APPLICATION OF 2D RESISTIVITY METHOD FOR DAM SITE INVESTIGATION IN MIRAKAN AREA, QUSHTAPA DISTRICT, ERBIL, IRAQI KURDISTAN REGION

Subsurface investigations have significant and substantial effects on the design of dams or any hydraulic structures. The proposed dam is located to the north of Mirakan village within Qushtapa district, Erbil governorate, Iraq. 2D resistivity method was applied to determine the suitability of the Mirakan site for dam construction. The results (Pseudo-sections) indicated that the top surface layer of the studied area shows medium to high resistivity which ranges from 75 to more than 360 ohm.m, and it consists mainly of coarse grain materials like gravels, pebbles and, boulders, while in some location fine grain materials like clays and silts appeared. The thickness of this top surface, recent sediment is ranging from 8 to 18 m. The middle part of the Pseudo-sections displays a layer of high resistivity ranges 40– 65 ohm.m, it has been detected within Bi Hassan Formation; it is mainly composed of the conglomerate. The lower part of the pseudo-sections illustrates a layer of low resistivity which represents the fine material deposits of the Bi Hassan Formation and the resistivity ranges from 7–20 ohm. m. There are no faults have been detected in this location as well as there is no evidence of the existence of cavities and there are no risky sources. So, the study area is suitable for dam construction. To sum up, the results reveal that the studied area has the potential to withstand the weight of the dam and natural disasters.


INTRODUCTION
The need for water in daily life activities, industries, and irrigating the farmlands nowadays become one of the very important goals for the local authorities and geopolitical DOI: 10.46717/igj.52.2.5Ms-2019-12-28 analysists, so the scientists have to do many types of research to show how they can manage it.
Accordingly, dam construction is important to achieve the mentioned aims.To identify the weakness zones of the subsurface geology that negatively affect the dam foundation, it is necessary to do many surveys, like geophysical, geological, geotechnical and hydrogeological surveys.In general, there are many geophysical methods which are used for dam site investigation.These methods are seismic reflection and refraction, two-dimension resistivity, gravity, electromagnetic, and ground penetrating radar.The current study located within Qushtapa district about 45 km far from Erbil city.The location of the site investigation was taken by a global positioning system (GPS) device with the coordinates of X=424686 and Y= 3980851 (Fig. 1).
The method which was used in this survey is two-dimension electrical resistivity.The survey was done by moving the cables past one end of the line by several units of electrode spacing, after completing a sequence of measurements (Fig. 2).This method is based on assumption that various entities like minerals, solid bedrock, sediments, air, and water-filled structures are detectable electrical resistivity contrast relative to the host medium (Panek et al., 2010).Much more effective and economical use of the geophysical survey is how to reduce the overall amount of drilling and how to improve the placement of test borings (Metwaly and Fouzan, 2013).An electrode arrangement along with the top of the dam core is the most practical and satisfactory method of installing resistivity monitoring systems on existing dams (Sjodahl et al., 2006).After the field investigation, the resistance measurements are usually reduced to apparent resistivity values.The raw data obtained for this work, comprising of measured apparent resistivity, were processed using the computer program RES2DINV version 3.4.Generally, the program automatically creates two-dimension models by dividing the subsurface into rectangular blocks (Loke, 1999).The data acquisition is completely controlled by computer software which checks that all electrodes are connected and properly grounded before measurement starts.After adequate grounding is attained software scans through the measurement protocol selected by the user (Dahlin, 1996).The first-qualitative result of a twodimension resistivity survey is a pseudo-section along with the profile (Knodel, 2007).Two previously researches concerning 2D electrical resistivity and engineering site investigations have been performed in Erbil governorate (Gardi, 2014).The 2D electrical resistivity tomography survey was used to discover the pollution risk of the soil which infiltrate to the groundwater at wastewater valley located southwest of Erbil city, whereas (Gardi et al. (2017) have conducted engineering site investigation for Siktan proposed dam using 2D electrical resistivity tomography to evaluate the geo-structural setting of the subsurface bedrock along the dam axis and abutment of the proposed dam site.The objectives of this research are to determine the subsurface structures like faults, caves, fractures and to know whether the area is suitable for dam construction or not.

GEOLOGICAL SETTING
The Mirakan dam is located within an Unstable Shelf Zone which is affected by Alpine Orogeny in Chamchamal-Butma Subzone.It has structural trends that are parallel to the Zagros-Taurus suture belts.The Butma-Chamchamal subzone is the NE unit of the Foothill Zone.
Butma-Chamchamal Subzone with thick Pliocene molasses dominated by a conglomerate and the strata are essentially horizontal (Jassim & Goff, 2006).There are large and long depressions filled by Pliocene and partly Pleistocene sediments (Buday, 1980).Erbil plain is a wide syncline located between two main anticlines, Pirmam Dagh from east and Khurmala-Avana from the west (Hassan, 1998).The study area is located within Erbil syncline.The most important formation which is present in the study area are: A-Bi-Hassan Formation (Upper Pliocene): It consists of variable units of the conglomerate in a different color, and grain size of clay, sandstone.The formation composed almost purely of terrigenous clastic from silt size to a boulder (Buday and Jassim, 1987) (Fig. 3).This conglomerate also occurs as channel filling within the claystone (Sissakian and Youkhana, 1986).The thickness of the formation is very variable due to primary differences and partly due to erosion too.The lower part of the formation is gradational and is put at the base of the first pebbly or conglomeratic bed.The formation is overlain by terrace gravels and/or alluvial deposits (Buday, 1980).According to (Mahmood,1986) the shape analysis, sedimentary structures, and field description of the stratigraphic section suggest that Bai Hassan Formation was deposited in a fluviatile environment which interchanges with alluvial fans or lacustrine environments.
B-Quaternary deposit: The deposition and stratigraphic sequence of Quaternary sediments depend on the climatic oscillations, causing periodically repeated phases of accumulation and erosion.Besides, especially in the mountain's areas of Iraq, the general uplift had played an important role too (Buday, 1980).
These deposits are non-effected by Alpine orogeny, consist of clay, loam silt, and sand (Lak, 2007).Stratigraphy of the quaternary deposit is unconformable with the underlying unit (vertically and horizontally appeared gravel alternative (repeat) coarse, medium, and fine grain size) (Buday and Jassim, 1987).

