The probability of return to the initial position for a particle diffusingin porous media

A recent approximate calculation (by Mitra and coworkers) of the probabilit y density of return to the initial position by time t for a particle diffus ing in a porous medium is shown to be incorrect and to yield systematic err ors whose magnitudes depend on the nature of the sample and on t. Another a pproach to approximate calculation of this characteristic is proposed, and the resultant errors are estimated. The value that is proposed for calculat ion is the probability of the return of the diffusing particle to the plane that is perpendicular to the pulse gradient of the magnetic field and pass es through the point of the initial particle position. These characteristic s are compared by the example of self-diffusion of a liquid in sandstones.