Summary. Here we assess the potential of the visco-acoustic frequency domain full-waveform inversion (FWI) to reconstruct P-wave velocity (VP) and P-wave atten

The Teisseyre‐Tornquist Zone (TTZ) is generally regarded as a fossil plate boundary in Europe that extends 2000 km from the Baltic Sea to the Black Sea. We used an integrated approach merging potential fields and seismic data to explore crustal architecture across the TTZ in central Poland. The aim of the study was to test whether the TTZ coincides with an early Paleozoic (Caledonian) suture formed through the closure of the Tornquist Ocean along the SW Baltica margin. The suture is presumed to separate the East European Craton (EEC) from the Paleozoic terranes of Western Europe. Two seismic reflection lines from the PolandSPAN™ experiment were used to image the deep structure at the SW margin of the EEC. Lines PL‐5300 and PL‐5400 run NE‐SW in central and northern Poland, respectively. The seismic interpretation down to top of basement was integrated with 2‐D gravity and magnetic modeling to highlight the structure of the deep crust. Both the gravity and magnetic models show a suture that welds together two blocks at the base of crust. However, top of basement above the suture dips uniformly to the SW and is overlain by undisturbed lower Paleozoic and younger sediments. By implication, the suture must have developed in the Precambrian and both crustal blocks amalgamated belong to the EEC. Consequently, the Caledonian suture, formed by the closure of the Tornquist Ocean between Avalonia and Baltica, must be located farther southwest beneath thick upper Paleozoic and Mesozoic sediments.

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.