The Archie Equation

• S_w = [ (a / F^m)*(R_w / R_t) ]^(1/n)
• S_w: water saturation
• F: porosity
• R_w: formation water resistivity
• R_t: observed bulk resistivity
• a: a constant (often taken to be 1)
• m: cementation factor (varies around 2)
• n: saturation exponent (generally 2)