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› Seismic Explorations
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› M.A.S.W. (Multichannel Analysis of Surface Waves)
The MASW (Multichannel Analysis of
Surface Waves) technique has the objective
to identify variation profiles
with the depth of the speeds of volume
waves (Vp and Vs). The method
is based on the known relations between
these speeds and the dispersion
of surface (or Rayleigh) waves
observed when propagating through
a stratified elastic medium. The analysis
can be based on signals produced
with a borehole on site by acquisition
device (with a ram or explosion), or on
the recording of the vibrations produced
by far away sources (rives, industrial
activities, traffic, etc).
In the first case, we are talking about
active MASW, with which it is possible
to explore a few tens of metres of
sub-soil, and in the second case, we
are talking about passive MASW, that
allows to reach greater depths, in particular
conditions.
Passive MA.S.W. is used with the purpose
to obtain a speed profile 1D of
the elastic waves of cut S. The technique
consists in the recording of the
“seismic noise” in temporal windows
and following study of the signal
processed. It is carried out by arranging
a bidimensional geophonic chain
with low resonant frequency in line
or in “array” (circular and irregular geometries)
and measuring the environmental
noise. From the F-K analysis
(frequency-space) of the wave-trains,
it is possible to obtain a dispersion
curve of surface waves that leads
to the calculation of the speed profile
of the shear waves and estimate
of a coverage in relation to the semispace.
› S.A.S.W. (Spectral Analysis of Surface Waves)
The Spectral analysis of Surface Waves
SASW allows to determine the speed
profile of the shear waves of a particular
ground. The method is based on
the use of the dispersive properties
of surface waves (Rayleigh) generated
by a surface impulsive input. The
depths explored vary from a few centimetres
(road floors) to a few tens of
metres. SASW is carried out by placing
2 geophones in line, in the ground,
with oscillation frequency from 14
to 1 Hz, and registering the seismograms.
The speeds profile of waves Vs
is obtained from studying the phase
speeds of Rayleigh waves. The data
elaboration consists in determining
the Coherence function, the Cross
Power Spectrum phase and the construction
of the dispersion curves
of the Speed during the experimental
phase in depth. In conclusion, the
propagation phenomenon of the surface
waves is simulated with the purpose
to identify the rigidity profile
reproduced by the experimental dispersion
curve.