Efficient Joint Analysis of Surface Waves and Introduction to Vibration Analysis: Beyond the Clichés by Giancarlo Dal Moro

Author:Giancarlo Dal Moro
Language: eng
Format: epub
ISBN: 9783030463038
Publisher: Springer International Publishing

In the introductory chapter, we showed that, for a standard MASW survey, the number of channels does not significantly affect the quality of the phase velocity spectra.

On the other side, is much important to get a profound understanding of the scheme reported in Fig. 1.​8. What are those merging polygons? A polygon represents an observable, i.e. “something” that can be analyzed in order to get information about the subsurface conditions (in our case the VS values). An large number of observables necessarily reduces the ambiguities of the solution.

Since these facts are often not sufficiently clear, let us clarify this point a bit further. When we consider the phase-velocity spectrum obtained from a standard MASW survey (e.g. 24 vertical geophones) and the HVSR curve, we deal with two polygons (i.e. observables to jointly analyze).

In case we consider the HVSR and 12 vertical geophones (instead of 24) we still deal with the same two polygons/observables (increasing the number of geophones does not help in increasing the robustness of the retrieved VS model).

If, together with the phase-velocity spectrum from the standard MASW (with vertical geophones, i.e. ZVF component) and the HVSR, we also consider the dispersion curve obtained from the ESAC, fundamentally we are still playing with just two polygons. In fact, both ESAC and ZVF-MASW provide information about the vertical component of Rayleigh waves: ESAC is potentially able to retrieve information about lower frequencies (but only if the array is sufficiently large) but the ambiguity of the effective curve retrieved from ESAC is somehow larger when compared with the dispersion obtained from active data (see Chap. 2).

In case, instead of using a set of vertical geophones, we use 12 horizontal geophones and compute the phase velocity spectra for Love waves and for the radial component of Rayleigh waves (see Sect. 1.​2 and Appendix A), we can eventually work with three observables: the HVSR and the dispersive properties of the radial component of Rayleigh waves and Love waves.

Through the HS approach, we can define up to five observables: the group velocity spectra of the three components (Z, R and T) and the RPM and RVSR curves (i.e. the Rayleigh-wave Particle Motion and the Radial-to-Vertical Spectral Ratio curves—see next sections). In the conceptual scheme reported in Fig. 1.​8 we have thus five polygons which, by adding the HVSR curve (from the passive data recorded by the same 3C geophone), would become six.

We might summarize these observations by saying that more does not necessarily mean better. Multi-component analysis based on the smart explotation of the data gathered by a single 3C geophone can in fact provide much more information so that one of the best known aphorisms of Mies van der Rohe (one of the fathers of modernist architecture) seems to apply perfectly: less is more.

While planning a seismic survey, it is important to choose carefully the most appropriate methodologies. We should be ready to understand whether, for those specific conditions and goals, the ESAC + HVSR approach is better or worse than the HS + HVSR solution (just to mention two possible approaches).

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