2-D inversion of magnetotelluric data at Dar-Chioukh region (Djelfa, Algeria) Naila KERBADJ1, Abderrezak BOUZID2, Hakim SAIBI3,*, Mouhand Ouabdallah BOUNIF1, Sofiance BOUGCHICHE3, and Yocef KEBEDE4 1Department of Geophysics, Univ. of Sc. and Tech. Houari Boumediene, Algiers, Algeria. 2CRAAG, Algiers, Algeria. 3UAE University, Al-Ain, UAE. 4Lab. of Exploration Geophysics, Kyushu University, Japan

Summary Measurements from eight magnetotelluric (MT) stations have been taken at Dar-Chioukh in Djelfa, Algeria in March 2008. This study presents the results of two-dimensional (2-D) inversion analysis of MT data in order to understand the geological features and investigate the 2-D resistivity structure beneath the study area. The Q-function and Bahr’s analysis were used to estimate strike and dimensionality of MT data. The results show a 2-D behavior with an electrical direction striking in NE-SW in agreement with geological and topographic maps. Rebocc-MT2D computer program was used to invert the MT data. The 2-D MT inversion results show successive conductive and resistive layers of about 200 m to 700 m thick. The conductive layers represent the groundwater aquifers and the different conductivities are interpreted by the different orders of salinity of the groundwater aquifers. The deep conductive layer (depth of 1 to 1.5 km) could represent a salt dome intrusion and/or a salty water aquifer. This research could be helpful for future hydrocarbon and water explorations in Djelfa region. Introduction Magnetotelluric (MT) is an electromagnetic geophysical method and applies to a vast range of different exploration surveys in hydrocarbon, geothermal (Kebede and Saibi, 2016), mining and groundwater investigations (Abdelzaher et al., 2012). In this study, 2-D inversion models of the rotationally invariant determinant of the apparent resistivity of MT were calculated using the Rebocc-MT2D computer program. We conducted strike and dimensionality analysis to assess to what extent the data can be interpreted in a 2-D sense on a regional scale and to define the appropriate profile direction. Following the approach of Zhang et al. (1987) makes it possible to estimate the strike direction based on multi-site and multi-frequency averaging. Geological and tectonic settings Being an integral part of the Ouled-Nail Mountains, the region of Djelfa, is located in the north eastern Saharan Atlas. This is in the central part of north Algeria at equal distance from its eastern and western borders, between 2° and 4° E longitudes and 34° and 35° N latitudes, covering an area of 32,257 km². This area is limited to the north by the North Atlasic Accident and to the south by the South

Atlasic Accident (Fig. 1). The primary geological information for the area was derived from the work of Pouget (1971) and the geological map of Algeria at scale 1/500,000 (Peron, 1883). The Saharan Atlas is characterized by large synclinal and anticline folds and is specified by two main structural directions. The first one, in the western part, which is represented by elongated structures in a NE-SW direction. The second, in the eastern part, is represented by an E-W direction structures (Cornet, 1951).

Figure 1: Mean geological units of northern Algeria. The study area, located at 34 km from the Djelfa town, is a village called Dar-Chioukh (Fig. 2). It is represented in the form of a vast syncline between two narrow anticlines marked by faults of NE-SW direction. Its central part is a depression filled by continental deposits of the Quaternary (Soltanian) discordant on the Cretaceous series. The structures that form the two flanks of this syncline are: Djebel Testara in the South, Djebel Sahari and Djebel Chebeibita in the North, where the deposits of the Cretaceous have a wide extension. The northern flank is distinguished from the southern flank by the presence of many metrical folds affecting mainly the Cenomanian formation. These folds end up disappearing at the approach of the Turonian formation. The anticlines which limit the syncline of Dar-Chioukh are affected by important faults visible at the outcrop (Chibane, 2010). The tectonic behaviour of the region is very complicated, marked by the traces of several important deformation phases that took place in the Mesozoic and the Cenozoic. The sedimentary series extends from the Cretaceous to the Quaternary. The formation of Cretaceous age constitutes the framework of the mounts of Djelfa and the substratum of the cuvettes where the Neogene and Quaternary series are deposited in discordance (Emberger, 1960).

2-D magnetotelluric inversion at Djelfa (Algeria) for geological investigations

The groundwater resources in this region come from four distinct aquifers; the lower Albian, the Turonian, the Maastrichtian and the Tertiary continental. The flow of water in the study area is temporary, the waters are accumulated in the depressions where they are maintained for a period of a few days to a few weeks in the rivers or in the plains where they evaporate and infiltrate (Chibane, 2010).

Figure 2: Simplified geological map of the study area. Black circles represent the MT stations. Methodology The magnetotelluric data was collected in March 2008 in the Djelfa region (south of Algiers). Thus, the MT five-component time series (2 telluric and 3 magnetic) were recorded on 8 sites forming a SE-NW profile of 3 km long. The interstation distance varies from 0.2 to 1.6 km (Fig. 3). In order to better constrain the underlying basin structure, the profile was chosen to be perpendicular to the direction of the geological structures elongation.

