  A narrow rectilinear valley in the French Pyrenees, affected in the past by
damaging earthquakes, has been chosen as a test site for soil response
characterization. The main purpose of this initiative was to compare
experimental and numerical approaches. A temporary network of 10 stations has
been deployed along and across the valley during two years; parallel various
experiments have been conducted, in particular ambient noise recording, and
seismic profiles with active sources for structure determination at the 10
sites. Classical observables have been measured for site amplification
evaluation, such as spectral ratios of horizontal or vertical motions between
site and reference stations using direct S waves and S coda, and spectral
ratios between horizontal and vertical (H/V) motions at single stations using
noise and S-coda records. Vertical shear-velocity profiles at the stations have
first been obtained from a joint inversion of Rayleigh wave dispersion curves
and ellipticity. They have subsequently been used to model the H/V spectral
ratios of noise data from synthetic seismograms, the H/V ratio of S-coda waves
based on equipartition theory, and the 3D seismic response of the basin using
the spectral element method. General good agreement is found between
simulations and observations. The 3D simulation reveals that topography has a
much lower contribution to site effects than sedimentary filling, except at the
narrow ridge crests. We find clear evidence of a basin edge effect, with an
increase of the amplitude of ground motion at some distance from the edge
inside the basin and a decrease immediately at the slope foot.
