TOMO3D: new 3-D joint refraction and reflection travel-time tomography parallel code for active-source seismic data - synthetic tests

International audienceWe present a new 3-D travel-time 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 propagation velocity distribution of the subsurface and the depth and geometry of reflecting boundaries. This code is based on its renowned 2-D version TOMO2D from which it inherited the methods to solve the forward and inverse problems. The forward travel-time calculations are done using a hybrid ray-tracing technique combining the graph or shortest path method and the bending method. The LSQR algorithm is used to perform the iterative inversion of travel-time residuals to update the initial velocity and depth models. In order to cope with the increased computational demand due to the incorporation of the third dimension, the forward problem solver, which takes by far most of the run time, has been parallelised with a combination of both MP and MPI standards. This parallelization distributes the ray tracing and travel time calculations among the desired/available computational resources, allowing the user to set the number of nodes, CPUs and cores to be used. The code's performance is illustrated with increasingly complex synthetic cases including a checkerboard anomaly and two reflectors. The code is designed to invert for a single reflector at a time which is why a data-driven layer-stripping strategy is proposed for cases involving several reflectors. This strategy builds the final model by successively extending it to a deeper reflector with each new inversion incorporating the previous results. This allows us to recover strong velocity discontinuities that would otherwise be smoothened while at the same time reducing the negative effect of the ambiguity between layer thickness and propagation velocity. If time permits, we will also show some preliminary results for a 3-D real data se