Verification of a Distributed Fiber Optic Sensing Slope Stability Monitoring Solution

Abstract

ABSTRACT: This paper describes the testing of a novel ground condition and slope stability monitoring system based on Distributed Rayleigh Sensing (DRS) on a landslide observatory operated by the British Geological Survey (BGS). The DRS system uses the backscattered light from buried fiber optic cables to determine the strain and temperature at any point along the fiber up to distances of over 50 km and has been shown to respond to ground movement, moisture content changes and temperature variations. The output of the fiber is compared to that expected based on the known geology of the slope and other instruments such as tiltmeters and moisture content sensors. In addition to long-term strain and temperature measurements, the system can also sense acoustic vibrations and can be used to make active and passive seismic surveys to give a comprehensive 3D picture of the subsurface state. The results show that the DRS system can provide improved spatial and time resolution and sensitivity to give a more comprehensive and detailed picture of slope behavior than would otherwise be possible. Such detail allows improved landslide prediction methods and early warning systems to be developed. 1. INTRODUCTION Slope stability monitoring is important for failure detection and prediction and for greater understanding of slope behavior. Slope stability depends on several slope characteristics and there are a wide range of measurement methods that can increase such understanding. Fiber optic based sensing approaches are widely used for infrastructure monitoring and are increasingly being used for structural integrity and slope stability assessment due to their high sensitivity and spatial coverage and resolution. Such systems usually either operate at acoustic frequencies (as in Distributed Acoustic Sensing (DAS) systems) or at lower frequencies as Distributed Strain Sensing (DSS) systems. DAS systems operate 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 until recently had limited applicability for longer term condition monitoring and event prediction. However, recent advances in the technology 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 slope stability monitoring.