Resistivity:

The electrical survey is geophysical method used to explore underground geology/structures by measuring the electrical responses of the ground. The electrical resistivity surface imaging is a specialized technique for interpreting the distribution of potential and finally imaging the underground structure by sending direct current of low frequency alternating through a pair of current electrodes and measuring the electrical potential through a pair of potential electrodes. Electrical resistivity is defined as the resistance of a unit volume of a material, which is different from the resistance of the material. While resistance depends on the shape of a material, resistivity is an intrinsic characteristic of the material. Electrical resistivity is the reciprocal of electrical conductivity. A change in electrical resistivity resulting from underground geological variations is fundamental part of the method. There are two methods that AL-AHMED uses for electrical resistivity test as below :

1. Electrical Resistivity Test (Field & Lab):

In this method we can get the apparent resistivity to depths of up to 20m by using the Wenner Four-Electrode Method according to ASTM G-57



2. Electrical Resistivity Imaging Test (Tomography):
For an electrical resistivity imaging survey, 84 electrodes with equal spacing between each other are installed and connected to the resistivity meter simultaneously. The data acquisition is controlled by a pre-programmed command file. For each measurement, a DC electric current is injected into the ground through two electrodes and the resulting electric potential is measured between another two electrodes and This process continues automatically. An apparent resistivity value is derived from injected current, measured voltage and geometric factor. Measured data are inverted to produce true subsurface resistivity distribution and resistivity distribution is correlated to subsurface geology by a data interpreter. By using the Super Sting apparatus from AGI Company (in USA), one of the most famous company around the world in this field, we can use different methods to get the results to depths of up to about 60m like :
• Wenner
• Schlumberger
• Dipole-Dipole
• Pole-Dipole
• Pole-Pole
• Bipole-Bipole
• Dipole-Gradient
• Bipole-Bipole