Relationship between surface ground motion and strain observed by a helically wound fiber-optic cable

Abstract This study used numerical tests and in situ observations to characterize the relationship between ground motion and strain measured using a newly developed borehole fiber-optic strainmeter with a helically wound sensor. This strainmeter is designed for the broadband observations across geodetic and seismic ranges, featuring a sensor fiber helically wound to accommodate a long baseline within the confined space of a borehole, thereby enhancing the sensitivity to multiple components of strain tensors. Therefore, the sensitivity of the strain tensor and its relationship with ground velocity were analyzed through numerical tests and compared with in situ records. The sensitivities of specific components within the strain tensor were evaluated based on the geometrical configuration of the sensor, revealing that areal and normal strains in the horizontal plane were particularly sensitive in these borehole fiber-optic strainmeters. To validate these theoretical characteristics, in situ records from the borehole fiber-optic strainmeter were comparatively analyzed with data from existing linear strainmeters and seismometers. The borehole fiber-optic strainmeter was installed in a 20-m-deep borehole at the Kamioka Mine in central Japan in March 2023. The strain and velocity measurements were concurrently conducted using a collocated linear strainmeter and a broadband seismometer, utilizing the P- and S-phases of teleseismic and local earthquakes as comparative signals. This comparison confirmed the sensitivity to areal and normal strains detected by the linear strainmeter. Additionally, the sensitivity of the strainmeter to strain in various directions was assessed by comparison with a seismometer. These analyses also indicated that the contrast in elastic properties between the strainmeter sensor and the surrounding rocks affected the scaling of the strain amplitude in situ. Moreover, the inhomogeneity of the surrounding media induced variations in strain amplitudes, especially in the shear waves. Graphical Abstract