Inversion of qP wave travel times from the URL experiment

Summary

The qP wave travel time data measured during an experiment in the vicinity of the Underground Research Laboratory (URL) in Manitoba, Canada are inverted for elastic parameters. Preliminary results of the inversion are presented. No a priori assumptions about heterogeneity and anisotropy of rocks surrounding the tunnel of the URL is made except the variation of elastic parameters in each grid box, of which the model box is composed, is tri-linear. The qP-wave travel time data from 36 events recorded at 16 receivers yield a very good spatial and angular coverage of the volume, in which the elastic parameters are sought. At the top and in the middle, the resulting model has anisotropy close to orthorhombic symmetry. At the bottom, a different anisotropy can be observed. Maximum velocities at the top and in the middle of the model are slightly deviating from the direction parallel to the axis of the tunnel, with minimum velocities in the direction perpendicular to it. At the bottom, prominent maxima and minima of phase velocities can be found in the plane perpendicular to the axis of the tunnel, along directions deviating about 30⁰ from horizontal. The strength of anisotropy decreases slightly with increasing depth. It is shown that a single iteration, from a refernce isotropic medium to an anisotropic medium is insufficient even if the inverted medium is weakly anisotropic. Thus, an iterative inversion procedure must be used. Obtained results indicate that the medium surrounding the tunnel is wekaly anisotropic and that the elastic parameters related to qS-wave propagation cannot be uniquely determined from only qP-wave travel time inversion.

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