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resistivity index

1. n. [Formation Evaluation]

The ratio of the true resistivity to the resistivity of the same rock filled with water. The resistivity index can be expressed as I = Rt / Ro, where Ro is the water filled resistivity and Rt is the true resistivity. It is related to the water saturation by the saturation exponent, n (I = Sw-n), and is a key component of the Archie equation.