Robust Acoustic Waveform Inversion from Underground Galleries to Image Strike-slip Faults in Clay-rock Formations

Deep argillaceous formations are potential host media for high-level long-life radioactive waste due to their confining properties. In order to improve its ability to expertise possible projects of radioactive waste disposal in a geological clay-rock formation, the French Institute for Radioprotection and Nuclear Safety led various experiments in the undeground Tournemire Experimental Platform (TEP). The presence of strike-slip faults with small vertical offsets in clay-rock formations, as they are observed in the TEP, must be well assessed since they could modify the confining properties of the host rock. The TEP is composed of several galleries excavated in a 200 m thick Toacian clay-rock layer. Various studies have shown this type of fault is difficult to detected with seismic reflection from the surface or from galleries. Thus we tried to image a 10 m thick strike-slip fault with a transmission survey involving several galleries. We present here how the use of a robust adaptation of weighted acoustic full waveform inversion can provide a high resolution P-wave velocity model and revealed a much more complex structure than expected.