3D Mapping of liquefaction phenomena induced by May 2012 Emilia earthquakes (Po Plain, Northern Italy)

Coseismic effects may have a very short life; most of the sand boils and cracks triggered by the main shocks of the May 20 th and 29 th Emilia earthquakes have disappeared just a few days after, both because of the atmospheric agents, and due to human intervention. In order to create an accurate database of these small micro-morphologies, several photogrammetric surveys were carried out using digital reflex cameras. Fixing in three dimensions the impact that shocks have made on the landscape allows to extract many morphometric parameters with high precision and, in a relatively simple way. Structure From Motion (SFM) algorithms are well known computer vision technique for the ability of reconstruct sparse point cloud from overlapping photographs; when they are combined with fixed calibrated optical and complemented with robust stereo-matching algorithms, detailed three dimensional models can be built with great resolution and accuracy. In order to obtain high resolution DEMs several convergent images were taken trying to cover all the angles and the whole area of the object. The first step called alignment, the reconstruction of the photograph shooting position is done by SFM that detect some images key points and subsequently correlates the movement of these along the image sequence. Once the basically geometry of the scene is known, dense stereo matching algorithm processes the scene trying to correlate every pixel for each photo in order to reconstruct a dense point cloud. Finally the obtained point cloud is triangulated and a detailed mesh is constructed. In order to build a local reference system suitable for use in GIS different targets of known size were distributed around every scene and ground control points were collected. More zenithal pictures were used to orthophotos production. Using this methodology we were able to develop DEMs with resolutions ranging from one millimeter for the small forms to some centimeter for the big ones.