GNGTS – Atti del 19° Convegno Nazionale / 04.07
G. P. Deidda (1), G. Ranieri (1) and E. Piano (2)
(1)
(2)
D.I.T., Università di Cagliari
Libero professionista
ESTIMATION OF NEAR-SURFACE SHEAR-WAVE VELOCITY
BY A JOINT ANALYSIS OF RAYLEIGH WAVES
AND SH-WAVE REFLECTION EVENTS
Shear wave velocity is one of the key parameters used to evaluate near-surface
stiffness and in general, for measuring low-strain soil elastic behaviour. In
geotechnical engineering, it is classically estimated through invasive seismic
methods such as cross-hole and down-hole measurements. Since their employment
requires the use of boreholes, they do not permit tests to be performed rapidly and at
low costs as requested in near-surface investigations. Non-invasive seismic methods
(refraction, reflection and surface waves), which are conducted from the free surface,
are more advantageous as they offer the possibility of testing larger portions of soil,
without the needs of boreholes, and so they are usually more cost-effective. Among
these, SH-wave reflection and surface-wave methods seem to be the most
interesting since they have the potential to detect a relatively soft layer underlying a
stiffer layer – a capability that can not be offered by refraction seismics.
The use of Rayleigh waves for characterisation purposes in geotechnical
engineering had a strong impulse with the introduction of the Spectral Analysis of
Surface Waves (SASW) method (Nazarian et al., 1983; Stokoe and Nazarian, 1983)
and a significant advance was made in the last 20 years (e.g., Nazarian and Stokoe,
1984, 1986; Stokoe and Nazarian, 1985; Gabriels et al., 1987; Tokimatsu et al.,
1991; Stokoe et al., 1994; Park et al., 1998).
Despite the wide success, two critical aspects still remain in the estimation of
near-surface shear-wave velocity by inversion of Rayleigh waves: one is the
necessity of assuming a consistent soil model constituted by a succession of parallel
layers, each one constituted by a homogeneous and isotropic elastic material; the
other concerns the non-uniqueness of the solution, which is a major difficulty for
every inversion problem. In fact, the inversion process needs a starting hypothesis of
stiffness profile and this needs to be chosen carefully because it can strongly
influence the final result. Any information about the site, such as interface position or
number of layers, is very important to set some constraints on the solution, in order
not only to mitigate the non-uniqueness but also to speed up the convergence of the
inversion process. This extra information, which is very often achieved from
boreholes, can be obtained using SH-wave shallow seismic reflection. When low
velocity materials occur (a quite common condition in near surface), the degree of
resolution and accuracy associated to SH-wave reflection imaging is certainly
sufficient to set an initial well-constrained earth model for the iterative inversion
process of surface waves.
On that account, to better estimate near-surface shear-wave velocity profiles,
we propose a joint analysis of surface-wave (Rayleigh-wave) and SH-wave reflection
data. We analysed experimental data from two test sites for which geological
information and seismic velocities, obtained from previous surveys, are available. For
each test site both surface-wave and SH-wave reflection data were acquired along
GNGTS – Atti del 19° Convegno Nazionale / 04.07
the same line. Dispersion curves of Rayleigh waves, obtained from multichannel
analysis in the f-k domain, were inverted with several different starting earth models.
At first, using no a priori information the initial model was obtained applying the
simple inversion procedure of Steady State Rayleigh Waves method (Jones, 1958;
1962). Another model, obtained by time-to-depth conversion of SH-wave stacked
section, was then used as an initial guess. Then, inversion results were compared
through the analysis of resolution matrices and rms errors associated to the inversion
processes. Analysis of the resolution matrix (R) provided an indication of the
uniqueness (in an ideal case, if R equals the identity matrix then the calculated
solution is the true and unique solution), as well as a measure of how well or poorly
the solution was determined. Analysis of rms error, instead, gave information about
the effectiveness of the fitting between numerical and experimental data. In all cases,
using the seismic depth sections as initial models, inverted earth model solutions
were found to have better accuracy and better convergence than all other solutions.
Therefore, the proposed joint analysis technique seems to be more effective
than the surface waves method alone, as it has the capability to provide more
realistic subsurface models. Such capability is surely interesting when geometrical
behaviour is also important, as in the case of site effects investigations.
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