Quality Improvement of the French Permanent Broadband Stations with Shallow Posthole Installations

Within the framework of the RESIF (Reseau sismologique et geodesique francais) research infrastructure, a major extension of the French permanent broadband network (RLBP) has been initiated in 2011 with the objective to reach ~190 stations by 2020. This network will allow a more homogeneous covering of metropolitan France, encompassing a wide variety of tectonic and geological units. Presently the RLBP network consists in 124 operating stations. For each new potential site a test was conducted for ~3 weeks, usually burying the broadband sensor at 50-80 cm depth directly in the ground or within a light temporary vault. For the final set-up, we tested various hosting infrastructures at three prototype sites and decided to promote the installation of a posthole sensor in a steel-cased shallow borehole, 5-20m deep, for all the sites in open areas. 43 stations have already been installed this way and have been operated for 5 to 20 months. RESIF is a national research infrastructure dedicated to the observation and understanding of the structure and dynamics of the Internal Earth. RESIF is based on high-tech observation networks, composed of seismological, geodetic and gravimetric instruments deployed in a dense manner throughout France. The data collected make it possible to study with high spatial and temporal resolution the deformation of the ground, surface and deep structures, seismicity on a local and global scale and natural hazards, and more particularly seismic events, on French territory. RESIF is part of the European (EPOS - European Plate Observing System) and global systems of instruments used to image the Earthu0027s interior as a whole and to study many natural phenomena.