A method for determining the porosity of a subsurface geological formation traversed by a borehole is provided. The method generally comprises obtaining a means which relates ranges of apparent formation porosity (. phi. sub. a) as determined by a neutron-neutron log, formation fluid salinities, formation total capture cross sections (. SIGMA. ), formation matrix and fluid constituents, and true formation porosity (. phi. sub. T) according to a predetermined equation. The predetermined equation relates the apparent porosity to a function of a modified migration length which is obtained in a semi-empirical manner (i. e. physics modifed by data). The modified migration length includes a slowing down length and a diffusion length, but causes at least the diffusion length to be a function of the slowing down length. The means which relates the apparent porosity to salinity,. SIGMA. , matrix and fluid constituents, and. phi. sub.