Monitoring Seasonal Strain Changes with Distributed Fiber Optic Sensing at a Landslide Site

Abstract

Strain changes were measured at 6-hour intervals using TW-COTDR (Tunable Wavelength Coherent Optical Time Domain Reflectometry) in a landslide area in Shimane Prefecture, Japan, from March 2018 to March 2019. A total length of 364 m of optical fiber cable was installed in two ways: 1) vertically within a 16-m-deep borehole and 2) horizontally along the ground surface in a trench 30 cm deep dug into the surface layer of the landslide mass. The measurement results revealed that strain changes measured in the borehole near the surface are more sensitive to the presence or absence of precipitation. In contrast, in the deeper section near the slip surface (approximately 8 m deep) and below, the correlation with precipitation was weaker, but the strain gradually increased throughout the year, reaching a maximum increase of about 750 . Along the ground surface, some areas showed an unclear relationship with the presence or absence of precipitation, and local differences were observed in the magnitude of strain change.