Comparison between Dipole-dipole and Pole-dipole Arrays in Delineation of Subsurface Weak Zones Using 2D Electrical Imaging Technique in Al- Anbar University, Western Iraq

Abstract

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...

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