The use of the reservoir saturation tool is used for formation evaluation behind casing. The tool provides measurements of the formation capture cross section, porosity, and carbon and oxygen in one trip in the wellbore. This data combination is useful for water saturation evaluation in old wells in which modern open hole logs have not been run. The tool can be configured for WFL measurement of both upward and downward flow inside and behind pip. This adaptability enables the diagnosis of any flow condition. The tool measures the phase velocity of water.