International audienc

S U M M A R YA new three-dimensional delay traveltime tomography is performed to image the intermediate structure of the western Gulf of Corinth. A large data set, collected in 1991 during a two-month passive tomographic experiment, has been reanalysed for the reconstruction of detailed Vp and Vs images. An improved tomography method, based on an accurate traveltime computation, is applied to invert simultaneously delayed P and S first-arrival traveltimes for both velocity and hypocentre parameters. We perform different synthetic tests to analyse the sensitivity of tomography results to the model parametrization and to the starting 1-D model selection. The analysis of the retrieved Vp and Vs models as well as deduced Vp/Vs and Vp · Vs images allows us to interpret and delineate the distribution of lithological variation, porosity/crack content and fluid saturation in the upper 9-11 km of the crust beneath the gulf. The tomographic models image a rather complex crustal structure, which is characterized by a vertical change in both velocity features and seismicity distribution. We identify a shallower zone of the crust (0-5 km depth), in which velocity distributions seem to be controlled by the still active N-S extensional regime and a deeper zone (7-11 km depth), which matches the seismogenic zone. The correlation between this latter and a specific unit of the Hellenic mountain structure (the Pyllite-Quartzite series) allows us to suggest a possible explanation for seismicity concentration in a narrow band at 7-9 km depth. Finally, the occurrence of clusters showing low-angle normal fault mechanisms in areas characterized by high Vp/Vs values indicates a possible role of fluids in triggering brittle creep along the identified low-angle normal faults.

Computationally efficient 3-D frequency-domain full waveform inversion (FWI) is applied to ocean-bottom cable data from the Valhall oil field in the visco-acoustic vertical transverse isotropic (VTI) approximation. Frequency-domain seismic modelling is performed with a parallel sparse direct solver on a limited number of computer nodes. A multiscale imaging is performed by successive inversions of single frequencies in the 3.5-10 Hz frequency band. The vertical wave speed is updated during FWI while density, quality factor Q P and anisotropic Thomsen's parameters δ and are kept fixed to their initial values. The final FWI model shows the resolution improvement that was achieved compared to the initial model that was built by reflection traveltime tomography. This FWI model shows a glacial channel system at 175 m depth, the footprint of drifting icebergs on the palaeo-seafloor at 500 m depth, a detailed view of a gas cloud at 1 km depth and the base cretaceous reflector at 3.5 km depth. The relevance of the FWI model is assessed by frequency-domain and time-domain seismic modelling and source wavelet estimation. The agreement between the modelled and recorded data in the frequency domain is excellent up to 10 Hz although amplitudes of modelled wavefields propagating across the gas cloud are overestimated. This might highlight the footprint of attenuation, whose absorption effects are underestimated by the homogeneous background Q P model (Q P = 200). The match between recorded and modelled time-domain seismograms suggests that the inversion was not significantly hampered by cycle skipping. However, late arrivals in the synthetic seismograms, computed without attenuation and with a source wavelet estimated from short-offset early arrivals, arrive 40 ms earlier than the recorded seismograms. This might result from dispersion effects related to attenuation. The repeatability of the source wavelets inferred from data that are weighted by a linear gain with offset is dramatically improved when they are estimated in the FWI model rather than in the smooth initial model. The two source wavelets, estimated in the FWI model from data with and without offset gain, show a 40 ms time-shift, which is consistent with the previous analysis of the time-domain seismograms. The computational efficiency of our frequency-domain approach is assessed against a recent time-domain FWI case study performed in a similar geological environment. This analysis highlights the efficiency of the frequency-domain approach to process a large number of sources and receivers with limited computational resources, thanks to the efficiency of the substitution step performed by the direct solver. This efficiency can be further improved by using a block-low rank version of the multifrontal solver and by exploiting the sparsity of the source vectors during the substitution step. Future work will aim to update attenuation and density at the same time of the vertical wave speed.

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