A method is disclosed for the non-destructive determination of the pore size distribution and the distribution of fluid flow velocities using NMR spin echo signal measurements. In one embodiment, the invention involves simultaneously injecting acoustic energy (208), generating a magnetic field having a gradient, and injecting radio-frequency electromagnetic energy (206) into a subsurface region, using for example an NMR tool and a well logging tool having an acoustic transmitter placed in a wellbore (202). The spin echo signal generated in the subsurface region is measured (210) by the NMR tool for at least two values (212) of the magnetic field gradient. A relationship is provided by which the pore fluid velocity distribution may be calculated (214) from the inverse Fourier transform of the spin echo data. The pore size distribution may then be calculated from the pore fluid velocity distribution.