  The inference of internal properties of the Sun from surface measurements of
wave travel times is the goal of time-distance helioseismology. A critical step
toward the accurate interpretation of travel-time shifts is the computation of
sensitivity functions linking seismic measurements to internal structure. Here
we calculate finite-frequency sensitivity kernels in spherical geometry for
two-point travel-time measurements. We numerically build Green's function by
solving for it at each frequency and spherical-harmonic degree and summing over
all these pieces. These computations are performed in parallel
("embarrassingly"), thereby achieving significant speedup in wall-clock time.
Kernels are calculated by invoking the first-order Born approximation
connecting deviations in the wavefield to perturbations in the operator.
Validated flow kernels are shown to produce travel times within 0.47% of the
true value for uniform flows up to 750 m/s. We find that travel-time can be
obtained with errors of 1 millisecond or less for flows having magnitudes
similar to meridional circulation. Alongside flows, we also compute and
validate sensitivity kernel for sound-speed perturbations. These accurate
sensitivity kernels might improve the current inferences of sub-surface flows
significantly.
