5:15 PM - 7:15 PM
[STT42-P07] Evaluation of noise level in Tonankai cable DAS observations
Keywords:DAS, noise level
The Meteorological Research Institute (MRI) conducts DAS observations using the dark fiber of an optical submarine cable at Tonankai where the JMA has laid near the Nankai Trough off the coast of Omaezaki City, Shizuoka Prefecture. This cable had a trouble in 2022 when the cable was damaged at about 4 km off the Omaezaki coast, but was restored in the spring of 2024. MRI restarted DAS observations using the Tonankai cable again in September 2024. As a result, it became possible to continuously record of DAS data from the station building in Omaezaki to a distance of 77 km. DAS records have the advantage of acquiring waveforms with high spatial density. But they have a lower dynamic range than seismographs, and their coupling to the ground is unknown in many cases due to poor installation environments. DAS measurement also has the disadvantage of measuring only one component (linear strain in the cable direction). Especially for submarine cables, it is difficult to directly observe the installation environment, and verification of the noise level is important. In this presentation, we will present the results of an investigation into the noise level of DAS observations conducted on the Tonankai cable.
The noise level was evaluated by the amplitude spectrum of the strain rate; data from each DAS channel was divided by every 5 minutes, and the average amplitude spectrum was calculated for one day to minimize the effect of noise level by time, such as day and night. The results show that at water depths shallower than about 100 m, the amplitude around the 10-second period is larger, indicating that the ocean wave noise is larger. As the water depth increases, the noise amplitude around 10 seconds becomes smaller, and the amplitude for a few seconds increases, as well as the overall amplitude level. The increase in the overall amplitude level may be due to the attenuation of the laser light as the distance to the reflection point increases, resulting in a smaller signal-to-noise ratio. Next, we verified whether stacking between close proximity channels decreases the noise level. It was found that stacking reduced the amplitude level in channels far from the laser source and at high frequencies, but not in other channels, suggesting that correlated noise predominates between channels. Observations were made by two gauge lengths (approximately 19 m and 39 m) and noise levels were compared. We will also present a comparison with the Tonankai5OBS record on the cable and what kind of seismic waveforms were obtained by the DAS observations.
The noise level was evaluated by the amplitude spectrum of the strain rate; data from each DAS channel was divided by every 5 minutes, and the average amplitude spectrum was calculated for one day to minimize the effect of noise level by time, such as day and night. The results show that at water depths shallower than about 100 m, the amplitude around the 10-second period is larger, indicating that the ocean wave noise is larger. As the water depth increases, the noise amplitude around 10 seconds becomes smaller, and the amplitude for a few seconds increases, as well as the overall amplitude level. The increase in the overall amplitude level may be due to the attenuation of the laser light as the distance to the reflection point increases, resulting in a smaller signal-to-noise ratio. Next, we verified whether stacking between close proximity channels decreases the noise level. It was found that stacking reduced the amplitude level in channels far from the laser source and at high frequencies, but not in other channels, suggesting that correlated noise predominates between channels. Observations were made by two gauge lengths (approximately 19 m and 39 m) and noise levels were compared. We will also present a comparison with the Tonankai5OBS record on the cable and what kind of seismic waveforms were obtained by the DAS observations.