Seismic Survey Across An Environment With Very High Poisson’S Ratio

During the course of a seismic survey across a region characterized by extremely high<br>Poisson’s ratios we observed an unusual phase. This phase arrives between the direct<br>P-wave and ground roll and has a significant amplitude at short offsets. Numerical<br>modelling indicates that it is a non-geometric shear wave generated by the interaction of<br>the explosive seismic source with the free surface. For the processing and interpretation<br>of very shallow seismic data, the presence of such non-geometric waves causes problems,<br>because the additional complexity in the seismic record may be interpreted in terms of<br>structural features in the subsurface. Another interesting phenomenon in high Poisson’s<br>ratio environments is the dramatic rotation of the P-wave polarization at the free surface.<br>This effect has to be taken into account when analysing multi-component recordings.