  Recent observations of helioseismic holography revealed that magnetic fields
inclined to the line-of-sight direction could cause systematic variations in
measured acoustic phase shifts, and that the surface magnetic field may shift
the phases and impair the coherence of acoustic waves (known as "showerglass
effect"). We examine how these affect time-distance helioseismology
measurements. It is confirmed that the inclined magnetic field could cause
variations in acoustic travel times inside sunspot penumbra, however,
inversions of the measured times show that this effect only slightly shifts the
location of negative sound-speed variations near the solar surface, but does
not change the inverted deeper interior structures. Regarding to the
showerglass effect, we find that outgoing and incoming travel time
perturbations through sunspots are significantly smaller than those reported
from helioseismic holography. In addition, our second-skip cross-correlation
experiments demonstrate that inside sunspots, the half of the double-skip
travel times are very similar to the mean single-skip travel times. We conclude
that these surface magnetism effects do not cause considerable systematic
errors in time-distance helioseismology of active regions.
