Toward a robust workflow for deep crustal imaging by FWI of OBS data: The eastern Nankai Trough revisited

Abstract

Crustal‐scale imaging by the full‐waveform inversion (FWI) of long‐offset seismic data is inherently difficult because the large number of wavelengths propagating through the crust makes the inversion prone to cycle skipping. Therefore, efficient crustal‐scale FWI requires an accurate starting model and a stable workflow minimizing the nonlinearity of the inversion. Here we attempt to reprocess a challenging 2‐D ocean‐bottom seismometer (OBS) data set from the eastern Nankai Trough. The starting model is built by first‐arrival traveltime tomography (FAT), which is FWI assisted for tracking cycle skipping. We iteratively refine the picked traveltimes and then reiterate the FAT until the traveltime residuals remain below the cycle‐skipping limit. Subsequently, we apply Laplace‐Fourier FWI, in which progressive relaxation of time damping is nested within frequency continuation to hierarchically inject more data into the inversion. These two multiscale levels are complemented by a layer‐stripping approach implemented through offset continuation. The reliability of the FWI velocity model is assessed by means of source wavelet estimation, synthetic seismogram modeling, ray tracing modeling, dynamic warping, and checkerboard tests. Although the viscoacoustic approximation is used for wave modeling, the synthetic seismograms reproduce most of the complexity of the data with a high traveltime accuracy. The revised FWI scheme produces a high‐resolution velocity model of the entire crust that can be jointly interpreted with migrated images derived from multichannel seismic data. This study opens a new perspective on the design of OBS crustal‐scale experiments amenable to FWI; however, a further assessment of the optimal OBS spacing is required for reliable FWI.

Application of common reflection angle migration for imaging deformation structures in an inner accretionary wedge, Nankai Trough, Japan

“…b). Górszczyk, Operto and Malinowski () shows similar deformed thrusts in the inner accretionary wedge based on FWI velocity model in the Eastern Nankai Trough subduction zone.…”

Seismic wavefield imaging of Earth’s interior across scales

“…FWI is computationally and algorithmically demanding, and, although these demands hampered progress for many decades, these restrictions have been largely overcome in the past decade, due to advances in algorithms and computing technology. Today, FWI is widely used in applications including medical imaging [10][11][12][13][14] , nondestructive testing [15][16][17][18][19][20][21][22][23][24] , near-surface characterization [25][26][27][28][29][30] , onshore and offshore exploration seismology [31][32][33][34][35][36][37][38][39] , deep crustal seismic imaging [40][41][42][43][44][45][46][47] , earthquake seismology [48][49][50][51][52][53][54] and ambient-noise seismology 55,56 . A comprehensive ove...…”

Crustal-scale depth imaging via joint FWI of OBS data and PSDM of MCS data: a case study from the eastern Nankai Trough

“…The aim of the processing of the SFJ-OBS dataset was to obtain crustal-scale P-wave velocity model using FWI, such that it could provide insight into deep crustal targets with a wavelength-scale resolution. The dataset was recently reprocessed by Górszczyk et al (2017).…”

“…To support interpretation of upper mantle one can additionally calculate refraction ray-tracing in the final 10 FWI model and track the rays corresponding to Pn arrival, which tend to be channelled along the Moho boundary (see Fig. 13(e) in Górszczyk et al, 2017, for the illustration).…”

“…Note also the challenging picking of first arrivals between 35km and 45km offset, where weak-amplitude first arrivals correspond to head waves that propagate along a thrust in the backstop. See Górszczyk et al (2017) for details. Inset in (b) presents resolution gap between tomographic velocity models and migrations images ( Fig.1.13 of Claerbout, 1985) 23 Solid Earth Discuss., https://doi.org/10.5194/se-2019-33 Manuscript under review for journal Solid Earth The symbols + denote the velocity nodes.…”