  As helioseismology matures and turns into a precision science, modeling
finite-frequency, geometric and systematical effects is becoming increasingly
important. Here we introduce a general formulation for treating perturbations
of arbitrary tensor rank in spherical geometry using fundamental ideas of
quantum mechanics and their extensions in geophysics. We include line-of-sight
projections and center-to-limb differences in line-formation heights in our
analysis. We demonstrate the technique by computing a travel-time sensitivity
kernel for sound-speed perturbations. The analysis produces the spherical
harmonic coefficients of the sensitivity kernels, which leads to better-posed
and computationally efficient inverse problems.
