DAS for vibration monitoring and passive seismic exploration of rock

Monitoring by passive acoustic vibrations, generated by the moving infrastructure or mining works, is a powerful method for extensively imaging the rock quality. Passive acquisition, where the underground excavation activities act as a source of acoustic energy, enables investigation of the deeper portion of the rock volume. 

Optical fibers have several advantages as they are low cost, small and lightweight, robust to high temperatures, immune to electromagnetic interference, versatile, and in addition have a low environmental impact. They are therefore a suitable sensing solution for the demanding underground environment. 

The DAS technology allows for continuous monitoring of large areas, up to several kilometers in range, with a dense spatial sampling (1-10 m) along the fiber (corresponding to one geophone every 1-10 meter). The system is capable of measuring vibrations ranging from a few millihertz to tens of kilohertz using a single optical fiber and requires access to only one end of the fiber. In addition, the active elements of the sensor, e.g., the electronic readout system and the data processing unit, can be placed kilometers away from the designated sensing area, increasing the safety of the sensor operators and guaranteeing long-term monitoring since the sensing probe can be permanently installed in the area of interest. 

In this scenario, the advantages of DAS technology compared to traditional seismic sensors are the higher spatial resolution and the capability to provide several measurement points in a single and totally passive optical fiber that works simultaneously as sensing probe and data transmission cable. Such features enable less complex installation of acoustic monitoring in complex environments, such as in boreholes or along tunnels, and will most likely improve the ability of 3D characterization of the rock volume.

The same techniques, developed and fine-tuned in the present project, could also be applied during the excavation of underground infrastructures in urban environments, where the geological model can be updated while the excavation face moves forward.