(1)	$\begin{array}{l}\displaystyle s_w = \left[ \frac{\sqrt{\frac{1}{R_t}}}{ \frac{V_{sh}^{\left( 1-0.5 \, V_{sh} \right)}}{\sqrt{R_{sh}}} + \sqrt{ \frac{\phi_e^m}{A \, R_w}}} \right]^{\, (2/n)}\end{array}$

where

$\begin{array}{l}s_w\end{array}$	formation water saturation
$\begin{array}{l}\phi_e\end{array}$	effective porosity
$\begin{array}{l}V_{sh}\end{array}$	shaliness
$\begin{array}{l}R_t\end{array}$	specific electrical resistivity from OH logs
$\begin{array}{l}R_w\end{array}$	specific electrical resistivity of formation water
$\begin{array}{l}R_{sh}\end{array}$	specific electrical resistivity of wet shales
$\begin{array}{l}m\end{array}$	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

Reference

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