Seismic reflection imaging of deep crustal structures via reverse time migration using offshore wide-angle seismic data on the eastern margin of the Sea of Japan

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 Abstract

Characteristics of crustal structures in the Yamato Basin, sea of Japan, deduced from seismic explorations

“…The total crustal thickness is approximately 15–16 km (Nishisaka et al, 2001; Nishizawa & Asada, 1999) and resembles that of Domain I (Sato et al, 2014). Around Domain I, Shiraishi et al (2022) pointed out that the depth of Moho discontinuity obtained by seismic reflection imaging technique with reverse time migration using wide‐angle seismic data corresponds to that estimated by seismic refraction tomography and a diffraction stack migration using OBS data of Sato et al (2014). Moreover, the P‐wave velocity in the lowermost part of the crust in the northernmost basin off Akita is relatively high at approximately 7.1–7.2 km/s (Nishisaka et al, 2001; Nishizawa & Asada, 1999).…”

Deep crustal structures with reverse time migration applied to offshore wide-angle seismic data: Equatorial and North-West Brazilian margins

“…In the case of pre-stack experiments, the shot profile reverse-time migration consists of a non-reflective two-way wave equation modeled with a finite difference (FD) method of the scalar acoustic wave equation. The method implemented is described in steps Nolet, 1986;Shiraishi et al, 2022). By adopting the source-receiver reciprocity on the relation between airgun shots and OBS's for each spacial coordinate in the model (x in equations ( 1), ( 2) and ( 3)), the downgoing wavefields -Fi(x,t), equation (1) -are extrapolated forward in time from OBS locations (XOBSi (i=1,2,…,M), equation ( 1)).The upgoing wavefields -Bi(t,x), equation( 2) -are extrapolated backwards in time from airgun shot locations (Xshotj, equation ( 2)) (e.g.…”

“…By adopting the source-receiver reciprocity on the relation between airgun shots and OBS's for each spacial coordinate in the model (x in equations ( 1), ( 2) and ( 3)), the downgoing wavefields -Fi(x,t), equation (1) -are extrapolated forward in time from OBS locations (XOBSi (i=1,2,…,M), equation ( 1)).The upgoing wavefields -Bi(t,x), equation( 2) -are extrapolated backwards in time from airgun shot locations (Xshotj, equation ( 2)) (e.g. (Schnurle et al, 2006;Shiraishi et al, 2022)):…”

“…RTM has been largely applied to reflection datasets of different sources and geometry acquisitions where the instruments have short distances between them (Chang and McMechan, 1987;Liu et al, 2011). The application of the method to wide-angle seismic (WAS) data is uncommon due to low folds in Ocean Bottom Seismometers (OBS) deployment with wide spacing (Górszczyk et al, 2017;Kamei et al, 2012;Nakanishi et al, 2008;Shiraishi et al, 2022).…”

Application of Reverse Time Migration to Faults Imaging in Rakhine Basin, Myanmar

“…The algorithm correctly images steep dips (even when angles exceed 90 °) and complex stratigraphic structures [3,[9][10][11][12]. RTM generally works well not only for seismic imaging of steeply dipping faults but also at crustal scales [13][14][15][16]. Since RTM employs an adjoint operator instead of a Hessian matrix, it has an intrinsic amplitude fidelity but suffers from resolution problems.…”