  The subsurface structure of an "average" supergranule is derived from
existing HMI pipeline time-distance data products and compared to the best
helioseismic flow model detailed in Duvall and Hanasoge (2013). We find that
significant differences exist between them. Unlike the shallow structure
predicted by the model, the average HMI supergranule is very extended in depth,
exhibiting horizontal outflow down to $7$--$10$~Mm, followed by a weak inflow
reaching a depth of $\sim20$~Mm below the photosphere. The maximal velocities
in the horizontal direction for the average supergranule are much smaller than
the model, and its near-surface flow field RMS value is about an order of
magnitude smaller than the often-quoted values of
$\sim250-350$~$\rm{m\,s^{-1}}$ for supergranulation. Much of the overall HMI
supergranule structure and its weak flow amplitudes can be explained by
examining the HMI pipeline averaging kernels for the near-surface inversions,
which are found to be very broad in depth, and nearly identical to one another
in terms of sensitivity along the $z$-direction. We also show that
forward-modeled travel times in the Born approximation using the model (derived
from a ray theory approach) are inconsistent with measured travel times for an
average supergranule at any distance. Our findings suggest systematic
inaccuracies in the typical techniques used to study supergranulation,
confirming some of the results in Duvall and Hanasoge (2013).
