11:15 AM - 11:30 AM
[SSS07-03] Deep crustal seismic profiling from airgun-OBS surveys offshore Niigata-Yamagata: reflection imaging via reverse time migration
Keywords:airgun-OBS survey, deep crustal structure, seismic reflection profile, reverse time migration, Japan Sea
In offshore seismic exploration, estimating lithosphere-scale velocity structures by Traveltime tomography and waveforms inversion is general application to the data of ocean bottom seismographs (OBS) with airgun shooting. On the other hand, reflection imaging from the airgun-OBS data, is not general because of low folds from the sparse OBS deployment. However, reflections in the long offset data, which may contain information from the deep like the Moho discontinuity and deeper structures, is important challenge for further understanding of the deep earth structures. Furthermore, reflection profiling of crustal structures from the airgun-OBS is useful when multichannel seismic (MCS) surveys with hydrophone streamer cables are not available due to weather conditions and difficulties on the equipment or vessels.
In this study, we applied reflection imaging by reverse time migration (RTM) to the airgun-OBS data acquired in the Japan Sea by JAMSTEC. The RTM method is based on numerical modeling of the wave equation to simulate forward wavefields from source with a function and backward wavefields from receivers with observed seismic records, then reflections are imaged by the time integration after correlating the two wavefields. Although this method requires more computation costs than conventional ray-based imaging methods, it can provide higher quality images in complicated structures. In case of the airgun-OBS data, the source-receiver reciprocity can be adopted on the relation between airgun shooting and OBSs to save of the computation cost.
We applied this method to the airgun-OBS data of two survey lines in the western offshore area of Niigata and Yamagata: EMJS1003 (297 km) in 2010 and SJ1901 (366 km) in 2019. These two lines were crossed each other at approximately 220 km offshore from the coast line. Airguns were fired every 200 m for both survey lines. OBSs were deployed with 5 km spacing on EMJS1003 and with variable spacing of 2km, 8 km, and 16 km from near the coast to the offshore on SJ1901. Reflections from the Moho were observed on most of the OBS records in both surveys. While MCS data was available on the same line of EMJS1003, MCS survey was not conducted on SJ1901 due to weather condition. Velocity models obtained by traveltime tomography (Sato et al., 2014, JGR; No et al., 2020, SSJ fall meeting) were available for wavefield modeling in the RTM application.
The Moho reflections were clearly imaged on seismic reflection profiles for both lines. The depth of the Moho was traced from approximately 20 km in the Yamato Basin to approximately 30 km near the coast through the Sado Ridge and the Mogami Trough. This result was consistent with the crust thickness inferred from analyses of the airgun-OBS data of EMJS1003 (Sato et al., 2014). The reflection profile from the OBS data showed clear image of the Moho that was difficult to be imaged by the MCS survey on EMJS1003. We also obtained the reflection profile on SJ1901 where the MCS data was not available. These results showed high potential for application of our method to airgun-OBS survey data with different varieties of OBS spacing. Updating velocity models by waveform inversion and using surface-related multiples by mirror imaging or interferometric OBS imaging are useful for better reflection profiles including sediment layers and structures in the crust. Complimentary improvement of subsurface information by both MCS and OBS surveys is important for further understanding of the deep structures.
In this study, we applied reflection imaging by reverse time migration (RTM) to the airgun-OBS data acquired in the Japan Sea by JAMSTEC. The RTM method is based on numerical modeling of the wave equation to simulate forward wavefields from source with a function and backward wavefields from receivers with observed seismic records, then reflections are imaged by the time integration after correlating the two wavefields. Although this method requires more computation costs than conventional ray-based imaging methods, it can provide higher quality images in complicated structures. In case of the airgun-OBS data, the source-receiver reciprocity can be adopted on the relation between airgun shooting and OBSs to save of the computation cost.
We applied this method to the airgun-OBS data of two survey lines in the western offshore area of Niigata and Yamagata: EMJS1003 (297 km) in 2010 and SJ1901 (366 km) in 2019. These two lines were crossed each other at approximately 220 km offshore from the coast line. Airguns were fired every 200 m for both survey lines. OBSs were deployed with 5 km spacing on EMJS1003 and with variable spacing of 2km, 8 km, and 16 km from near the coast to the offshore on SJ1901. Reflections from the Moho were observed on most of the OBS records in both surveys. While MCS data was available on the same line of EMJS1003, MCS survey was not conducted on SJ1901 due to weather condition. Velocity models obtained by traveltime tomography (Sato et al., 2014, JGR; No et al., 2020, SSJ fall meeting) were available for wavefield modeling in the RTM application.
The Moho reflections were clearly imaged on seismic reflection profiles for both lines. The depth of the Moho was traced from approximately 20 km in the Yamato Basin to approximately 30 km near the coast through the Sado Ridge and the Mogami Trough. This result was consistent with the crust thickness inferred from analyses of the airgun-OBS data of EMJS1003 (Sato et al., 2014). The reflection profile from the OBS data showed clear image of the Moho that was difficult to be imaged by the MCS survey on EMJS1003. We also obtained the reflection profile on SJ1901 where the MCS data was not available. These results showed high potential for application of our method to airgun-OBS survey data with different varieties of OBS spacing. Updating velocity models by waveform inversion and using surface-related multiples by mirror imaging or interferometric OBS imaging are useful for better reflection profiles including sediment layers and structures in the crust. Complimentary improvement of subsurface information by both MCS and OBS surveys is important for further understanding of the deep structures.