ALBERT

Description: We present a new 3-D traveltime tomography code (TOMO3D) for the modelling of active-source seismic data that uses the arrival times of both refracted and reflected seismic phases to derive the velocity distribution and the geometry of reflecting boundaries in the subsurface. This code is based on its popular 2-D version TOMO2D from which it inherited the methods to solve the forward and inverse problems. The traveltime calculations are done using a hybrid ray-tracing technique combining the graph and bending methods. The LSQR algorithm is used to perform the iterative regularized inversion to improve the initial velocity and depth models. In order to cope with an increased computational demand due to the incorporation of the third dimension, the forward problem solver, which takes most of the run time (~90 per cent in the test presented here), has been parallelized with a combination of multi-processing and message passing interface standards. This parallelization distributes the ray-tracing and traveltime calculations among available computational resources. The code's performance is illustrated with a realistic synthetic example, including a checkerboard anomaly and two reflectors, which simulates the geometry of a subduction zone. The code is designed to invert for a single reflector at a time. A data-driven layer-stripping strategy is proposed for cases involving multiple reflectors, and it is tested for the successive inversion of the two reflectors. Layers are bound by consecutive reflectors, and an initial velocity model for each inversion step incorporates the results from previous steps. This strategy poses simpler inversion problems at each step, allowing the recovery of strong velocity discontinuities that would otherwise be smoothened.

Description: Geophysical data from the MEDOC experiment across the Northern Tyrrhenian backarc basin has mapped a failed rift during backarc extension of cratonic Variscan lithosphere. In contrast, data across the Central Tyrrhenian have revealed the presence of magmatic accretion followed by mantle exhumation after continental breakup. Here we analyse the MEDOC transect E–F, which extends from Sardinia to the Campania margin at 40.5°N, to define the distribution of geological domains in the transition from the complex Central Tyrrhenian to the extended continental crust of the Northern Tyrrhenian. The crust and uppermost mantle structure along this ~400-km-long transect have been investigated based on wide-angle seismic data, gravity modelling and multichannel seismic reflection imaging. The P -wave tomographic model together with a P -wave-velocity-derived density model and the multichannel seismic images reveal seven different domains along this transect, in contrast to the simpler structure to the south and north. The stretched continental crust under Sardinia margin abuts the magmatic crust of Cornaglia Terrace, where accretion likely occurred during backarc extension. Eastwards, around Secchi seamount, a second segment of thinned continental crust (7–8 km) is observed. Two short segments of magmatically modified continental crust are separated by the ~5-km-wide segment of the Vavilov basin possibly made of exhumed mantle rocks. The eastern segment of the 40.5°N transect E–F is characterized by continental crust extending from mainland Italy towards the Campania margin. Ground truthing and prior geophysical information obtained north and south of transect E–F was integrated in this study to map the spatial distribution of basement domains in the Central Tyrrhenian basin. The northward transition of crustal domains depicts a complex 3-D structure represented by abrupt spatial changes of magmatic and non-magmatic crustal domains. These observations imply rapid variations of magmatic activity difficult to reconcile with current models of extension of continental lithosphere essentially 2-D over long distances.

Description: Ongoing works on full waveform inversion (FWI) are yielding an increasing number of objective functions as alternative to the traditional L2-waveform. These studies aim at designing more robust functions and inversion strategies to reduce the intrinsic dependence of the FWI results on (1) the initial model and (2) the lowest frequency present in field data. In this work, we perform a comparative study of five objective functions in time domain under a common 2-D-acoustic FWI scheme using the Marmousi model as benchmark. In particular, we compare results obtained with L2-based functions that consider the minimization of different wave attributes; the waveform-based, non-integration-method; instantaneous envelope; a modified version of the wrapped instantaneous phase and an improved version of the cross-correlation travel time (CCTT) method; and hybrid strategies combining some of them. We evaluate the robustness of these functionals as a function of their performance with and without low frequencies in the data and the presence of random white Gaussian noise. Our results reveal promising strategies to invert noisy data with limited low-frequency content (≥4 Hz), which is the single strategy using the instantaneous phase objective function followed by the hybrid strategies using the instantaneous phase or CCTT as initial models, in particular the combinations [I. Phase + Waveform], [CCTT + Waveform] and [CCTT + I. Phase].