Monitoring the Vertical Distribution of Rainfall‐Induced Strain Changes in a Landslide Measured by Distributed Fiber Optic Sensing With Rayleigh Backscattering

Abstract

Distributed fiber optic sensing with Rayleigh backscattering, which has been recognized as a novel technique for measuring differences in temperature or strain, was adopted in a borehole to a depth of 16 m in an actual landslide to detect a vertical profile of strain changes. Strain changes were measured every with a spatial resolution of 10 cm and strain resolution of 1.87 με. The measurements provided a clear-cut vertical profile of the strain changes caused by rainfalls that cannot be detected by conventional methods. The results show that there are two types of deformation in the landslide mass: (1) sliding at the boundary between tuff and mudstone and (2) creep in mudstone layers. Activation of deeper sections of the landslide by heavy rainfalls has also been detected.Plain Language Summary This paper reveals in great detail the deformation process of a landslide corresponding to rainfall, which has never been detected in previous research, through the monitoring of the vertical distribution of strain changes with distributed fiber optic sensing analyzing Rayleigh backscattering. The measurements were completed on 18 October 2017. Therefore, this paper needs to rapidly report the results. This paper visualizes the unknown deformation of a mudstone layer below a slip plane that has been gradually activated by heavy rainfalls. The detection of the deformation of deeper zones leading to a larger landslide is a significant advance in disaster prevention and mitigation. The applicability of this technique to strain measurements for landslides shown in this paper suggests possible strain measurements for rocks in many research fields, including in studies of the deformations of active faults and volcanos. The results shown in this paper may also be useful for researchers working in optics, photonics, and physical as well as geophysical research fields where they are trying to develop new technologies.

Distributed Fiber Optic Sensing System for Well‐Based Monitoring Water Injection Tests—A Geomechanical Responses Perspective

“…In this case, DFOS technology is increasingly in demand in the oil and gas industry and is deemed as an ideal sensing solution to detect fluid migration and evolution in situ to benefit from its electromagnetic interference (EMI) immunity, long‐lasting durability, installation ease, and small physical footprint when compared with that of traditional electrical sensing and foil strain gauge. Among various DFOS technologies, Brillouin optical time‐domain reflectometry (BOTDR), optical time‐domain reflectometry (OTDR), and optical frequency domain reflectometry are frequently used to simultaneously measure parameters including strain, temperature, and pressure along the entire fiber length (Kogure & Okuda, ; Luca Schenato, ). Both are superior to other optical sensors, such as quasi‐distributed fiber Bragg grating (FBG) that is used for local measurement and Raman‐based distributed temperature sensing (DTS) that is used only for temperature (Arnon et al, ; Sun et al, ; Sun et al, ;Sun et al, ; Tyler et al, ).…”

Vertically Distributed Sensing of Deformation Using Fiber Optic Sensing

“…Our data showed a near-surface zone at approximately 0-to 6-m depth where the Brillouin frequency shift fluctuated markedly over time ( Figure S8b), which was primarily attributed to the variation of shallow soil temperature (Bense & Kooi, 2004;Kogure & Okuda, 2018). This can be readily addressed by employing an additional FO temperature cable insensitive to mechanical strains (Habel & Krebber, 2011;Schenato, 2017).…”

“…Previous research showed vertically distributed strain sensing of a shallow landslide up to ~15 m (Kogure & Okuda, ). In their study, the measured strains were not converted to actual deformation, probably due to the complicated correlation between axial linear strain and shear deformation.…”

“…In addition, abnormal strain values were registered at a depth of~0-6 m as shown in Figure S8b (shaded area). This could be due to the variation of shallow soil temperature, which was also observed during the monitoring of a landslide triggered by rainfall infiltration using the coherent optical time domain reflectometry technique (Kogure & Okuda, 2018). Because strain changes in this near-surface zone were already comparable to those induced by groundwater abstraction, they were not included in the color map.…”

Toward Distributed Fiber‐Optic Sensing of Subsurface Deformation: A Theoretical Quantification of Ground‐Borehole‐Cable Interaction

“…Despite lack of standardized backfilling procedure, current practice indicates that FO borehole backfilling materials are of similar stiffness as surrounding strata and so E b and μ b are considered to be within 0.01–40 GPa (Plinninger et al, 2010) and 0.15–0.45, respectively. The radius of borehole, r b , is assumed to range from 25 to 200 mm (Kogure & Okuda, 2018; Zhang et al, 2018), and the ground‐borehole interface adhesion coefficient, k b , from 1 to 1,000 GPa/m (Xiang & Wang, 2018).…”