[S-SS02] Seismological advances in the ocean

[SSS02-03] Seismic velocity structure survey off Sanriku using marine controlled sources and distributed acoustic sensing technology on seafloor optical fiber cable

*Masanao Shinohara¹, Ryosuke Azuma², Hiroki Takano¹, Raymundo Plata-Martinez³, Yasunori Sawaki³, Pousali Mukherjee³, Kazuya Tateiwa², Yoshihiro Ito³, Yusuke Yamashita³, Rie Nakata¹, Kimihiro Mochizuki¹ (1.Earthquake Research Institute, the University of Tokyo, 2.Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, 3.Disaster Prevention Research Institute, Kyoto University)

Keywords:Seismic velocity structure survey, Distributed acoustic sensing, Marine controlled seismic source

Recently Distributed Acoustic Sensing (DAS) measurement which utilizes an optical fiber itself as a sensor becomes popular for various fields and begins to be applied to seismic observations. A DAS measurement is one of optical fiber sensing technologies. A coherent laser pulse is launched into to a single mode optical fiber intermittently and repeatedly, and the backscattered light is sensed as a function of time at the launching end. When a small deformation of a fiber occurs by a vibration near the fiber, a pattern change of the backscattered light is observed. Shortest spatial sampling of the observation reaches a few meters for a long distance and high temporal sampling rate is achieved. From these characteristics, a DAS measurement enables a dense seismic observation as a linear array. A length of the array which has a short interval of sensor corresponds to a length of a deployed optical fiber. The seafloor seismic tsunami observation cable system using an optical fiber for data transmission was deployed off Sanriku by Earthquake Research Institute, the University of Tokyo in 1996, and we made several pilot observations of a DAS measurement using a spare fiber of the Sanriku seafloor observation system. As results, many earthquakes including micro-earthquakes occurring near the cable system and a deep earthquake around Japan were recorded. It is found that the current DAS measurement has enough sensitivity for seafloor seismic observation. In addition, spatially high-density observation has advantages for studies of seismic structure beneath the cable. Therefore, we carried out seismic survey using controlled sources and DAS measurement on the seafloor optical fiber cable to determine a structure with high resolution.

The survey was performed from November 3 to 8, 2020 during the research cruise KH20-11 using R/V Hakuho-maru. Nine pop-up type OBSs were deployed on the profile on November 3 and 4. Three OBSs had broadband seismometer with a natural period of 120 s. After the deployment of the OBSs, we started shooting of the controlled seismic sources. We used two type of seismic sources; four large airguns (each airgun had a chamber volume of 1500 cu in.) and two GI-guns (each gun had capacity of 355 cu in.). Shooting of large-airgun array were carried out on November 5 and 6 along the profile with a length of about 200 km. Shot interval was about 40 s. Two GI-guns were shot on November 6 and 7. A shot interval was 20 s and a length of the profile was about 90 km. Cruising speed was 4.5 kts and a hydrophone streamer was towed during both shootings. The OBSs recording airgun signals were recovered from November 6 to 8. Corresponding to the airgun shooting, the DAS measurements were conducted at the landing station of the Sanriku cable system. Because the cable system has six spare fibers, we made observation with two systems of DAS measurement concurrently. Each system used a dedicated fiber and recorded the data independently. The continuous recording started on November 4 and stopped on November 8. The data were recorded for 100 km or 80 km long with a spatial resolution of 5 m and sampling frequency of 500 Hz. The gauge length was set to 40 m. The airgun signals were clearly recorded by both DAS systems.