Electrodes c1 and c2 are separated by a distance (a), and electrodes p1 and p2 are separated by a distance (b). the distance between the potential electrodes pair c1-c2 is the same as the distance between the potential electrodes pair P1-P2 and the distance between the centers of the respective pairs is (n+1) a, the “a” spacing is initially still fixed and the “n” factor is increased from 1 to 2 to 3 until 6 in order to increase the depth of investigation. the apparent resistivity determined by this arrangement is: pa = πa n(n+l)(n+2) . ( V/I)

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The depth of this array depends on the “n” factor and the “a” factor . Dipole-dipole array has a shallower depth compared with the Wenner array. But , this array has a better horizontal data coverage than the Wenner (Figure ). One possible disadvantage of this array is the very small signal strength for large values of the “n” factor. The voltage is inversely proportional to the cube of the “n” factor. This means that for the same current, the voltage measured by the resistivity meter drops by about 200 times when “n” is increased from 1 to 6. to overcome this problem , we need to increase the “a” spacing between the C1-C2 and P1-P2 dipole pair to reduce the drop in the potential when the overall length of the array is increased to increase the depth of investigation. Figure 9 shows two different arrangements for the dipole-dipole array with the same array length but with different “a” and “n” factors. The signal strength of the array with the smaller “n” factor (Figure 9b) is about 28 times stronger than the one with the larger “n”

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With the proper field equipment and survey techniques, this array has been successfully used in many areas to detect structures such as cavities where the good horizontal resolution of this array is a major advantage. The plotting location of the corresponding datum point (based on the median depth of investigation) used in drawing the apparent resistivity pseudosection is also shown in Figure 8c. Note that the pseudosection plotting point falls in an area with very low sensitivity values. For the dipole-dipole array, the regions with the high sensitivity values are concentrated below the C1-C2 electrodes pair and below the P1-P2 electrodes pair. In effect, the dipoledipole array gives minimal information about the resistivity of the region surrounding the plotting point, and the distribution of the data points in the pseudosection plot does not reflect the subsurface area mapped by the apparent resistivity measurements. Note that if the