def resistivity_index(rt, ro):
    """
    Archie Resistivity Index (I)

    Parameters
    ----------
    rt : float
        True formation resistivity (ohm.m)
    ro : float
        Resistivity of water saturated formation (ohm.m)

    Returns
    -------
    float
        Returns Archie resistivity index (I)
    """
    return rt/ro