Figure 3: MT sites on Goolge Earth image. The time series was recorded using the V5 system 2000 of Phoenix Geophysics. AMT (Audio-Magnetotelluric or high frequency) rrecordings were combined with MT

measurements to give broadband recordings at each station. The AMT measurements were taken over a period of 30 minutes to a few hours, whereas the conventional MT measurements were carried out over a duration of 2 hours to 20 hours. The measurement of the two electric field components was realized using two pairs of impolarizable electrodes. Three coils were used to measure the three magnetic field components. To shift from AMT to MT in a given station, the magnetic sensors are changed while keeping the electrodes in their places. The measurements were carried out with a single MT station without the use of a reference station. The time series were then processed using a robust code provided by the manufacturer. This code is based on the method of Jones and Jödicke (1984), more particularly on method 6 of Jones et al. (1989). The AMT and MT data is then merged to form broadband soundings with frequencies ranging from 10,000 to 0.01 Hz (Fig. 4).

Figure 4: apparent resistivity and phase of the xy and yx magnetotelluric impedance tensor component of site dj-7. Results and Discussions Strike and dimensionality analysis were conducted to assess to what extent the data can be interpreted in a 2-D sense on a regional scale. The degree to which the data supports an interpretation, in terms of 2-D models, is

2-D magnetotelluric inversion at Djelfa (Algeria) for geological investigations

important for assessing the quality of the final model. Following the approach of Zhang et al. (1987) makes it possible to estimate the strike direction based on multi-site and multi-frequency averaging. According to Bahr’s (1988) recommendation, the regional 2-D indicator (3D/2D skew) should be below 0.3 for regional 2-D structure (Fig. 5).

Figure 5: Bahr 3-D/2-D skew values for the profile and threshold of 0.3 is marked by the change of color from blue to red. Most of the skews are less than 0.3. The Bahr strike is defined for underlying 2-D regional structure in the presence of 3-D near-surface inhomogeneities. In the Q-function analysis, the strike direction is found by minimizing the object function (Q-function) as discussed by Smirnov and Pedersen (2009) (Fig. 6).

Figure 6: Rose diagrams of the regional strike according to Bahr, and Q-function strike for the combination of stations. The period range is 0.0078–10500 Hz (127.8-0.0001 sec). The Bahr and Q-function regional strike shows NE-SW direction estimated from the impedance tensor which is

subject to 90 degree ambiguity. This ambiguity can be resolved using induction arrows (or by referring to the geological structure), which is in a 2-D environment and perpendicular to the geoelectric strike direction, so we need to rotate the impedance tensor in order to get good model for TE and TM mode. By rotating the impedance tensor by -45 degree, we can keep the strike direction parallel and perpendicular with E-field and/or H-field. In the inversion of the MT data using Rebocc-MT2D, the starting model was given a homogeneous half space with 100 Ω-m. The determinant phase, and apparent resistivity data, were inverted simultaneously with 10 iterations and the root mean square (RMS) error values between the models and observed data were 0.9 (Figs. 7 and 8).

Figure 7: 2-D inversion model of determinant of impedance Z from the profile of the study area after rotating by -45 degree.

Figure 8: Interpretation of the 2-D MT inversion model. R1 and R2 are resistive zones, C1 and C2 are conductive zones.

2-D magnetotelluric inversion at Djelfa (Algeria) for geological investigations

The 2-D resistivity model shows a succession of conductive and resistive layers. Thus, it is possible to clearly distinguish a surface conductive layer of ~400 m thick corresponding to the Maastrichtian aquifer, resting on a resistive layer (R1-R2) of 400 m thickness. A third layer is characterized by a decrease of resistivity with depth up to 1000 to 1500 m (Turonian aquifer). Then, a very sharp drop in resistivity probably due to a salt oversaturation of the Albian aquifer causes a drastic attenuation of the electromagnetic waves making it difficult to constrain the geological structure beyond 1500 m depth. Moreover, a strong lateral change in the resistivity (C1) is observed SE of the study area near dj-8. Although difficult to constrain by our data because it is located at the end of the profile. It could be a fault along which salty waters rise. Under the central part of the profile (beneath dj-5 and dj-6), another sub-vertical conductive structure (C2), but less accentuated than C1, passes through the resistive layer and separates it into two parts, R1 to SE and R2 to NW. Moreover, this conductive structure (C2) is located just above a substantial rise of the deep conductive layer reaching an amplitude of 400 to 500 m, which could result from a phenomenon of Triassic salt diapirism. Conclusions The analysis of dimensionality and strike of MT data indicates 2-D character for most period bands, and the strike is almost in the NE-SW direction, matching with the NE-SW orientation of the geological structure. The 2-D MT inversion result shows a succession of conductive and resistive layers. The conductive layers are attributed to the groundwater aquifers with different salinities. From the top to down, the conductive layers present the Maastrichtian, the Turonian, and the Albien aquifers. The Albian aquifer presents the lowest resistivity and is well known for its high water salinity. At 1 km depth beneath MT stations dj-5 and dj-6, an existence of a salt intrusion is highly possible, as it exists at nearly 25 km away from the study area. Between MT station dj-7 and dj-8, a near-vertical fault can be interpreted, at the lateral change of electrical resistivities. It is recommended that for future work, more MT sites are tested for a better inversion study including sensitivities. References Abdelzaher, M., Nishijima, J., Saibi, H., El-Qady, G.,

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Acknowledgments The field survey was carried out with the support of CRAAG. We thank B. Chibane, A. Djenidi, and M.S. Guettouche for their support. The third author HS acknowledges the financial support of UAEU by Research Start-Up Grant No. 8, Fund 31S264 (2016).