(1)	$\begin{array}{l}\displaystyle \frac{1}{R_t} = \frac{\phi_e^m \, s_w^n }{A R_w} \quad \Rightarrow \quad s_w = \Big ( \frac{A}{\phi_e^m} \; \frac{R_w}{R_t} \Big) ^{1/n}\end{array}$

where

$\begin{array}{l}s_w\end{array}$	formation water saturation
$\begin{array}{l}\phi_e\end{array}$	effective porosity
$\begin{array}{l}R_t\end{array}$	specific electrical resistivity from OH logs
$\begin{array}{l}R_w\end{array}$	specific electrical resistivity of formation water
$\begin{array}{l}A\end{array}$	dimensionless constant, characterizing the rock matrix contribution to the total electrical resistivity	0.5 ÷ 1, default value is 1 for sandstones and 0.9 for limestones
$\begin{array}{l}m\end{array}$	formation matrix cementation exponent	1.5 ÷ 2.5, default value is 2
$\begin{array}{l}n\end{array}$	formation matrix water-saturation exponent	1.5 ÷ 2.5, default value is 2

Archie model is usually used in:

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