This chapter describes the processing using the tau-p (or 'slant stack')
method and the interpretation of six expanding spread profiles (R1
to R6) recorded in 1984 on the Aquitaine shelf of the Bay of Biscay
during the ECORS Bay of Biscay Survey. Slant stacking transforms
the initial t-x time/distance sections in tau-p sections (tau is
the intercept of the raypath and p the raypath parameter). An inversion
by linear programming enabled us to determine velocity models as
function of depth. This method, which does not assume homogeneous
layers, offers the advantage of identifying velocity gradients, and
serves to express the uncertainties in the interpretation of the
data in the final model. The result of inversion is a set of models
within a range of velocities and depths, of which the width depends
on the quality of the data. The models obtained reveal a crust about
35 km thick under the Armorican and the Cantabrian shelves, whereas
the Moho rises spectacularly below the Parentis basin, by about 15
km. Our models generally show low velocity gradients (about 0.02
s-1) in the crust. The upper crust is characterized by velocity ranging
between 52 to 59 and 6.0 to 6.2 km/s beneath the Armorican shelf,
and between 5.9 and 6.5 km/s beneath the Cantabrian shelf. They assume
values of 6.5 km/s or more in the lower crust, In this layer, under
the Parentis basin and the Landes high, some models show high velocity
gradients (about 0.3 s 1). They might be linked to the uplift of
the Moho under these structures. The tau-p method helps to identify
sedimentary layers accurately. They are characterized by high velocity
gradients. The Tertiary has a high gradient of about 1s-1 in the
Parentis basin, and the velocities range from 2 km/s to up to 5 km/s.
In the Mesozoic series, the velocitiy goes from 5.0 to 5.7 km/s (with
a gradient of about 0.2 to 0.3 s-1) at a depth of about 12 km (profile
R3) and 15 km (profile R4). Under the Landes high, whereas reflection
shooting fails to detect any individual structure, our results indicate
the presence of velocities between 4.7 and 5.7 km/s to a depth of
about 8 km. These velocities, which are too low for metamorphic basement,
can be explained either by tectonic stacking or, as we believe, by
the presence of a Palaeozoic basin about 7 km thick.