分散型光ファイバーセンシングを用いた深部基盤岩内の地質水文モニタリング(Geohydrological monitoring in deep basement rocks using distributed fiber-optic sensing)

島根大学大学院自然科学研究科地球科学専攻
同上

博士課程 1 年（助成時）
博士課程 2 年（現 在）ACHARYA ASHIS

1.

Introduction

Rapid landslides such as steep channel debris flows can devastate populations and economies because of their high flow
velocities, high impact forces and long run-out distances. Monitoring and development of early warning for such landslides
needs quick data acquisition which is still a challenging task with traditional monitoring tools. The development of novel
distributed fibre-optic sensing (DFOS) techniques has made significant technological advancements, from traditional slope
monitoring tools to real-time measurements of multiple parameters such as strain or temperature. These sensing methods
fulfil the demands of extensive and distributed monitoring of geological structures across vast distances, offering
measurements with excellent resolution. DFOS-based fibre-optic sensors, when embedded into sliding masses, function
akin to the nerves in the human body, adeptly capturingextensive information to assess the health condition of thelandslide. As a novel step towards predicting theoccurrence of deep landslides, this study attempts tomonitor

thestrain(toexamine

thedeformation

characteristics of basement rocks) and temperaturechanges (to locate seepage phenomenon) within a deepslope.
2.

Laboratory feasibility study

This study demonstrates the ability of the Rayleigh-based
phase-noisecompensatedopticalfrequency-domainreflectometry (PNC-OFDR) sensing method to monitor the
distributed temperature field with an ultra-short dataacquisition period of 2 ms, a spatial resolution of 2 cm, anda temperature resolution of 0.1 °C. Two FR PVC cables, asillustrated in Fig. 1, were used in the experiments. One wasfor heating (heating cable; H-cable) and the other for temperature sensing (temperature measurement cable; T-cable). The
H-cable was embedded within a cylindrical concrete mortar specimen and subjected to various heating powers. The Tcable was placed adjacent to the heating cable to monitor the temperature distribution continuously. Two water-holding
boxes were installed along the specimen at two positions to retain water. The copper wire in the H-cable implanted inside
the specimen is the heat source for the distributed temperature profile registered by the T-cable. Due to the high heatAfter 120 min
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capacity
and thermal conductivity of water in the PET boxes, the20local temperature field of the specimen will be affected2010
by 10the presence of the water boxes. If the anomaly of temperaturefields is identified with the FO sensing system by00

measuring the local spectral shift, the water-supplied position canbe located and monitored. Fig. 2 presents the temporal-10-10.

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evolution of temperature alongthe (m)specimen during a heating period of 3 hrs at four different
electrical
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Distance (m)Distance (m)Temperature change along the specimen detected by the PNC-OFDR system

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1010-1010.

Field study

Field deployment involved installing monitoring systems
within a 50-meter-deep borehole, where optical cables forheating, temperature sensing, and strain sensing wereembedded (Fig. 3). The hardened anchorage formed bycement slurry backfill facilitates both seepage detection(a) Drilling boreholethrough active heating and rock deformation monitoring(b) Installing optical cables

via strain sensing. Our future tasks involve the heating
tests of boreholes at different electrical powers to locate
the groundwater and other seepage phenomena. We
continuously record strain and temperature within theborehole and delve into the relationship between bedrock(c) ackfilling of borehole

deformation, seasonal variations, and groundwater flowlevels.

Fig(d) Final top view of the borehole withinstalled cables

Various steps of drilling and cable installation

The insights gained from this study have broad applications beyond landslide prediction, extending to groundwater
management, civil engineering projects, and urban infrastructure development.