We present a novel approach to slope stability monitoring using a Distributed Rayleigh Sensing system on conventional, commercial fiber optic cables. Commercially this technology is generally deployed for characterising short time period strains or Distributed Acoustic Sensing (DAS) and is not usually considered for static strain monitoring. Advances in equipment and characterisation at ultra low frequencies tending towards 0 Hz provides a mechanism to record static strain from an arbitrary starting point together with local temperature and acoustic monitoring from a single instrument / fiber pair. By exploiting differences in cable characteristics we can extract each measurand from what would be a combined single measurand. The long range of Rayleigh based systems enables the linear monitoring of dams, levees and embankments over many miles. We describe a long term deployment at the UK's National Landslip Laboratory with the British Geological Survey which experiences annual slips and correlate data from the purely glass sensor with those from other instrumentation.
Distributed Acoustic Sensing (DAS) systems are widely used for infrastructure monitoring. These systems usually operate at acoustic frequencies by monitoring phase changes in Rayleigh backscattered light from pulses transmitted down an optical fiber. These phase changes occur due to small strain and temperature induced variations in the refractive index of the fiber. Most systems based on Rayleigh sensing are AC coupled to strain and are not used to perceive strain changes over longer timescales. As such, they have primarily been used for event detection and have had limited use for longer term condition monitoring and event prediction. However, DC coupled and stabilized approaches have enabled absolute strain changes to be measured over much longer timescales using Distributed Rayleigh Sensing (DRS) systems. This leads to a wider range of applications, for example structural integrity and stability monitoring, which is the subject of this paper.
