P-wave, as well as horizontally and vertically polarized S-wave, tomographic data were collected between two borehole pairs. This enabled the joint-inversion of the three datasets. By employing structural constraints, the S-wave traveltimes were coupled to the more accurate P-wave traveltimes during the inversion. Thereby, the traveltime and anisotropic artefacts, initially observed in the individually inverted S-wave tomograms, were significantly reduced and the correlation with the borehole logs improved, while the resolution of the jointly inverted P-wave tomogram was only marginally affected. The joint inversion proves successful in determining the S-wave velocity distribution more accurately than individual inversions. In addition, the jointly inverted tomograms were used to detect aquifer heterogeneities, caused by differences in clay content, and to distinguish areas of relatively high effective pressure. Comparison of the jointly inverted S-wave tomograms suggests the effect of S-wave anisotropy, which showed substantial velocity differences of approximately -10% to +10%. The anisotropy may have been caused by the presence of water-filled pores, micro-cracks and preferred mineral alignment (mainly clay) in the media.
