Ground penetrating radar is commonly used to locate fractures in rocks. However a much more detailed analysis is required to study the characteristics like the filling material and the fracture width. The reflected signal contains information about the characteristics of the fracture (width, filling), but a detailed analysis is required to assess the influence of these characteristics on the reflections. Ground penetrating radar measurements with a 1 GHz antenna have been carried out in limestone. A finite difference time-domain technique was used to model the behaviour of the radar signal in the same geological conditions.