2023_programme: Broadband hydro-pressure measurements by a fiber-optic submarine cable in the Nankai Trough, Japan

• Session: 03. Comprehensive Nuclear-Test-Ban Treaty Monitoring and its Civil and Scientific Applications
  Organiser(s): Georgios Haralabus, Mario Zampolli and Peter Nielsen
• Lecture: Broadband hydro-pressure measurements by a fiber-optic submarine cable in the Nankai Trough, Japan [invited]
  Paper ID: 1890
  Author(s): Matsumoto Hiroyuki, Araki Eiichiro, Yokobiki Takashi
  Presenter: Matsumoto Hiroyuki
  Abstract: Distributed Acoustic Sensing (DAS) is becoming a popular technique to detect vibrating signals along a fiber-optic cable because the number of sensing channels is much larger than that of individual seismometers. Since 2019, we conduct long-term DAS measurements using a 50-100 km abandoned submarine cable. Our primary objective is to monitor seismic activity in the Nankai Trough seismogenic zone, Japan. In the early stages of our DAS observation, hydroacoustic signals originating from the air-gun were examined and compared with the co-located hydrophones. Our field observations show that hydroacoustic signals detected by DAS correlate well with hydrophones, suggesting that DAS can detect explosive phenomena in the water-column. Additionally, strain is observed along the submarine cable where the fiber-optic cable runs along relatively shallow water area, which is attributed to ambient temperature. Therefore, we deployed temperature sensors during the DAS measurement and directly compare DAS with co-located temperature sensors. Simultaneous measurement of ambient temperature and DAS were conducted for a period of 50 days to qualify the contribution of ambient temperature change on DAS in this region. Our in-situ observations suggest that the coefficient between strain and temperature is 4 to 7 micro-strain per degree Celsius. In 2022, a stable laser device is used as an incident source resulting in the identification of infra-gravity waves (predominant period > 100 s) during the typhoons (tropical storms). Our study supports that the CTBT IMS hydroacoustic hydrophone stations have the potential for supporting parallel observations for various undersea phenomena using DAS.
• Corresponding author: Dr Hiroyuki Matsumoto
  Affiliation: JAMSTEC
  Country: Japan
  e-mail: