  I analyse the maps recording the travel-time shifts caused by averaged plasma
anomalies under an "average supergranule", constructed by means of statistical
averaging over 5582 individual supergranules with large divergence signals
detected in two months of HMI Dopplergrams. By utilising a three-dimensional
validated time-distance inversion code, I measure the peak vertical velocity of
117+/-2 m/s in depths around 1.2 Mm in the centre of the supergranule and
root-mean-square vertical velocity of 21 m/s over the area of the supergranule.
A discrepancy between this measurement and the measured surface vertical
velocity (a few m/s) can be explained by the existence of the large-amplitude
vertical flow under the surface of supergranules with large divergence signals,
recently suggested by Duvall & Hanasoge (2012).
