Microscopically the volume of a single pore in a porous media may be divided into two regions; surface area  and bulk volume  (Figure 1).
The surface area is a thin layer with thickness  of a few molecules close to the pore wall surface. The bulk volume is the remaining part of the pore volume and usually dominates the overall pore volume. With respect to NMR excitations of nuclear states for hydrogen-containing molecules in these regions, different relaxation times for the induced excited energy states are expected. The relaxation time is significantly shorter for a molecule in the surface area, compared to a molecule in the bulk volume. This is an effect of paramagnetic centres in the pore wall surface that causes the relaxation time to be faster.
The inverse of the relaxation time , is expressed by contributions from the bulk volume , the surface area  and the self-diffusion :
: with 
where  is the thickness of the surface area,  is the surface area,  is the pore volume,  is the relaxation time in the bulk volume,  is the relaxation time for the surface,  is the gyromagnetic ratio,  is the magnetic field gradient (assumed to be constant),  is the time between echoes and  is the self-diffusion coefficient of the fluid. The surface relaxation can be assumed as uniform or non-uniform.
The NMR signal intensity in the  distribution plot reflected by the measured amplitude of the NMR signal is proportional to the total amount of hydrogen nuclei, while the relaxation time depends on the interaction between the nuclear spins and the surroundings. In a characteristic pore containing for an example, water, the bulk water exhibits a single exponential decay. The water close to the pore wall surface exhibits faster  relaxation time for this characteristic pore size.