Geophysical study of the Chame-Rezan Dam Site, Sulaimaniyah City, NE IRAQ

A 1D and 2D resistivity surveys were carried out over the area of Chame Rezan dam site which is located about 33 km to the Northwest Sulaimaniyah City, NE Iraq. Schlumberger array were conducted in 34 locations with AB/2 spacing equal to 200m, as well as two representative Winner-Schlumberger profiles were laid out with electrode spacing equal to 5 m. The study concluded that the combination of 1D and 2D resistivity surveys is successful in identifying and producing a complete high-resolution image of the subsurface. The results were obtained show excellent correlation with the several bore holes were drilled on the recommendation of the surveys. The depths obtained by interpretation of 1D sounding points is slightly smaller than the actual depths recorded from the boreholes at the location of both dam abutments where the dip of the beds is ranging from 40 to 70 degrees. Consequently more reliable depths are obtained in the same locations from 2D resistivity profiles as well as the range of resistivities and lateral changes were obtained by 2D tomography are most reliable. In addition, the study reveals to the existence of a recent sediments has thickness ranging from 7 to 25 m overlain a low resistive Kolosh Formation.


Introduction
The geoelectrical survey was conducted by 1D vertical electrical sounding and 2D Resistivity Tomography for the proposed Site of Chame Rezan Dam and its surrounding areas. The area is located about 33 km to the northwest of Sulaimani City, northeastern part of Iraq (Fig.1), at the intersection of latitude 35 o 49' 57" and longitude 45 o 02' 12". On topographic map, it is appearing a u-shaped valley. It has relatively wide inlet and also wide outlet (mouth) forming suitable discharge site for dam building as well there is enough wideness behind and at upstream of the gorge to be suitable for dam building. The study area is located within high folded zone.
Therefore, the area was highly deformed due to active tectonic movements during the past geological time. As a result, several secondary folds have been seen especially on the northeastern limbs of the anticlinal and synclinal structures.
The main aim of the survey is to study the feasibility of the area for construction of a large dam as well as the comparison of results obtained by both 1D and 2D surveys. The area and it is surrounding had not being studied in geological and geophysical points of view, so the current study is considered as a first attempt in such field. In recent years, application of geoelectric study by 1D and 2D resistivity studies for feasibility of dam sites have widely increased , several researches were carried out related to this topics such as [ 1,2,3,4 ] Electrical resistivity surveys have progressed from the conventional vertical soundings, which provide layer depths and resistivity at a single place, to techniques that provide 2D and even 3D high resolution electrical images of the subsurface; [ 5,6 ]. This development started with the introduction of geoelectrical tomography field systems and was soon followed by post-processing and inversion algorithms, to transform the measured apparent resistivity pseudo-section in a true resistivity cross-section model or ERT [ 7 ] Several researches were carried out using both 1D and 2D for comparing the 1D and 2D inversion results of the subsurface resistivity distribution in areas where the lateral resistivity variation cannot be neglected, such as , [ 8,9,10 ].

Methodology:
The SYSCAL Jr switch-72, a new modern computerized static type resistivity meter was used, the complete system consists of the resistivity meter (from IRIS company), portable computer, switching unit (link box), six reels of multi-core cables with take-out at electrode points, 72 electrodes and their joining wire, car battery and chargers.
Three profiles were laid out and surveyed continuously by 1D resistivity using Schlumberger array and two other profiles were surveyed by 2D resistivity tomography using Wenner-

Results and Discussion
The apparent resistivity field curves are classified into several types. Generally they are refers to Elevation (m)

2D Traverse
the Kolosh Formation. The first layer represent a top soil covers the surface has high resistivity representing by yellow to red color. The second layer, which is representing by red, yellow and green colors represents the clay and rock fragments of the recent sediments, which show high resistivity ranging from 40 to 350 Ohm.m., Fig (11). Below the whole area a layer of low resistivity is appeared that composed mostly of black Shale, it shows resistivity ranging from 7 to 20 Ohm.m. This layer extends to deeper portion of the area.

Traverse-1
Traverse-2 The length of this traverse is equal to 315 m, it holds eleven sounding points. This traverse is shows two main zones, the first represents by variable color which represents the location of clay and rock fragments within recent sediments. The second zone is of low resistivity representing the upper part of the Kolosh Formation, Fig (12).
The section show obviously increasing of the recent deposits towards both right and left bank of the proposed dam.

Traverse-3
The length of this traverse is approach 318 m. It is show several anomalies of low and high resistivity, Fig (13). The first anomaly is appeared at shallow depth and exactly below the sounding points 33 and 17. The lower layer display low resistivity, this refers to the existence of the Kolosh Formation at depths ranging from 7 to 24 m.
According to the resistivity recorded in this location the whole area under consideration composed mainly of recent sediments that contain coarser materials than finer materials such as large boulder and limestone blocks of the Sinjar Formation. All the mentioned sections denote to the decreasing of the recent deposits toward the trough of the valley and exactly near the stream.

Apparent iso-resistivity map:
Apparent iso-resistivity map of AB/2=10 m is drawn for approximate depth level equal to 5 m.
Generally, apparent resistivity increase toward both right and left bank of the proposed dam as shown in Fig (14), this refers to the increase of the thickness of the recent deposits.
The central part of the area shows moderate resistivity ranging from 20 to 160 Ohm.m, it is denote to thin soil cover that saturated with groundwater. At the northeastern part of the area the resistivity is rapidly decrease to about 400 Ohm m due to existence of large blocks of limestone rocks.  For the depth level of 20 m apparent iso-resistivity map of AB/2=50 m is drawn, Fig (16).
Approximately, the same trend of the previous depth level maps is observed with slight change in the resistivity magnitudes. The right bank still shows high resistivity ranging from 40 to 240 Ohm.m. It refers to greater thickness of the recent deposits at this location.
The whole subsurface is composed of black Shale of the Kolosh Formation of low resistivity.
The resistivity changes through the whole area are from 7 -240 Ohm.m, the variation of the resistivity indicates that the subsurface is characterized by more heterogeneity.  where the dip of the beds is around 15 degrees. The conclusion is that however the amount of dip is increase the interpretation of 1D sounding gives depths smaller than the actual depth of investigation.

Fig.(18) Depth map of the Kolosh Formation based on 1D resistivity interpretation
Resistivity Tomography 2D model interpretation was performed using the last new version of software package "RES2DINV" version 3.54.53. It performs smoothness-constrained inversion using finite difference forward modeling and Quasi-Newton techniques [12]. An important factor in the inversion process of 2D imaging data is the quality of the field data. Good quality data usually show a smooth variation of apparent resistivity values in the pseudosection. All the soundings give very good quality. Therefore little appearance of irregularity and spots that shows high or low resistivities than surrounding has been seen in the pseudosections.
The bad data points are removed from the data set to avoid their effect in the process of inversion.
These data are exterminated by the aid of plotting the data in profile from that helps to highlight the bad datum points and remove them from the data set.

Resistivity Tomography of Profile-1:
It is run normal to the valley on an irregular area; it has length of about 355 m. Very good data quality is obtained, as shown in Fig (19). The inverse section of the sounding shows appearance