2013 Honorary Lecturer

Near Surface

Laura Valentina Socco

Politecnico di Torino, Italy

Surface wave analysis for near-surface characterization: Introduction, theme and variations

2013 HL Valentina Socco

While seismologists have been using surface waves to image the Earth's crust since the ‘60s and geotechnical engineers developed surface-wave analysis methods to characterize the soil in the ‘80s, we have had to wait until the last decade to see surface-wave analysis becoming a standard method in seismic exploration. Nowadays, surface waves, which are traditionally considered noise in seismic gathers, have shown the potential of estimating the shear properties of the shallow subsurface. They are, hence, routinely used in many near-surface applications that range from seismic hazard studies to the building of weathering-layer velocity models for seismic reflection corrections.

Different kinds of surface waves can be gathered, according to the environment and the subsurface conditions: Rayleigh waves, from land surveys, Scholte waves, in marine environments, Love waves, when horizontally polarized sources and receivers are used, Lamb waves or P-guided waves, when very high-impedance contrasts are present. Regardless of the kind of waves, the most established analysis method is based on the inversion of the geometrical dispersion of surface waves. This approach, which exploits the relationship between the vertical distribution of the seismic properties in the subsurface and the phase velocity of different surface wave harmonics, only considers the kinematics of the propagation and is based on several simplified assumptions. I will examine the potentials and limitations of this approach and different technical solutions for acquisition, processing, and inversion in the context of different applications.

The most important assumptions that are usually made in surface-wave analysis are that the site is 1D and that the experimentally retrieved dispersion curve coincides with the surface-wave fundamental mode. Both assumptions are often violated and specific measures have to be taken to handle multimodal propagation in laterally varying environments. Several technical solutions have been proposed and I will discuss them to define the best practice for surface-wave analysis in complex geologic conditions. Recently, tomographic techniques, which are usually applied in seismology, have been adapted to active seismic data. Their potentials and requirements will be presented.