aquifer ranged from 15 to 50 m, while its depth from the surface lies between 10 and 60 m. The ERT not and store water on which approximately 20–30 % of only provided precise near surface information, but was also very the Tunisia population depends, as stated in classical useful for establishing the 3D geometry and the position of several textbooks on karst. These carbonate reservoirs (Abiod potential cavities and karts. The results show the presence of small to large isolated cavities at various depths. The low resistivity of and Boudabbous/El Gueria formations) in the study cavities in the Boudabbous Formation has been explained by the area constitute approximately less than 1 % of all groundwater saturation. The ERT technique could be effectively limestone outcrops in Tunisia. Hydro-chemical used for 3D detection of underground limestone cavities. analyses aid in the identification of the mechanisms causing the accumulation of carbonates in ground- water and the water type. The stable isotopes of water have been analyzed in investigations of karst aquifers to understand the recharge processes, reservoir mixing and the inter- 1 Water Researches and Technologies Center Borj-Cedria actions between groundwater and surface water and (CERTE), BP 273, Soliman 8020, Tunisia. E-mail: belgacem.red- [email protected]; [email protected]; [email protected] to identify water sources in karst springs (KATZ et al. 2 Advanced Geosciences Europe-Spain, Madrid, Spain. 1997;VANDENSCHRICK et al. 2002;MALOSZEWSKI et al. E-mail: [email protected] 2002;PERRIN et al. 2003;ANDREO et al. 2004;LONG 3 King Abdulaziz University, Jeddah, Saudi Arabia. E-mail: [email protected] and PUTNAM 2004;SCHWARZ et al. 2009;HAMED et al. B. Redhaounia et al. Pure Appl. Geophys. 2014…). Geophysical methods are very useful for et al. 2010;ARIDHI et al. 2011;REDHAOUNIA et al. studying karst terrains because of the intrinsic 2012, 2013a, b;ANENE et al. 2013). heterogeneity of the medium (SACKS and TIHANSKY From the structural point of view, the Meso– 1996;Sˇ UMANOVAC and WEISSER 2001;VAN SCHOOR Cenozoic formations in northwestern Tunisia are 2002;VOUILLAMOZ et al. 2003;SMITH 2005;JARDANI intensely folded and faulted. The tertiary formations et al. 2006;NEUKUM et al. 2010). However, the are structured in a series of anticlines and synclines in electrical resistivity tomography (ERT) imaging the NE–SW direction. The main structural features provided significant information of the structural are anticlines and synclines, faults and fractures. The setting of the Amdoun region characterized by high structural and geomorphological features have groundwater potential, fractured and chalky lime- affected the limestone sedimentary rocks in the stone bodies due to the elevated density and Amdoun study area and have a great impact on the concentration of open fracture network within the karst processes. The oldest rocks are the E–W Tri- very brittle chalk of the lower Eocene (Boudabous/El assic diapiric outcrop of the Jebel Ezzouza, and Gueria Formations) and Upper Cretaceous (Abiod except for the lithological sequences in this study Formation). These two units are characterized by area it varies in age from the Upper Cretaceous to the open fractured limestones, interconnected drainage Quaternary (GOTTIS and SAINFELD 1956). The major paleochannels, shallow and deep karst and caves part of the Amdoun basin (35 % of the carbonate (REDHAOUNIA et al. 2014b, c;AYADI et al. 2014). units) is underlain by carbonate rocks with moderate The present study is an approach using the ERT to high degree of karstification. The densities of caves imaging technique to identify and characterize the and karstic springs are the hydro-geological features discontinuities, faults and water investigation of the used to check the calculated degree of karstification. fractured and karstified limestone aquifers in the These types of surface karst features are common in Amdoun Mounts. The conducted research demon- the Amdoun study area where relatively pure car- strated that the ERT method was an effective tool for bonate bedrock is present and precipitation levels are imaging the subsurface in the karst terrain. high (REDHAOUNIA et al. 2014c). Karsts developed in the Upper Cretaceous limestones as well as in the tertiary carbonate (Eocene Formations). The karst 2. Geology of the Study Area systems in the Upper Cretaceous limestones are independent of those in the carbonate limestones Tunisia is located at the intersection between the (Abiod Formation). The most extended caves systems African, and European and Mediterranean plates develop in the Upper Cretaceous and constitute the (Fig. 1a). Its structural framework is characterized by regional karst system which can be divided into a transition from the Saharan platform in southern several subsystems. Tunisia to Alpine folded structures (Atlasic domain and Tellian troughs) in the north (BOUAZIZ et al. 2002). The Amdoun region is divided both geologi- 3. Hydrogeological and Petrophysical cally and geographically into two zones roughly of Characteristics equal importance, characterized by highly faulted outcrops of Oligo-Miocene Numidian Flysch series The region where the aquifer is located presents a unconformably over folded and faulted para-au- typically Mediterranean climate, where cool and tochthonous to autochthonous Cretaceous to Eocene rainy seasons alternate with hot dry seasons. The units (Fig. 1b, c). The Nappes Zone (Alpine thrust average temperature is 16.5 °C and the potential belt) and the Imbricated Zone is immediately situated evapotranspiration estimates range between 1000 and -1 and affected by thrusts and reverse faulting (GOTTIS 1800 mm year . The mean precipitation is -1 and SAINFELD 1956;ROUVIER 1985;KHOMSI 1998;EL 1400 mm year and there are marked seasonal EUCHI et al. 1998, 2004;KHOMSI et al. 2009;RIAHI variations. Most rainfall takes place in the autumn. Electrical Resistivity Tomography (ERT) Applied to Karst Carbonate Aquifers Figure 1 a Geographical location of the study area; b general geological map of the Amdoun study area; c geological map of the study area including ERT survey of the three sites, 1, 2, and 3; d positioning plan of ERT profiles in site 3 Amdoun carbonate aquifers are made up of Upper represented more than 200 9 106 m3 year-1 with Cretaceous limestones (Abiod Formation) of three more than 13 % of groundwater resources of Tunisia. members whose thickness ranges between 200 and These formations present as very karstifiable 250 m and presents anticlines structures in the axis geologic units in the Amdoun Mounts, because the direction NE–SW (Jebel Sabbah). The Boudabbous karstic rock masses are pervaded by a network of perched carbonate aquifer, about 4.7 km2, is com- open discontinuities and fractures with calcite filling pletely isolated from other