Abstract
We applied reverse time migration (RTM) to offshore wide-angle seismic data acquired with airgun shots and sparsely deployed ocean bottom seismographs (OBSs) for reflection imaging of the Moho discontinuity in the eastern margin of the Sea of Japan. While seismic tomography is generally applied to wide-angle seismic data for estimating regional velocity, reflection imaging is uncommon due to the low folds from wide-spacing OBS deployment. The long offset reflection data obtained by airgun-OBS surveys are promising for profiling deep crustal structures, which may be able to add constraints on the velocity structures estimated by tomographic inversion. Furthermore, reflection imaging from wide-angle seismic data is useful when only airgun-OBS data are acquired without any MCS data due to weather conditions or restrictions of using streamer cables. In this study, we validated the feasibility of RTM, which is an effective reflection imaging method based on wavefield modelling with the two-way wave equation, using offshore wide-angle seismic data acquired along two crossing survey lines off Niigata–Yamagata. Airgun shot intervals were 200 m in both surveys, and the OBS spacings were 5 km along a 297-km-long line and 8 km or 16 km along a 366-km-long line, except for OBSs near the coast. By applying RTM with velocity models estimated by traveltime tomography of the same OBS data, we successfully imaged clear reflections around depths of 20–30 km. We confirmed that reflections observed in the long offset range were effective in imaging the deep structures that were not imaged by the MCS survey in this region. The depths of reflectors were traced from approximately 20 km in the offshore area to approximately 30 km near the coast, which corresponds to the Moho discontinuity. The depth variation is consistent with the crustal classification that was inferred based on tomography analyses: thick oceanic crust in the Yamato Basin and rifted continental or island arc crust beneath the areas from the Sado Ridge to the coast. Our results from two surveys with different OBS spacings suggested the high potential of the application to a wide variety of wide-angle seismic data for crustal-scale seismic exploration.Graphic
Highlights
In crustal-scale offshore seismic exploration, regional seismic velocity is generally estimated by traveltime tomography and waveform inversion using wide-angle seismic data acquired with airgun shots and ocean bottom seismographs (OBSs) (e.g., Nakanishi et al 2008; Kamei et al 2012; Górszczyk et al 2017)
Reflections observed in the long offset range of airgun-OBS data are promising for containing information from deep structures, such as the Moho discontinuity, and they may add reflection boundary constraints on the velocity structures estimated by tomographic inversion from wide-angle seismic data (e.g., Zelt et al 1998; Dessa et al 2004; Qin et al 2020)
We applied the reverse time migration (RTM) method to airgun-OBS data with regional velocity models obtained by traveltime tomography from the same datasets, which is a straightforward approach to investigate deep crustal structures using wide-angle seismic data
Summary
In crustal-scale offshore seismic exploration, regional seismic velocity is generally estimated by traveltime tomography and waveform inversion using wide-angle seismic data acquired with airgun shots and ocean bottom seismographs (OBSs) (e.g., Nakanishi et al 2008; Kamei et al 2012; Górszczyk et al 2017). In some situations with limited survey periods, only airgun-OBS data are available without any MCS data due to the following reasons: weather conditions, restrictions of using streamer cables by fisheries or marine traffic in survey areas. To overcome these problems, subsurface imaging by directly using reflections recorded in airgun-OBS data is important. In most cases of crustal-scale seismic exploration using OBSs with wide spacings of more than a few kilometres, reflection imaging by using primary reflections (Dessa et al 2004; Qin et al 2020) and using multiple reflections by mirror imaging (Grion et al 2007) and interferometric imaging (Shiraishi et al 2017) are useful methods for obtaining reflection profiles, including shallow subsurface profiles, from nondense OBS data to better understand crustal structures in addition to detailed profiles by MCS surveys
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