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.

The electrical resistivity method has been applied to detect the subsurface karst hazards using Dipole-dipole and Wenner – Schlumberger arrays with a-spacing equal to 1m and n-factor 6 at three selected stations within the State Company for Glass and Refractories in Al-Ramadi City, Iraq. The results indicate that the area formed a weakness zone as two separated zones, the first at a depth in the range of 2.5-5 m within the quaternary deposits and the second weakness zone within the Injana Formation deposits at an approximate depth range of 7 – >12 m. The investigation advises conducting physical, chemical, and engineering site investigations to decide the suitable treatment techniques to solve these problems.