5:15 PM - 7:15 PM
[STT42-P02] FEM simulation of tidal responses of pore-pressure and strain records at a deep seafloor borehole observatory
Keywords:Fiber optic strainmeter, borehole observatory, tidal response, pore pressure
Tidal response is one of the most popular signals in geodetic observations. For tilt and strain observation, tidal responses have been used for estimation of coupling condition of the sensor and elastic properties around observatories taking advantage of its stationarity. Thus, we estimated response of the sensor instruments and effect of geological structures around the observatory for a deep seafloor borehole observatory, C9038B, by simulating tidal responses using the Finite Element Method.
C9038B observatory was installed in a 500 m deep borehole drilled on the seafloor off the Kii Channel in the Nankai Trough. The borehole was drilled in a sedimentary V-shaped ditch, 650 meters deep and 6 km wide. Pore pressure and strain around 450 m below the seafloor are observed. The strain sensor was cemented in the sedimentary layer with structures used for the installation. A closed section filled with fluid for pore pressure measurement was made below the section with the strain sensor.
One of the characteristics of the C9038B observatory was the use of the fiber sensing technique for precise and stable measurement of strain. The detectability was lower than 0.1 nano strain. With the advantage of the low noise environment of the deep borehole, the strainmeter is expected to detect small signals of future slow-slip events around the observatory. Pore pressure observations at existing borehole observatories in the Kumano Basin, in the Nankai Trough, could detect slow-slip events (Araki et al., 2017; Ariyoshi et al., 2021).
Tidal responses are the most dominant signal in the pore pressure and the strain records at the C9038B observatory. Typical amplitudes of the tidal signals are ~10 kPa and ~300 nano strains for the pore pressure and the strain records, respectively. Amplitude ratio of the tidal response between seafloor pressure and pore pressure is 0.7. The strain records were well correlated with variations in seafloor pressure. An extension was shown with an increase in seafloor pressure. The sensitivity was -2.2× 10-11 [strain/Pa].
To understand the mechanism of the tidal responses in pore pressure and strain records, responses to vertical loading on the seafloor were simulated by the Finite Element Method (FEM) in two steps. The first step was response of geological structure around the C9038B observatory. The second step was response of instruments to measure strain. The ratio between seafloor strain and pore pressure was simulated in the first step. Using strain field at the point of the strain sensor, sensitivity in the strain record was simulated. The geological structure around the observatory was simplified as a cuboid of 50 km square and 20 km depth with a 6 km width and 650 m depth V shape ditch in the center. The instrumental model is made of a 10 m square cuboid with a simplified structure of the strain sensor at the center. Elastic parameters were given based on seismic surveys and drilling parameters. COMSOL Multiphysics 6.3 was used for FEM simulations.
Results of the FEM simulation were consistent with observations even simple models were used for the simulation. The ratio in pressure between the seafloor and sensor depth was 0.7. This was equal to the observation. The sensitivity in strain was -1.9× 10-11 [strain/Pa] were obtained. This was 80% of the observation.
Distribution in strain field obtained by the geological model suggested that geological structures around the sensor affect the magnitude of the strain measurement. Especially in the areal strain, the strain at the sensor position was 1.8 times larger than that on the seafloor. Contrarily, pressure and vertical strain showed limited effect on the sedimentary layer. In addition, structures of the instrument also showed effects on strain measurement. Strain measured by the sensor was 1.1 times larger than that in the surrounding media.
C9038B observatory was installed in a 500 m deep borehole drilled on the seafloor off the Kii Channel in the Nankai Trough. The borehole was drilled in a sedimentary V-shaped ditch, 650 meters deep and 6 km wide. Pore pressure and strain around 450 m below the seafloor are observed. The strain sensor was cemented in the sedimentary layer with structures used for the installation. A closed section filled with fluid for pore pressure measurement was made below the section with the strain sensor.
One of the characteristics of the C9038B observatory was the use of the fiber sensing technique for precise and stable measurement of strain. The detectability was lower than 0.1 nano strain. With the advantage of the low noise environment of the deep borehole, the strainmeter is expected to detect small signals of future slow-slip events around the observatory. Pore pressure observations at existing borehole observatories in the Kumano Basin, in the Nankai Trough, could detect slow-slip events (Araki et al., 2017; Ariyoshi et al., 2021).
Tidal responses are the most dominant signal in the pore pressure and the strain records at the C9038B observatory. Typical amplitudes of the tidal signals are ~10 kPa and ~300 nano strains for the pore pressure and the strain records, respectively. Amplitude ratio of the tidal response between seafloor pressure and pore pressure is 0.7. The strain records were well correlated with variations in seafloor pressure. An extension was shown with an increase in seafloor pressure. The sensitivity was -2.2× 10-11 [strain/Pa].
To understand the mechanism of the tidal responses in pore pressure and strain records, responses to vertical loading on the seafloor were simulated by the Finite Element Method (FEM) in two steps. The first step was response of geological structure around the C9038B observatory. The second step was response of instruments to measure strain. The ratio between seafloor strain and pore pressure was simulated in the first step. Using strain field at the point of the strain sensor, sensitivity in the strain record was simulated. The geological structure around the observatory was simplified as a cuboid of 50 km square and 20 km depth with a 6 km width and 650 m depth V shape ditch in the center. The instrumental model is made of a 10 m square cuboid with a simplified structure of the strain sensor at the center. Elastic parameters were given based on seismic surveys and drilling parameters. COMSOL Multiphysics 6.3 was used for FEM simulations.
Results of the FEM simulation were consistent with observations even simple models were used for the simulation. The ratio in pressure between the seafloor and sensor depth was 0.7. This was equal to the observation. The sensitivity in strain was -1.9× 10-11 [strain/Pa] were obtained. This was 80% of the observation.
Distribution in strain field obtained by the geological model suggested that geological structures around the sensor affect the magnitude of the strain measurement. Especially in the areal strain, the strain at the sensor position was 1.8 times larger than that on the seafloor. Contrarily, pressure and vertical strain showed limited effect on the sedimentary layer. In addition, structures of the instrument also showed effects on strain measurement. Strain measured by the sensor was 1.1 times larger than that in the surrounding media.