Giancarlo Dal Moro

Some Final Notes

In this final chapter, we will consider few further facts useful to master surface-wave analysis and have a final holistic overview on the main topics and evidences illustrated in the previous chapters, thus being able to properly plan a survey and analyze the data in a way to obtain a robust and consistent subsurface model not affected or biased by misinterpretations or ambiguities.

Horizontal-to-Vertical Spectral Ratio

This chapter presents the fundamental aspects related to the computation of the horizontal-to-vertical spectral ratio , especially in light of its use (and abuse) for the determination of the vertical shear-wave velocity profile. Proper data acquisition and processing are described also with respect to the evaluation of the meaningfulness of the curve itself.

Understanding Surface-Wave Phenomenology

Surface waves propagate according to several modes and the dominating ones depends both on the site (the local stratigraphy), the acquisition geometry (i.e., the distance between source and receiver(s), and the considered components (Rayleigh waves have both a vertical and a radial component) and source. The way modes interlace and manifest in the velocity spectra can be extremely complex and even counterintuitive. This chapter aims at giving some evidence of such a complexity thus also suggesting possible alternative approaches for both acquisition and analysis procedures. The most appropriate solution for a given site will depend on both the specific stratigraphic characteristics and both on the goals of the survey.

Introducing Surface Waves

In this first chapter, we introduce basic aspects regarding surface wave generation and propagation. The comprehension of these facts will be crucial to then carry out the analysis of their dispersion (and attenuation) and, consequently, define a good subsurface model.

Inversion and Joint Inversion

In this chapter, the fundamental concepts of inversion and joint inversion will be introduced, also putting in evidence the conceptual relationships between (automatic) inversion and (direct) modeling .

Full Velocity Spectrum Inversion and Other Unconventional Approaches

An introduction to unconventional approaches to surface-wave analysis of complex dataset that cannot be solved via modal dispersion curve analysis: effective dispersion curves, Full Waveform inversion, and Full Velocity Spectrum inversion.

Chapter 2 – Data Acquisition

Although surface-wave analysis is often performed while considering (only) the vertical component of Rayleigh waves, this chapter will present some alternative ways to acquire data that are functional for more stringent analyses capable of solving the problems briefly introduced in the previous section and widely presented in chapters 3 and 5. In this chapter, a number of practical suggestions aimed at properly acquiring active and passive datasets are proposed, giving some emphasis to multicomponent analysis necessary to properly constrain an inversion procedure.

Shear-Wave Velocity Reconstruction via Unconventional Joint Analysis of Surface Waves: A Case Study in the Light of Some Theoretical Aspects

In site characterisation studies, the unambiguous determination of the shear-wave velocity (VS) vertical profile is a crucial point often accomplished via surface-wave analysis. The determination of the dispersive properties eventually inverted for the determination of the VS vertical profile, can be performed both via active and passive methodologies and, for land data, while considering both Rayleigh and/or Love waves. Because of its constitutive equations, Rayleigh-wave propagation is often characterized by a complex phenomenology determining non-trivial mode excitement (thus complex velocity spectra), while Love waves typically result so-to-speak simpler. These aspects logically suggest the use of a joint approach capable of reducing the non-uniqueness of the solution and solving possible interpretative issues particularly problematic when the inversion is performed according to the classical approach (picking of interpreted dispersion curves and successive inversion). After the presentation of a synthetic dataset shown to put in evidence the above-mentioned problematic aspects, a case study solved while adopting a non-ordinary approach (the joint inversion of the whole Rayleigh- and Love-wave velocity spectra accomplished by considering the Full Velocity Spectrum approach) is presented.