The “Glass Beads” Coupling Solution for Borehole and Posthole Accelerometers: Shaking Table Tests and Field Retrievability


 Borehole accelerometers are designed to record strong ground movements at depth. They have become an important complement to surface accelerometers for seismic and earthquake engineering applications. Borehole accelerometers present several implementation challenges, including their coupling with the geological environment. One possible coupling solution is the use of small glass beads, which are placed inside the borehole casing with the purpose of filling in any empty space between the sensor and the casing walls. We carried out a test on a shaking table, over a wide range of peak ground accelerations (PGA from 0.17 to 1.64g), allowing the comparison of the signals between a surface accelerometer and a borehole accelerometer coupled through the glass beads. These tests show that there is almost no difference between the surface and borehole accelerometer signals between 0.5 and 25 Hz, and only very small differences outside this band (0.2–0.5 Hz and 25–80 Hz). Furthermore, experience from multiple vertical accelerometric arrays show that an installation using glass beads is “reversible”, that is, 30 yr after the initial installation it can still be possible to easily extract the accelerometers for repair or replacement, without any problems or damage to the sensors.