  Helioseismic waves observable at the solar surface can be used to probe the
properties of the Sun's interior. By measuring helioseismic travel times
between different location on the surface, flows and other interior properties
can be inferred using so-called sensitivity kernels which relate the amount of
travel-time shift with variations in interior proporties. In particular,
sensitivity kernels for flows have been developed in the past, using either ray
or Born approximation, and have been used to infer solar interior flows such as
the meridional circulation which is of particular interest for understanding
the structure and dynamics of the Sun. Here we introduce a new method for
deriving three-dimensional sensitivity kernels for large-scale horizontal flows
in the solar interior. We perform global-Sun wave-propagation simulations
through 784 small flow perturbations placed individually in the interior of a
simulated Sun, and measure the shifts in helioseismic travel times caused by
these perturbations. Each measurement corresponds to a linear equation
connecting the flow perturbation velocities and the sensitivity kernels. By
solving the resulting large set of coupled linear equations, we derive
three-dimensional sensitivity kernels for horizontal flows which have a
longitudinal component (parallel to the wave's travel direction) and a
transverse component (perpendicular to the wave's travel direction). The
kernels exhibit a "banana" shape, similar to kernels derived using Born
approximation methods, and show that transverse components are not negligible
in inversions for interior flows.
