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Write-up on Inverse slope method of resistivity inversion_IVES

Write-up on Inverse slope method of resistivity inversion_IVES

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1 ELECTRICAL RESISTIVITY METHODS Electrical Resistivity Method is one of the Geophysical techniques used to investigate the nature of the subsurface formations. In Electrical Resistivity methods current sent into the ground through a pair of electrodes, called current electrodes, and resulting potential difference across the ground is measured with the help of another pair of electrodes, called potential electrodes. The ratio between the potential difference ( V) and the current (I) gives the apparent resistance, which depends on the electrode arrangement and on the resistivities of the subsurface formations. There are several types of electrode arrangements (configurations) of which Wenner and Schlumberger configurations are more popular. In Wenner Configuration all the four electrodes are kept along a line at equal distances called electrode separation ‘a’. For each measurement all the electrodes are moved simultaneously keeping the inter electrode spacings same. The current is sent normally through outer electrodes and potential difference is measured across the inner electrodes. The resistance is multiplied by the configuration factor 2 π a, to get the value of apparent resistivity ( ρ aw ) I V B A M N A B M N V ° A B N M O a a I a Wenner Array
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2 ρ aw = 2 π a R Where R= V/ I In Schlumberger Configuration all the four electrodes are kept in a line similar to that of Wenner but the outer electrode spacing is kept large compared to the inner electrode spacing, usually more than 5 times. For each measurement only current electrodes are moved keeping the potential electrodes at the same locations. The potential electrodes are moved only when the signal becomes too weak to be measured. The apparent resistivity for this configuration is computed with the formula; ρ as = π [(AB/2) 2 – (MN/2) 2 ]R MN There are two types of procedures for making resistivity observation, namely Resistivity Sounding (also called Vertical Electrical Sounding, VES) and Resistivity Profiling (Horizontal Electrical Profiling). Resistivity profiling is employed to determine the lateral variations in the resistivities thereby establishing the existence of vertical bodies like dykes, fracture zones, geological contacts etc. The Vertical Electrical Sounding is used to estimate the resistivities and thickness of various subsurface layers at a given location and is mainly employed in groundwater exploration to determine the disposition of the aquifers. Vertical Electrical Sounding (VES): In this approach, the center of the configuration is kept fixed and the measurements are made by successively increasing electrode spacings. The apparent resistivity values obtained with increasing values of electrode separations are used to estimate the thicknesses and resistivities of the subsurface formations.
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