ABSTRACT
At each stage of perturbation, the reorganizations of the grains can be precisely followed by taking pictures before and after a given perturbation by use of a high resolution CCD camera (1280 ∗ 1024 pixels2) with a fine sub-pixel analysis with a precision of 0.05
pixels (∼d2/600). Then the displacements of each grain are calculated in response to the up-motions and down-motions of the intruder (the corresponding displacements are noted respectively −→u ↑ and −→u ↓).
