5:15 PM - 6:45 PM
[STT36-P02] Relationship between surface ground motion and strain observed by a borehole fiber optic strainmeter
Keywords:Optical sensing, fiber optic strainmeter, borehole observation, strain measurement
The relationship between ground motion and strain records observed by a borehole fiber optic strainmeter was estimated by theoretical calculation based on sensor shape and comparisons to in-situ data observed by seismometers and linear fiber optic strainmeter.
The borehole fiber optic strainmeter was developed for long-term observation in the seafloor borehole at the Nankai trough subduction zone to observe seismic and geodetic phenomena occurring associated with subduction processes. The strainmeter is composed of approximately 200m of reference fiber in a pressure housing and the same length of sensing fiber which is closely wound around a stainless-steel mandrel like a coil. The two fibers compose a Michelson interferometer. This unique shape of the sensing fiber enables us to install a longer baseline in limited volume in the borehole and to obtain sensitivities in various directions. A borehole fiber optic strainmeter was installed in a 20m-deep borehole at a test site in Kamioka mine, the central part of Japan in March 2023 for testing (Machida et al., 2023, JpGU Meeting), and a modified one was installed in a 500m-deep borehole at Nankai Trough in November 2023 (Machida et al., 2023, AGU Meeting).
In this study, sensitivity of the borehole fiber optic strainmeter was estimated by theoretical calculation and comparison of in-situ data. Firstly, sensitivity in each component of the strain tensor was estimated based on the geometrical shape of the sensing fiber. Next, records of teleseismic signals and local earthquakes observed by the borehole fiber optic strainmeter, a broadband seismometer, and a linear fiber optic strainmeter were compared. In the comparison between strain records and velocity records, seismic signals are assumed to be plane waves. Under this assumption, strain can be expressed as a multiple of the inverse of phase velocity (slowness) and ground velocity.
As a result, normal and areal strains in the horizontal plane were sensitive components in measurements with the borehole fiber optic strainmeter. The theoretical calculation showed that normal strain and areal strain in the horizontal plane showed approximately 10 times more sensitivity to shear strain. Normal strain in the vertical component had almost no sensitivity. To confirm the tendency, velocity waveforms of the direct S phase in teleseismic signals were fitted with strain records in low frequency because almost linear ground motion can be obtained. As a result, the records are fitted within 20% of residuals in wave energy. However, the magnitudes of coefficients between velocity and strain are not those that are expected from plane wave assumption.
A mismatch in amplitude may suggest an effect of the borehole. Contrast in elastic modulus between the sensor and surrounding media can enhance or reduce the sensitivity. Now, we are trying to include the effect in the sensitivity estimation.
The borehole fiber optic strainmeter was developed for long-term observation in the seafloor borehole at the Nankai trough subduction zone to observe seismic and geodetic phenomena occurring associated with subduction processes. The strainmeter is composed of approximately 200m of reference fiber in a pressure housing and the same length of sensing fiber which is closely wound around a stainless-steel mandrel like a coil. The two fibers compose a Michelson interferometer. This unique shape of the sensing fiber enables us to install a longer baseline in limited volume in the borehole and to obtain sensitivities in various directions. A borehole fiber optic strainmeter was installed in a 20m-deep borehole at a test site in Kamioka mine, the central part of Japan in March 2023 for testing (Machida et al., 2023, JpGU Meeting), and a modified one was installed in a 500m-deep borehole at Nankai Trough in November 2023 (Machida et al., 2023, AGU Meeting).
In this study, sensitivity of the borehole fiber optic strainmeter was estimated by theoretical calculation and comparison of in-situ data. Firstly, sensitivity in each component of the strain tensor was estimated based on the geometrical shape of the sensing fiber. Next, records of teleseismic signals and local earthquakes observed by the borehole fiber optic strainmeter, a broadband seismometer, and a linear fiber optic strainmeter were compared. In the comparison between strain records and velocity records, seismic signals are assumed to be plane waves. Under this assumption, strain can be expressed as a multiple of the inverse of phase velocity (slowness) and ground velocity.
As a result, normal and areal strains in the horizontal plane were sensitive components in measurements with the borehole fiber optic strainmeter. The theoretical calculation showed that normal strain and areal strain in the horizontal plane showed approximately 10 times more sensitivity to shear strain. Normal strain in the vertical component had almost no sensitivity. To confirm the tendency, velocity waveforms of the direct S phase in teleseismic signals were fitted with strain records in low frequency because almost linear ground motion can be obtained. As a result, the records are fitted within 20% of residuals in wave energy. However, the magnitudes of coefficients between velocity and strain are not those that are expected from plane wave assumption.
A mismatch in amplitude may suggest an effect of the borehole. Contrast in elastic modulus between the sensor and surrounding media can enhance or reduce the sensitivity. Now, we are trying to include the effect in the sensitivity estimation.