Geoscience Reference
In-Depth Information
0.00
0.02
0.04
0.06
0.08
0.10
0.12
−
2.0
−
1.5
−
1.0
−
0.5
0.0
Time [sec]
0.5
1.0
1.5
2.0
Fig. 3.6. Seismic sectionbuilt fromcross-correlationfunctions between the vertical
component of pairs of stations.Each trace shows the average cross-correlation
between twostationsseparated the corresponding distance. Aclear pulse
is observed. It corresponds tothe fundamental mode of Rayleigh waves. Itsphase
velocity is about 270 m/s (After Ch´avez-Garc´ıa et al.,2006.)
Parkway basin, New Zealand. Each trace corresponds to a single station pair (Ch´avez-
Garc´ıa et al., 2006). A clear pulse emerges, corresponding to the fundamental mode of
Rayleigh waves. It is possible to measure phase velocity dispersion in this figure (it is
about 270m/s forthe dominant frequency of the pulse, between 3 and 4Hz).
Time domain cross-correlation of ambient noise between two sites should theoreti-
callycorrespondtothecompleteGreen's functionbetween them.However, observations
suggestthatitisonlythefundamentalmodepulseofsurfacewavesthatisrecovered.The
frequencyanalysisofthatpulseallowsmeasuringphase(forinter-stationdistancessmall
as compared to wavelength) or group (for very large inter-station distances as compared
with wavelength) velocity dispersion. Inversion of dispersion curves allows recovering a
shear-wave velocity profile.
We may note that all of these techniques give as a result a 1D profile. Indeed, all the
techniques that are based on measuring and analysing surface wave dispersion assume,
by definition, a 1D layered structure. If the structure being investigated has strong
lateralvariations,thesetechniqueswillfail.Ifthestructurehassmoothlateralvariations,
the repeated use of these techniques at different sites may be called for. Therefore, we
have implicitly assumed that ambient noise consists essentially of surface waves. The
fact that useful results have been obtained suggests strongly that ambient noise consists
effectively of surface waves and that body waves, if present, have smaller amplitudes.
