Geoscience Reference
In-Depth Information
Station 02
Station 06
NS
EW
S inc on NS
S inc on EW
NS
EW
S inc on NS
S inc on EW
10
10
1
1
0
1
3
4
0
1
2
Frequency [Hz]
3
4
2
Frequency [Hz]
Station 09
Station 17
NS
EW
S inc on NS
S inc on EW
NS
EW
S inc on NS
S inc on EW
10
10
1
1
0
1
2
Frequency [Hz]
3
4
0
1
2
Frequency [Hz]
3
4
Fig. 3.5. Solid lines:observed transferfunctions computed used spectral ratiosrelative
to areference sitefor soft soil stations inParkway basin. Dashed lines:computed
transfer functions using avery rough 3Dmodel of the basin. Dominant period was
used toconstrain the model. However, inaddition to the dominant peak, additional
features of theobserved transfer function arewell reproduced by the3D model
(After Ch´avez-Garc´ıa, 2003.)
The more reliable method to determine the structure and the properties of the subsoil
materials is seismic prospecting. However, it is seldom used in site effect studies. It may
be too cumbersome to apply in a city, or straightaway impossible to use because of cul-
tural noise in a city or its sheer size (e.g., Mexico City). It could be noted that, in some
experiments, when first arrivals could not be used, advantage was taken of the surface
waves (considered to be noise in seismic prospecting) generated by the source (e.g.,
Campos-Enr´ıquez et al., 2004). Other seismic prospecting methods have been developed
that explicitly exploit the surface waves such as SASW (e.g., Brown et al., 2002). How-
ever, the use of active sources is usually too expensive to be used generally in site effect
studies.
Ambient noise is much cheaper. For this reason, we have seen many applications of
microtremors to the determination of the subsoil structure. Horike (1985) showed a suc-
cessful application of frequency-wavenumber analysis of ambient noise. This technique
analysesthenoiserecordsobtainedatanarrayofstationsassumingtheyconsistofsurface
