2:45 PM - 3:00 PM
[SSS07-13] Development of new SPAC method using DAS and seismometer data: the influence of Love waves to estimation of Rayleigh-wave phase velocity by SPAC method using DAS data.
Keywords:Distributed acoustic sensing (DAS), Seismic interferometry, Seafloor cable, DAS and seismometer
Recently, Distributed Acoustic Sensing (DAS), which enables long-range observations over tens of kilometers at intervals of several meters using optical fibers as sensors, is being applied for earth sciences (e.g., Zhan 2019). The spatial dense DAS data in submarine cables with spatial autocorrelation (SPAC) method provide the S-wave velocity structure of the sedimentary layer and uppermost crust with high resolution (Fukushima et al. 2022). In the previous study, the SPAC method was applied the DAS data on the assumption that the extracted cross-spectrum consist of Rayleigh waves only (DAS-DAS). However, it is known that there is a disadvantage to the SPAC method using DAS data. In addition to Rayleigh waves, Love waves are also included (Nakahara et al. 2021). Consequently, the accuracy of the estimated phase velocities for Rayleigh waves is reduced due to the contamination of Love waves.
To address this shortcoming, we suggest a new SPAC method between DAS and the vertical component of seismometer data (DAS-vertical). The cross-spectrum obtained from a combination of DAS and vertical seismometer data contain only Rayleigh waves because Love waves have no vertical component. The SPAC method of DAS-vertical give more accurate phase velocities of Rayleigh waves than SPAC method using DAS data only. We also applied new SPAC method of DAS-vertical to DAS data and a vertical record of ocean bottom seismometers (OBSs) connected Sanriku seafloor cable which was also used for DAS measurement. We compared results of phase velocity estimation in the same section using two SPAC methods of DAS-DAS and DAS-vertical. Errors of phase velocities were estimated by the bootstrap method. The estimated phase velocities between 0.14 and 0.20 Hz form two methods coincided within the margin of the errors. However, the estimated phase velocities using DAS-DAS data are faster than the phase velocities using the method of DAS-Vertical at the frequency range from 0.10 to 0.14 Hz.
This result required us to numerically evaluate the influence of Love waves on phase velocity measurements of DAS-DAS analysis. In a numerical test, the theoretical cross-spectrum between the strain data of two DAS stations was calculated by following Nakahara et al. 2021 (Fig.A, equation A-1). We then estimated the phase velocities of Rayleigh waves in the frequency range of 0.10–0.20 Hz using the SPAC method of DAS-DAS. Assuming the conventional estimation with DAS–DAS data, we used only with the terms of Rayleigh waves (the first term in right hand side in Fig.A, equation A-2). The Rayleigh wave phase velocities estimated by the procedure generally agrees with the results obtained when actual observation data are used (Fig. B-1). The above numerical test revealed the influence of Love waves on the phase velocity estimation of Rayleigh waves with the CCFs of DAS–DAS. The difference depends on the ratio of the phase velocities between Rayleigh and Love waves, their amplitude ratio, and the maximum distance between two stations (i.e., array size). The numerical test also noticed that the phase velocity of Love waves is slower than that of Rayleigh waves over the whole considered frequency range here but the estimation of phase velocities from the SPAC method of DAS-DAS leads to a larger velocity. The phenomenon appears in different shapes of the cross-spectra of Rayleigh and Love waves. Compositions of the Bessel functions consisting of cross-spectra between Rayleigh and Love waves differ. As conclusion, our new SPAC method of DAS-vertical can estimate the phase velocities of Rayleigh waves more accurately than the conventional SPAC method of DAS-DAS.
To address this shortcoming, we suggest a new SPAC method between DAS and the vertical component of seismometer data (DAS-vertical). The cross-spectrum obtained from a combination of DAS and vertical seismometer data contain only Rayleigh waves because Love waves have no vertical component. The SPAC method of DAS-vertical give more accurate phase velocities of Rayleigh waves than SPAC method using DAS data only. We also applied new SPAC method of DAS-vertical to DAS data and a vertical record of ocean bottom seismometers (OBSs) connected Sanriku seafloor cable which was also used for DAS measurement. We compared results of phase velocity estimation in the same section using two SPAC methods of DAS-DAS and DAS-vertical. Errors of phase velocities were estimated by the bootstrap method. The estimated phase velocities between 0.14 and 0.20 Hz form two methods coincided within the margin of the errors. However, the estimated phase velocities using DAS-DAS data are faster than the phase velocities using the method of DAS-Vertical at the frequency range from 0.10 to 0.14 Hz.
This result required us to numerically evaluate the influence of Love waves on phase velocity measurements of DAS-DAS analysis. In a numerical test, the theoretical cross-spectrum between the strain data of two DAS stations was calculated by following Nakahara et al. 2021 (Fig.A, equation A-1). We then estimated the phase velocities of Rayleigh waves in the frequency range of 0.10–0.20 Hz using the SPAC method of DAS-DAS. Assuming the conventional estimation with DAS–DAS data, we used only with the terms of Rayleigh waves (the first term in right hand side in Fig.A, equation A-2). The Rayleigh wave phase velocities estimated by the procedure generally agrees with the results obtained when actual observation data are used (Fig. B-1). The above numerical test revealed the influence of Love waves on the phase velocity estimation of Rayleigh waves with the CCFs of DAS–DAS. The difference depends on the ratio of the phase velocities between Rayleigh and Love waves, their amplitude ratio, and the maximum distance between two stations (i.e., array size). The numerical test also noticed that the phase velocity of Love waves is slower than that of Rayleigh waves over the whole considered frequency range here but the estimation of phase velocities from the SPAC method of DAS-DAS leads to a larger velocity. The phenomenon appears in different shapes of the cross-spectra of Rayleigh and Love waves. Compositions of the Bessel functions consisting of cross-spectra between Rayleigh and Love waves differ. As conclusion, our new SPAC method of DAS-vertical can estimate the phase velocities of Rayleigh waves more accurately than the conventional SPAC method of DAS-DAS.