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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. New robust workflow minimizing nonlinearity of the crustal‐scale FWIExtensive quality control for validation of the starting and final FWI modelNew geological insight into structure of the eastern Nankai Trough

A tomographic P‐wave velocity model is inferred from a ground level‐to‐gallery vertical 500 m × 800 m seismic experiment conducted at the inter‐Disciplinary Underground Science and Technology Laboratory (LSBB, France). No initial knowledge of the velocity structure of the surrounding fractured‐porous carbonates was previously available. Ninety‐four shots at the surface were recorded by a line of 189 seismometers on the steep slope of the topographic surface and by a line of 150 geophones in an 800 m‐long, 250‐500 m‐depth gallery. The P‐wave velocities inferred from first‐arrival travel time inversion display a relatively large set of values ranging from 4000 to 6000 m/s. Such Vpvariations correlate well with the 5 to 20% porosity variations between the main geological units that consist of two sedimentary facies affected by a complex cemented fault zone. Taking advantage of the known geology of the site, this study explores the influence of the acquisition geometry impacted by the topography and of the near‐surface weathered zone onto the shallow Vptomography resolution ability. Considering the mesoscopic scale of the targeted medium, reliable imaging of hectometric geological bodies with 10% contrasts in porosities can be achieved only with the simultaneous association of (i) a high density of sources and receivers in the monitoring array geometry, and (ii) the equal consideration of surface‐to‐gallery and surface‐to‐surface first‐arrival travel times, as an essential constraint to correctly image the underlying structures.

Seismic refraction profiles were carried out in 1983 and 1987 throughout the Kerguelen Isles (southern Indian Ocean, Terres Australes & Antarctiques Franqaises, TAAF) and thereafter at sea on the Kerguelen‐Heard Plateau during the MD66/KeOBS cruise in 1991. These profiles substantiate the existence of oceanic‐type crust beneath the Kerguelen‐Heard Plateau stretching from 46°s to 55°S, including the archipelago. Seismic velocities within both structures are in the range of those encountered in ‘standard’ oceanic crust. However, the Kerguelen Isles and the Kerguelen‐Heard Plateau differ strikingly in their velocity‐depth structure. Unlike the Kerguelen Isles, the thickening of the crust below the Kerguelen‐Heard Plateau is caused by a 17 km thick layer 3. Velocities of 7.4 km s−1 or so within the transition to mantle zone below the Kerguelen Isles are ascribed to the lower crust intruded and/or underplated by upper mantle material. The crust‐mantle boundary below the Kerguelen‐Heard Plateau is abrupt and devoid of any underplated material. The difference in structure between the northern edge of the Kerguelen Plateau (including the archipelago) and the Kerguelen‐Heard Plateau may be related to variability of the time‐dependent hotspot activity. The Kerguelen‐Heard Plateau was emplaced during the Cretaceous time (110 Ma) when the volcanic output rate of the Kerguelen Plateau and the Ninetyeast Ridge was high (as well as high potential temperature). The northernmost Kerguelen Plateau and the archipelago were emplaced during Tertiary time (40–45 Ma), as the volcanic output rate reduced. Furthermore, intraplate volcanism continued in the Kerguelen archipelago for at least 40 Ma. The isostatic compensation of the Kerguelen Isles and the Kerguelen‐Heard Plateau is achieved by low‐density mantle material, as shown by refraction and geoid studies. The velocity‐depth structure below the Kerguelen Isles is similar to that found below intraplate oceanic islands such as Hawaii. Despite the differences in age, the crust below Iceland (0Ma) and the Kerguelen Plateau (100–120 Ma) are strikingly akin. The similarity between the Kerguelen‐Heard Plateau and Iceland, then, strongly supports a similar origin for both structures, the Kerguelen‐Heard Plateau being a fossil equivalent of present‐day Iceland. Crustal thickening beneath the Kerguelen‐Heard Plateau, which may result from an Iceland‐type setting (i.e. an active spreading centre over a hotspot), is mostly produced by thickening of layer 3, layer 2 representing 25 per cent only of the thickness of the igneous crust. The Kerguelen Isles, despite the initial volcanism near the active Southeast Indian Ridge, behave as a midplate volcanic island and are definitely not representative of the whole Kerguelen Plateau structure.