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[SSS03-P09] Seismic structure revealed by reflection survey using distributed acoustic sensing applying to seafloor cable off Sanriku
キーワード：Distributed acoustic sensing、Seafloor cable、Seismic reflection
Distributed Acoustic Sensing (DAS) has been applied to seismological observations in recent years. DAS measures strains at plural points along an optical fiber by detecting backscattered light generated at heterogeneity by laser pulse launched into the optical fiber. DAS measurement has advantages of a spatially high-density observation with an observational interval ranging from a few meters to several tens of meters for a long fiber with a length of greater than tens of kilometers. However, there are few previous experiments for marine seismic reflection surveys using DAS on seafloor cables. We conducted DAS observations using two spare (dark) optical fibers in a seafloor cable off the coast of Kamaishi, Iwate Prefecture installed by Earthquake Research Institute (ERI), the University of Tokyo. A seismic survey experiment was performed along a route of the seafloor cable from November 5 to 7, 2020. Two types of controlled sources were used: four Bolt 1500LL air guns (each chamber capacity 1500 cu.in.) and two Sercel GI guns (each chamber capacity 355 cu. in.). Simultaneous DAS observations were carried out at the landing station using two identical instruments connected to each end of a separate fiber, with a sampling rate of 500 Hz, a channel spacing of 5 m, a gauge length of 40 m, and a total observation distance of 100 km (80 km in some cases). The 1500 LL airgun survey line coincided with the seafloor cable laying route and was extended seaward. A total length of the survey line reached about 200 km. The shot interval was set to 40 s, which corresponded to 100 m. The GI-gun survey line was also along the seafloor cable route and was approximately 100 km long. A shot interval was about 50 m. During the airgun shooting, a streamer cable was towed from the R/V Hakuho-maru to simultaneously record reflected waves in addition to the DAS measurement. In this experiment, we obtained seismic data using DAS measurement for a total length of more than 80 km with a channel interval of 5 m on the seafloor. Therefore, the obtained DAS records were considered to be suitable for applying process of seismic reflection method. The purpose of this study is to apply basic seismic reflection survey processing to DAS data obtained by seafloor cable, and to obtain a high-resolution reflection processing images taking advantage of spatially high-density data. First, we have confirmed no significant difference for the records between the two instruments. Then, seismic reflection processing was applied to those data, and reflection profiles were obtained. We adopted the constant velocity stacking method for the velocity analysis because reflected waves were observed in limited offset distances on shot gathers. In the CMP gather, offset distances were limited to less than 3 km, and a constant velocity of 1.0 km/s for the 1500LL airgun and 0.8 km/s for the GI-gun were adopted for normal moveout correction and creating CMP stacked section. The obtained reflection profiles using DAS data were compared with those using data from the hydrophone streamer. As a result, we concluded that two profiles indicate the same structure below the seafloor, however, two-way travel times for the same reflection interfaces in two profiles differed. Two-way travels times from the seafloor to reflection interfaces in the DAS reflection profile are about 2 to 5 times larger than those seen on the reflection profile by the hydrophone streamer. In consideration of the long two-way travel times to interfaces and the low stacking velocities at CMP stacking, almost of reflections on the DAS profile are estimated to be related to converted S-waves. Finally, Vp/Vs was estimated by comparing the reflected profile of the DAS measurement with that by the hydrophone streamer.