GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 3.2 422 SURFACE-WAVE TOMOGRAPHY-DERIVED LINEAR INVERSION: THE CASE STUDY OF A LANDFILL I. Barone 1 , J. Boaga 1 , A. Carrera 1 , A. Flores-Orozco 2 , G. Cassiani 1 1 Università degli Studi di Padova, dipartimento di Geoscienze, Padova (Italy) 2 TU Wien, department of Geodesy and Geoinformation, Vienna (Austria) Surface wave tomography (SWT) is a powerful technique to image lateral seismic velocity variations in the shallow subsurface at very different scales. While seismological applications of SWT allow the characterization of the Earth crust down to the upper mantle (Trampert and Woodhouse, 1996; Boschi and Dziewonski, 1999; Shapiro et al., 2005), SWT applied at the local scale can identify relevant velocity variations in the near surface due to the presence of faults, lithological contacts, buried structures, etc. (Gouédard et al., 2012; Pilz et al., 2012; Mordret et al., 2019). In the present case, we demonstrate the applicability of the SWT for the delineation of changes in the waste composition in a municipal solid waste (MSW) landfill. The surface-wave inversion scheme presented here derives from SWT, and can be applied to any 2D active seismic dataset. Under the assumption of a straight-ray seismic propagation, the forward problem for one frequency becomes a linear system relating phase differences between adjacent receivers and wavenumbers. A norm damping regularization constraint is imposed to the system, which ensures a laterally smooth solution. The optimal damping parameter for each frequency of analysis is found through a minimization process taking into account only phase variations larger than the average wavelength. The linear system is solved using a weighted Least- Squares scheme, where the weights are the inverse of data variances. Model wavenumbers are converted to phase velocity values; accordingly, the obtained pseudo-section is a collection of local dispersion curves, which can be depth-inverted to retrieve a quasi-2D shear-wave velocity section. The inversion scheme described above has been applied to a 2D active seismic dataset acquired over the “Heferlbach” landfill in Mannsworth, 15 km from Vienna (Austria). The site Fig. 1: (a) Quasi-2D shear-wave velocity section obtained with the surface-wave tomography-derived linear inversion followed by depth inversion of individual dispersion curves. (b) Conductivity section obtained through inversion of ERT data. Note the lateral variation of both seismic velocity and conductivity around x =180 m. The higher conductivity values on the bottom of the municipal waste deposit could indicate the former presence of leachate.