Time-distance helioseismology is one of local helioseismology techniques to investigate solar interior structures and flow fields. Recent inversions, based on sensitivity kernels from ray approximation, Fresnel-zone approximation, and Born approximation, revealed shallow slow sound-speed region immediately below the sunspot's surface, and faster sound-speed below a depth of approximately 5 Mm. Converging and downward flows were found in the shallow depth beneath sunspots, while divergent flows were found below the depth of about 5 Mm. However, some observational effects, including inclined magnetic field effect, showerglass effect, masking effect, and travel time variations with acoustic frequency, may challenge the accuracy of the interpretation of time-distance measurements.
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