A novel surveillance system for installed fiber optic cables is presented. It is based on the analysis of the local stimulated Brillouin scattering interaction. The configuration of the instrument rely on the use of a single laser source and the required light signals are all generated using an electro-optic modulator, resulting in a high stability and an excellent reliability of the setup. Some results of the first field measurements are discussed. They have provided important information on the strain distribution actually experienced by the fibers, fiber uniformity, local birefringence, temperature variations. The system can operate over 100 km and the spatial resolution, which remains sub-metric over the first 10 km, is below 10 m over the full range.