9:00 AM - 9:15 AM
[SVC31-01] Feasibility study of DAS observations using Tonga seafloor cable to monitor volcanic activities
Keywords:Disasters from volcano eruptions, Remote monitoring, Seafloor observations
Background:
The devastating eruption at the submarine volcano, Hunga-Tonga Hunga-Ha’apai (HTHH), on 15 January 2022 influenced the world by the large tsunami, including the meteo-tsunami generated by the atmospheric wave. This event reminds us of the importance of monitoring the activities of shallow-sea volcanoes. Seismic observation is essential to monitor volcanic activities, but no seismic stations were operating in Tonga during the eruption. There are few lands near the volcanoes, and installing and maintaining stations on remote islands is expensive. Seismic observations based on Distributed Acoustic Sensing (DAS), a new technology developing in recent years, may solve this issue. DAS observations using the seafloor cable close to the volcano would enable monitoring with less cost and less affected by anthropogenic noise. Here we present a preliminary report of a feasibility study of DAS observations in Tonga, using the seafloor cable for domestic internet connections, which was cut by the eruption.
Observation:
In Tonga, a seafloor cable of international broadband communications connects Tongatap, the main island of Tonga, and Fiji, and domestic ones connect the island and Vava’u and Ha’apai. Both cables were damaged by the HTHH eruption. The international cable has already been repaired 1 month after the eruption, but the domestic ones are scheduled to repair in March 2023. DAS ordinary uses a dark fiber, but there is no spare in the Tonga cables. We have got permissions to use the domestic cables before the repair from Tonga Cable Ltd., the owner.
The domestic cables are missing between 30 and 100 km from Tongatapu. We installed a DAS interrogator at Tongatapu to obtain seismic data in the 30 km segment, of which end is located about 20 km from the center of HTHH. We conducted the observation from 6 to 13 February 2023. We also installed seismometers for the reference.
We used OptaSense QuantX for the DAS observation. We set the gauge length at 30.63 m, channel interval at 2.04 m (14960 channels), pulse rate at 3.125 kHz, and data was recorded at 312.5 Hz after decimation. We used seismometers Nanometrics Trelium Compact (120 s period) and Lenartz LE-3Dlite MKIII (1 s period), and the data was recorded at 200 Hz by Hakusan LS8800. We installed two seismometers at Tonga Cable Ltd., and two at about 7 km from this site.
Results:
Noise level of DAS records was relatively high close to Tongatap from oceanic surface waves, but lower at distant locations suitable for seismic observations in deep sea. We observed regional and teleseismic earthquakes. We also observed possible local events.
Prospectives:
In monitoring land volcanoes, we estimate their current status from multiparameter observations close to the summit crater and expect future activities. During the eruptions at Nishinoshima, an uninhabited island in the Ogasawara archipelago, efforts were made for the close observation by installing ocean bottom seismometers around the island and a seismo-acoustic online station on the island. However, continuous monitoring was impossible. In this study, we confirmed that DAS could detect signals from submarine volcanoes in real-time. DAS will improve the monitoring capabilities of volcanic and seismic activities in this region if the seafloor cables along and across the Tonga arc are available for DAS even in operation. The installation took only several hours, so DAS is also suitable for quick observations immediately after large events. To achieve this, we need to develop methods of real-time data analyses, transfer the analysis results, and DAS observations on fibers in operation. This study is the first step of the research.
Acknowledgements:
This study is funded by ERI of the University of Tokyo and JAMSTEC with the support of Tonga Geological Services (TGS), Tonga Cable Ltd., and Japan International Cooperation Agency (JICA). We also thank Mr. T. Yagi (ERI), V. Tovi (TGS), and members of TGS for their assistance.
The devastating eruption at the submarine volcano, Hunga-Tonga Hunga-Ha’apai (HTHH), on 15 January 2022 influenced the world by the large tsunami, including the meteo-tsunami generated by the atmospheric wave. This event reminds us of the importance of monitoring the activities of shallow-sea volcanoes. Seismic observation is essential to monitor volcanic activities, but no seismic stations were operating in Tonga during the eruption. There are few lands near the volcanoes, and installing and maintaining stations on remote islands is expensive. Seismic observations based on Distributed Acoustic Sensing (DAS), a new technology developing in recent years, may solve this issue. DAS observations using the seafloor cable close to the volcano would enable monitoring with less cost and less affected by anthropogenic noise. Here we present a preliminary report of a feasibility study of DAS observations in Tonga, using the seafloor cable for domestic internet connections, which was cut by the eruption.
Observation:
In Tonga, a seafloor cable of international broadband communications connects Tongatap, the main island of Tonga, and Fiji, and domestic ones connect the island and Vava’u and Ha’apai. Both cables were damaged by the HTHH eruption. The international cable has already been repaired 1 month after the eruption, but the domestic ones are scheduled to repair in March 2023. DAS ordinary uses a dark fiber, but there is no spare in the Tonga cables. We have got permissions to use the domestic cables before the repair from Tonga Cable Ltd., the owner.
The domestic cables are missing between 30 and 100 km from Tongatapu. We installed a DAS interrogator at Tongatapu to obtain seismic data in the 30 km segment, of which end is located about 20 km from the center of HTHH. We conducted the observation from 6 to 13 February 2023. We also installed seismometers for the reference.
We used OptaSense QuantX for the DAS observation. We set the gauge length at 30.63 m, channel interval at 2.04 m (14960 channels), pulse rate at 3.125 kHz, and data was recorded at 312.5 Hz after decimation. We used seismometers Nanometrics Trelium Compact (120 s period) and Lenartz LE-3Dlite MKIII (1 s period), and the data was recorded at 200 Hz by Hakusan LS8800. We installed two seismometers at Tonga Cable Ltd., and two at about 7 km from this site.
Results:
Noise level of DAS records was relatively high close to Tongatap from oceanic surface waves, but lower at distant locations suitable for seismic observations in deep sea. We observed regional and teleseismic earthquakes. We also observed possible local events.
Prospectives:
In monitoring land volcanoes, we estimate their current status from multiparameter observations close to the summit crater and expect future activities. During the eruptions at Nishinoshima, an uninhabited island in the Ogasawara archipelago, efforts were made for the close observation by installing ocean bottom seismometers around the island and a seismo-acoustic online station on the island. However, continuous monitoring was impossible. In this study, we confirmed that DAS could detect signals from submarine volcanoes in real-time. DAS will improve the monitoring capabilities of volcanic and seismic activities in this region if the seafloor cables along and across the Tonga arc are available for DAS even in operation. The installation took only several hours, so DAS is also suitable for quick observations immediately after large events. To achieve this, we need to develop methods of real-time data analyses, transfer the analysis results, and DAS observations on fibers in operation. This study is the first step of the research.
Acknowledgements:
This study is funded by ERI of the University of Tokyo and JAMSTEC with the support of Tonga Geological Services (TGS), Tonga Cable Ltd., and Japan International Cooperation Agency (JICA). We also thank Mr. T. Yagi (ERI), V. Tovi (TGS), and members of TGS for their assistance.