  Supergranules create a peak in the spatial spectrum of photospheric velocity
features. They have some properties of convection cells but their origin is
still being debated in the literature. The time-distance helioseismology
constitutes a method that is suitable for investigating the deep structure of
supergranules. Our aim is to construct the model of the flows in the average
supergranular cell using fully consistent time-distance inverse methodology. We
used the Multi-Channel Subtractive Optimally Localised Averaging inversion
method with regularisation of the cross-talk. We combined the difference and
the mean travel-time averaging geometries. We applied this methodology to
travel-time maps averaged over more than 10000 individual supergranular cells.
These cells were detected automatically in travel-time maps computed for 64
quiet days around the disc centre. The ensemble averaging method allows us to
significantly improve the signal-to-noise ratio and to obtain a clear picture
of the flows in the average supergranule. We found near-surface divergent
horizontal flows which quickly and monotonously weakened with depth; they
became particularly weak at the depth of about 7 Mm, where they even apparently
switched sign. To learn about the vertical component, we integrated the
continuity equation from the surface. The derived estimates of the vertical
flow depicted a sub-surface increase from about 5 m/s at the surface to about
35 m/s at the depth of about 3 Mm followed by a monotonous decrease to greater
depths. The vertical flow remained positive (an upflow) and became
indistinguishable from the background at the depth of about 15 Mm. We further
detected a systematic flow in the longitudinal direction. The course of this
systematic flow with depth agrees well with the model of the solar rotation in
the sub-surface layers.
