Application sur le site de Saint Gervais ( Article ECEES Genève)

Ce paragraphe a été inspiré de l’article n°1431 « Rapid screening of site effects through microtremors surveys in view of microzonation study » écrit pour la conférence ECEES de Genève 2006. Le site de Saint Gervais présenté ici, fait partie de l’étude présentée au paragraphe 5.2.1.

The steps of the protocol described in paragraph 5.1, were applied to a study over all the South of the Isère region, in the French Alps. Le port Saint Gervais was chosen as an example, it is located on the Isère valley, downstream of Grenoble and Moiran, a little bit upstream of Saint Marcelin, on the North of the limestone massif the Vercors.

A first measurement campaign was performed in order to find a 1D place by observing the variation of the shape of the H/V ratio over the all studied site. The H/V ratios were computed onto a 2 km long transverse profile of 24 points of measurement; this profile shows the evolution of the resonance frequency between 0.45 Hz and 1.2 Hz across the valley that represents the shape of a typical glacial valley (Figure 75). On the sides on the valley, the pick disappears because of the shallow top of the bedrock. From this observation we can deduce that there is an impedance contrast, at least of 3, between sediments and bedrock -it is a favourable case for array analysis- It allows us to choose an appropriate 1D place to deploy array measurements around the sensor n°14 (Figure 75). With the help of the abacuses and the available space, we deployed 3 circular arrays of 6 sensors with 15m, 30m and 60m radiuses.

A constant resonance frequency of 0.5 Hz was provided by the H/V ratio. That corresponds to a deep (at least 100meters) layer that was not reached by our arrays, so only the surface velocity of the first layer will be considered as a reliable result.

Figure 75: Results of the H/V ratios across the Isère valley at Saint Gervais. The upper figure shows the localisation of the transversal profile (red line) and the choice of the place for arrays measurements (red

circle). In the lower figure, the colour scale represents the amplitude of the H/V ratio; the abscise axis represent each measured points; the ordinate represents the frequency from 4 Hz to 0 Hz.

The dispersion curve estimated with FK technique and the dispersion curves corresponding to the models provided by the inversion of SPAC curves give consistent and complementary curves (Figure 78). The profiles from these two analyses also provide close results as we can see on Figure 76 and Figure 77.

A dispersion curve between 0.6 and 6.5 Hz is then defined mixing FK and SPAC results, its inversion provides a surface S-waves velocity between 270 m/s and 370 m/s (Figure 79). We do not consider the first jump as a really layer because the H/V ratio and the SH response are strongly divergent (Figure 80). With 0.5Hz resonance frequency, the layer thickness will be at least 135m that cannot be reached with our arrays.

Figure 76: SPAC Results: inversion of the autocorrelation curves with a initial parameters space with one layer overlaying a bedrock; a) Vp profiles, b) Vs profiles, c) dispersion curve picked in black points and

theoretical of the selected profiles in grey

Figure 77: FK Results: inversion of the dispersion curve estimated with FK, with an initial parameters space with one layer overlaying a bedrock; a) Vp profiles, b) Vs profiles, c) dispersion curves calculated in black

Figure 78: DISPERSION CURVE at Saint Gervais, Isère, France. The blue curve with error bars represents the dispersion curve estimated with the FK analyse. In red, picked dispersion curve corresponding to the models obtain by the inversion of the autocorrelation curves. The blue and green limits correspond to aliasing

(high frequency) and resolution (low frequency) limits of the 3 arrays used on the field for the measurements.

Figure 79 : Inversion of the final dispersion curve obtained by combining FK and SPAC techniques. (a) Vp profiles, (b) Vs profiles, (c) resulting dispersion curves in grey, target dispersion curve in black, limits in plain

line.

Figure 80: comparison of the H/V (black curves, plain line for the mean value, dashed lines for the standard deviation) with the SH response of the selected models (in grey)

Here are the main steps of the protocol with the associated score:

1. The H/V ratios show sharp and high peaks around 0.5 Hz in the area of array

recordings. [2 points]

2. In the 1D zone selected (Figure 79), we have chosen the appropriate arrays aperture using abacuses, with a radius of 15, 30 and 60 meters.

3. The analyses with FK and SPAC provide consistent results (cross-check in Figure 76, Figure 77 and Figure 78). [2 points]

4. Merging the dispersion curves we extend the frequency band from 0.6 to 6.5 Hz.

log(fmax)-log(fmin) = log(6.5) – log(0.6) = 1 point

5. The spread of the mean sub-surface velocity is about 90 m/s [1 point]

6. The coherence between the fH/V and the f0 of the SH response of the selected profiles is very low because the aperture of the array does not allow to reach the deep layer corresponding to the resonance frequency of 0.5 Hz [0 point]

7. Check if the flat slope of the dispersion curve is reached: DC(5) – DC(6.5) = 267– 236 = 31 m/s, less than 15 % of 236 m/s [1 point]

8. There is no geotechnical and geological information available for this area [0 point] The level of confidence considering all that points is 7 over 10.

The degree of confidence 7/10 corresponds to a valid result. The value of the velocity of the 1^st twenty is considered as a confident value.

5.2.3. Analyse de données de bruit de fond en réseau enregistrées