MATERIALS AND METHODS
In this study, the surveying was made as a two-dimensional resistivity imaging, with Wenner array (48 electrodes) along 235 m, the space between electrodes is 5 m.SYSCAL Pro resistivity meter was used in this survey, which consists of several electrodes located on a line at the same time, which carries out an automatic switching of these electrodes, which is automatic switch electrodes taken 360 data (Fig. 4).

Fig. 3: Geological map of the study area
Three profiles were taken, the first profile is parallel to the dam axis which starts from northwest to the southeast direction.The second profile is located on the left abutment which is directed from northeast to the southwest and the third profile is located at the down of the right abutment which is nearly parallel to the profile 2 (Fig. 5).The two-dimension resistivity profile on the right abutment could not be covered because of the narrowness and limited extent of the place which assigned (topography) for taking the profiles, so to make a good resistivity imaging survey must be far away from the noise source.The depth of the investigation in this study is assumed to be 40 m, being a fraction ( ) of the total length of the profile.
In profile 1, the first electrode located on the left abutment of the proposed dam with a height of 18.5 m.The profile is parallel to the projected dam axis and running semi-parallel to the strike of the outcrops (NW-SE).The outcrops of Bi Hassan Formation do not exist in the area and its surroundings because of the recent material deposits.For the profile 2, the first electrode is assumed to be on the downstream, it is placed on the left bank of the proposed dam, and it is running approximately vertical to the strike of outcrops.Profile 3 is positioned on the right bank of the proposed dam, and it is running approximately perpendicular to the strike of outcrops.In profile 2, which is applied to the left bank of the suggested dam site, the measured and calculated apparent resistivity sections show high resistivity data.The surface layer is of high resistivity, which is green to yellowish, reddish colors, it represents gravel of river traces of quaternary deposits.The thickness of this layer ranging 15-18 m (Fig. 7).The second zone consists of clay with low resistivity ranging 7-15 Ohm.m.This layer is appearing at the center of the section.The third zone of moderate resistivity has been identified on the downstream and upstream of the section.It represents a coarse material deposition cycle of Bai-Hassan Formation, mostly composed of the conglomerate.This zone extends to the depth, the maximum depth of investigation which is about 50 m.
In profile 3, which is located to the right bank of the suggested dam site, measured and calculated apparent resistivity sections show sensible good quality data.The surface layer is of high resistivity (green to yellowish, reddish colors); it represents boulder and gravel of river traces of quaternary deposits.The thickness of this layer ranging 6-8 m.The second zone is consisted of clay with low resistivity ranging from 5 to 15 Ohm.m.This layer is appearing at the left and right of the section.The third zone of moderate resistivity has been identified in the center of the section.It represents a coarse material deposition cycle of Bai-Hassan Formation, mostly composed of conglomerate (Fig. 8).

CONCLUSIONS
1-At the top surface layer the resistivity is high, it ranges between 75 to higher than 360 Ohm.m, it consists mainly of coarse grain materials such as boulder and gravel, while in some location silt and clay has been appeared.The quaternary thickness in profile 1 is ranging between 14-12 m while in profile 2 is ranging between 15-18 m, and in profile 3 the thickness of the quaternary is ranging between 6-8, and the distance 235 m for every profile.
2-A layer of high resistivity 40-65 Ohm.m has been detected within Bai-Hassan Formation; it is mainly composed of the conglomerate.
3-A layer of low resistivity was identified which represents the fine materials of the Bi Hassan Formation, and the resistivity ranges between 7-20 Ohm.m.
4-According to the three geo-electric sections, there is no faults have been detected in this location and no evidence of the existence of cavities and there are no any risky sources.So, the study area is suitable for dam construction.

Fig. 1 :
Fig. 1: Location map of the study area

Fig. 4 :
Fig. 4: 2D dimension resistivity meter n Fig. 5: the 2D Resistivity survey lines RESULTS AND DISCUSSION In profile 1, which is parallel to the dam site, three layers were recognized (Fig. 6); the first one represents recent sediments with resistivity ranging 95-290 Ohm.m in the center of the profile, it is composed of coarse sediments, boulder, gravel, pebble, sand, and silt.It has thickness ranging 4-12 m.The second layer composed of conglomerate with resistivity ranging 55-120 Ohm.m, the thickness of this layer ranging 10-12 m.The third layer is composed of claystone bedrock of Bai-Hassan Formation toward right abutment with resistivity ranging 8-10 Ohm.m.