Methods, tools, and systems for determining porosity in an earth formation are disclosed. Neutrons are emitted into the formation to induce inelastic scattering gamma rays and thermal capture gamma rays in the formation. The induced gamma rays are detected at a proximal gamma detector and a far gamma detector, which are spaced at different axial distances from the neutron source. A measured proximal-to-far inelastic ratio (a ratio of inelastic scattering gammas detected at the proximal and far detector) and a proximal-to-far thermal capture ratio (a ratio of thermal capture gammas detected at the proximal and far detector) are determined and used to calculate the formation porosity. Techniques are disclosed for removing borehole and casing configuration effects from the measured proximal-to-far thermal capture ratio, leaving only porosity dependence.