Bi-phasic flow propagating through a granular medium can be established as an unstable infiltration and can be precursors of non-negligible deformations. Capturing such infiltration through the lens of a high resolution camera, full-field strain maps are determined via digital image correlation. However, this image analysis technique is based on the fundamental assumption of gray level conservation between the reference and deformed images. In this paper, DIC algorithm is recalled and suitable controlling parameters are introduced to improve the accuracy of the output displacement fields and to remove the gray level variations induced by an air-water flow percolating through a Fontainebleau sand. In addition, an in-depth investigation on the full-field volumetric strain maps is conducted in order to extract the hidden relationship between the front propagation and the strains evolution.