We apply Ground Penetrating Radar (GPR) to evaluate rock characteristics of interest in geological, geomechanical, hydrogeological, engineering and environmental studies. Primary objective of the study is 3-D imaging and detailed reconstruction of the electromagnetic velocity and attenuation field to define rock volumes that can be considered homogeneous from the geological and geomechanical point of view. The study is based on a series of experiments carried out in two test sites: a limestone quarry (test site-1) and limestone/dolomite contacts (test site-2). Rock mass characterization is performed by linking rock mass properties that can be estimated from GPR data to petrophysical properties (e.g. porosity, fluid content, permeability) and to rock characteristics such as texture, density of fractures and filling materials.