10:45 AM - 12:15 PM
[SSS12-P11] Detailed P-wave velocity structure of the sedimentary layers and uppermost crust off Sanriku using DAS, OBS and controlled seismic sources.
Keywords:Distributed acoustic sensing, Controlled seismic sources, P-wave velocity structure
Recently, Distributed Acoustic Sensing (DAS), which enables long-range observations over tens of kilometers at intervals of several meters using optical fibers as sensors, is being applied for earth sciences (e.g., Zhan 2019). We have conducted the DAS observations using seafloor cables laid off the coast of Sanriku, Japan. The S-wave velocity (Vs) structure of the sedimentary layers and uppermost crust was estimated with high spatial-resolution by applying seismic interferometry to the DAS records (Fukushima et al. 2022). To obtain detailed distribution of Vp/Vs in sedimentary layer and uppermost crust is useful for other geophysical studies. However, estimation of detailed Vp/Vs in marine area is generally difficult. Because the high-resolution S-wave structure has been already estimated, we have a possibility to obtain a detailed spatial distribution of Vp/Vs by estimation of P-wave velocity (Vp) structure with spatially comparable resolution with the obtained Vs structure.
In this study, we estimated a detailed Vp structure using DAS and OBS records during a structure survey using controlled seismic sources in 2020 to obtain spatially detailed distribution of Vp/Vs in sedimentary layers and uppermost crustal Vp/Vs below the cable deployed off Sanriku, Japan. Controlled seismic sources used in this study are four Bolt 1500LL (each airgun had a chamber capacity of 1500 cu.in.). A shooting interval is about 100m. A DAS interrogator of AP Sensing GmBH was used, and a channel interval was set to approximately 5 m.
As the first step, Vp structure of shallow sediments was obtained using the t-sum inversion method (Shinohara et al. 1994). Then Vp structure in the lower sedimentary layers and uppermost crust was estimated using the seismic ray tracing method (Zelt and Smith 1992).
Shot gathers and receiver gathers were obtained for the DAS and OBS records, respectively. We transformed data in time-distance domain into t (intercept time) - p (ray parameter) domain. Considering that DAS has low sensibility for P-wave indicating vertically, we transformed gathering DAS data for adjacent 11 shots to increase signal-to-noise (S/N) ratio. As a result, the data of t-p domain using DAS data are consistent in those using OBS data. Then we estimated one-dimensional P-wave velocity (1-D Vp) structure in the shallow sedimentary layer by t-sum inversion using the data in t-p domain of gathering of DAS data for adjacent 11 shots. Horizontal position of 1-D was defined as the center of 11 shots. We estimated 1-D Vp structures at an interval of approximately 1 km and obtained two-dimensional (2-D) structure by smoothing from 1-D Vp structures. The estimated 2-D Vp structure is comparable to the detailed Vs structure from DAS observation (Fukushima et al., 2022) and results of previous seismic survey using OBS (Takahashi et al., 2004). DAS observation during seismic survey improves spatial resolution for Vp structure compared to that estimated from OBS data alone.
As second step, the Vp structure in the lower sedimentary layers and uppermost crust ware estimated by the seismic ray tracing method (Zelt and Smith 1992) using the first arrival time of the refracted waves. To obtain the first arrival times, we made receiver gathers for both DAS records and OBS records. Although DAS receiver gathers show lower S/N ratio than OBS receiver gathers of vertical components, refracted waves from lower sedimentary layer are clearly recognized. A resolution of Vp structure in the lower sedimentary layer can increase by using DAS data in addition to OBS data, because DAS data has spatially high density. According to estimation of Vp structure in sediment layers and uppermost crust with spatially high-resolution, we can evaluate detailed distribution of Vp/Vs in sediments and uppermost crust.
In this study, we estimated a detailed Vp structure using DAS and OBS records during a structure survey using controlled seismic sources in 2020 to obtain spatially detailed distribution of Vp/Vs in sedimentary layers and uppermost crustal Vp/Vs below the cable deployed off Sanriku, Japan. Controlled seismic sources used in this study are four Bolt 1500LL (each airgun had a chamber capacity of 1500 cu.in.). A shooting interval is about 100m. A DAS interrogator of AP Sensing GmBH was used, and a channel interval was set to approximately 5 m.
As the first step, Vp structure of shallow sediments was obtained using the t-sum inversion method (Shinohara et al. 1994). Then Vp structure in the lower sedimentary layers and uppermost crust was estimated using the seismic ray tracing method (Zelt and Smith 1992).
Shot gathers and receiver gathers were obtained for the DAS and OBS records, respectively. We transformed data in time-distance domain into t (intercept time) - p (ray parameter) domain. Considering that DAS has low sensibility for P-wave indicating vertically, we transformed gathering DAS data for adjacent 11 shots to increase signal-to-noise (S/N) ratio. As a result, the data of t-p domain using DAS data are consistent in those using OBS data. Then we estimated one-dimensional P-wave velocity (1-D Vp) structure in the shallow sedimentary layer by t-sum inversion using the data in t-p domain of gathering of DAS data for adjacent 11 shots. Horizontal position of 1-D was defined as the center of 11 shots. We estimated 1-D Vp structures at an interval of approximately 1 km and obtained two-dimensional (2-D) structure by smoothing from 1-D Vp structures. The estimated 2-D Vp structure is comparable to the detailed Vs structure from DAS observation (Fukushima et al., 2022) and results of previous seismic survey using OBS (Takahashi et al., 2004). DAS observation during seismic survey improves spatial resolution for Vp structure compared to that estimated from OBS data alone.
As second step, the Vp structure in the lower sedimentary layers and uppermost crust ware estimated by the seismic ray tracing method (Zelt and Smith 1992) using the first arrival time of the refracted waves. To obtain the first arrival times, we made receiver gathers for both DAS records and OBS records. Although DAS receiver gathers show lower S/N ratio than OBS receiver gathers of vertical components, refracted waves from lower sedimentary layer are clearly recognized. A resolution of Vp structure in the lower sedimentary layer can increase by using DAS data in addition to OBS data, because DAS data has spatially high density. According to estimation of Vp structure in sediment layers and uppermost crust with spatially high-resolution, we can evaluate detailed distribution of Vp/Vs in sediments and uppermost crust.