The presence of natural voids and fractures (weak zones) in subsurface gypsiferous soil and gypsum, within the University of Al-Anbar, western Iraq. It causes a harsher problem for civil engineering projects. Electrical resistivity technique is applied as an economic decipher for investigation underground weak zones. The inverse models of the Dipole-dipole and Pole-dipole arrays with aspacing of 2 m and an n-factor of 6 clearly show that the resistivity contrast between the anomalous part of the weak zone and the background. The maximum thickness and shape are well defined from 2D imaging with Dipole-dipole array, the maximum thickness ranges between 9.5 to 11.5 m. It is concluded that the 2D imaging survey is a useful technique and more effective for determining and mapping subsurface weak zones (voids, fracture and cavities), when taken in consideration using the suitable a-spacing and n-factor for each electrode array, especially with the Dipole-dipole array which provided the best imaging of the subsurface shape of the weak zones. Introduction:The subsurface voids, fractures, cavities and subsidence are natural phenomena that can occur in shallow geology sediments at different regions in the world. Cavities hazard assessment is one of the most difficult near subsurface investigations. It is clear that sinkhole formation is a dynamic process occurring over time, resulting in variations in the subsurface properties, such as porosity, fracture density, water saturation, etc. Roads and highway subsidence, building foundation collapse, and dam leakage are few of the problems related with cavities and sinkholes [1,2]. In the Southern part of Al-Jazeera, along the left bank of the Euphrates River (West Iraq), large caves are formed in gypsum beds of Fatha Formation and carbonate rock of Euphrates Formation. Few kilometers north of Hit a large cave is formed in the gypsum beds of the plateau that border Euphrates valley. The altitude of the entrance of the cave is about 130 m A.S.L The presence of natural voids and fractures in subsurface gypsiferous soil and gypsum, within Al-Anbar University causes a harsher problem for civil engineering. The electrical resistivity technique is applied as an economic decipher for investigation underground voids and weak zone.Selecting the correct geophysical tool for the detection of subsurface cavities and voids is not always straightforward, is of necessary importance in land-use planning [2]. The electrical resistivity method is considered as one of the promising geophysical methods that are used in the subsurface investigation because it gives a semi-true subsurface picture for buried structures with rapidity to calculate and determined the distribution of subsurface resistivity by making measurements on the ground surface [3,4]. In Dipole-dipole array, the spacing between current and potential electrodes are the same and remaining fixed for each spacing and n-factor [5]. Pole-dipole is another array that is using in shallow weak zones detection that has an approximately good i...

The 2D resistivity imaging technique was applied in an engineering study for the investigation of subsurface weakness zones within University of Anbar, western Iraq. The survey was carried out using Dipole-dipole array with an n-factor of 6 and a-spacing values of 2 m and 5 m. The inverse models of the 2D electrical imaging clearly show the resistivity contrast between the anomalous parts of the weakness zones and the background resistivity distribution. The thickness and shape of the subsurface weakness zones were well defined from the 2D imaging using Dipole-dipole array of 2 m a-spacing. The thickness of the weakness zone ranges between 9.5 m to 11.5 m. Whereas the Dipole-dipole array with a-spacing of 5 m and n-factor of 6 allocated the geoelectrical stratigraphic layers sequence in low-accuracy of weakness zones, but deeper than the inverse model of 2 m a-spacing. This survey was made to explain the correlation between the weakness zone and the deeper layers in the study area. It points out that the deeper layers were not affected in the weakness zones. The inverse model was produced using the Standard Least-Squares Inversion Method and the Robust Inversion Model Constraints Method. The first method had a gradational boundary of the weakness zones and the second had sharper and straighter boundaries of fractures and voids within the weakness zones.

Electrical resistivity methods are one of the powerful methods for the detection and evaluation of shallower geophysical properties. This method was carried out at Hit area, western Iraq, in two stages; the first stage involved the use of 1Dimensional Vertical Electrical Sounding (VES) technique in three stations using Schlumberger array with maximum current electrodes of 50m. The second stage included the employment of two dimension (2D) resistivity imaging technique using dipole-dipole array with a-spacing of 4m and n-factor of 6 in two stations. The 1D survey showed good results in delineating contaminated and clear zones that have high resistivity contrast. Near the main contaminated spring, the 2D resistivity imaging technique was applied in four sections length (100 m) using a dipole-dipole array position coincided with the three points VES. We compared the results of the interpretation of imaging the techniques 2D and VES. We found that the 2D imaging resistivity technique was better than VES survey in determining the distribution of pollution under the surface in the area surveyed. It was also found that the polluted water is located about 5 m below the surface. The largest amount of leakage was found towards the northeast and coincided with the direction of the groundwater movement. Spring water has leaked from outside the region through the Kubaisah area. Most of this water is contained in quaternary deposits and karst gypsum fractures.

Um El-Adam cavity is one of the well-known cavities inside gypsum rocks in the Hit region, western Iraq, where this was chosen as a case study to evaluate 3D resistivity imaging technique detection and imaging of this type of cavity in complicated lithology. 3D view fulfilled by collating four 2D resistivity-imaging lines. The 2D resistivity imaging survey was carried out by Dipole-Dipole array with (n) factor and electrode spacing (a) of 6 and 2m, respectively. Both conventional inversion methods obtained the 2D and 3D models: standard least-squares and robust constrain options. The two options were able to define clearly the cave, but the second was more accurate, the dimensions of the cave in the inverse model closer to the actual dimensions. Horizontal slices displayed the final 3D model to reveal a 3D resistivity distribution with depth. The effect of Um El-Adam cavity appears after 1.5 m on the model slices, which is represented by increasing resistivity contrast compared to surrounding sediments. At the seventh and eighth slices, it was found that the size of the cavity exceeded compared with the actual dimensions. The results of this study indicated the high potential of this method for the detection and delineation of subterranean caves.

K-3 solution cavity is located within Haditha area, Western Iraq. The measurements of Graphical Bristow's method were carried out by using Pole-dipole array, to evaluate the method to detect the dimension of a relatively large natural cave. The cave resulted due to the dissolve of carbonate rocks within Euphrates Formation (Miocene). The actual depth and height of the cave are 38.8m and 29.5m respectively. Two traverses with a-spacing equal to (5m) and length of about 100m are achived, in South-North and West-Eest direction above the cave site. The data interpretation detect the cavity elongate along West-Eest traverse of 58.6m and indicate an error not exceeded 3% in depth and 2% in height values. The results are concluded that this method is useful to be able to detect subsurface cavities and voids.