Detecting different water table levels in a shallow aquifer with combined P-, surface and SH-wave surveys: Insights from VP/VS or Poisson's ratios

When applied to hydrogeology, seismic methods are generally confined to the characterisation of aquifer geometry. The joint study of pressure-(P) and shear-(S) wave velocities (V-P and V-S) can provide supplementary information and improve the understanding of aquifer systems. This approach is proposed here with the estimation of V-P/V-S ratios in a stratified aquifer system characterised by tabular layers, well-delineated thanks to Electrical Resistivity Tomography, log and piezometer data. We carried out seismic surveys under two hydrological conditions (high and low flow regimes) to retrieve V-S from both surface-wave dispersion inversion and SH-wave refraction interpretation, while V-P were obtained from P-wave refraction interpretation. P-wave first arrivals provided 1D V-P structures in very good agreement with the stratification and the water table level. Both V-S models are similar and remain consistent with the stratification. The theoretical dispersion curves computed from both Vs models present a good fit with the maxima of dispersion images, even in areas where dispersion curves could not be picked. Furthermore, V-P/V-S and Poisson's ratios computed with V-S models obtained from both methods show a strong contrast for both flow regimes at depths consistent with the water table level, with distinct values corresponding to partially and fully saturated sediments. (C) 2014 Elsevier B.V. All rights reserved.