In time-distance helioseismology, computed travel time is believed to be the shortest time taken by a wave packet to travel between two spatial locations on the surface of the Sun separated by the shortest distance. Typically, it is computed by cross-correlating oscillation signals at the two locations and identifying the position of the envelope peak of the cross-correlation function. When the oscillation signals are measured in the region where the waves are propagating, correlation

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... do not necessarily provide the shortest travel time. Instead, they are shown to give the total time for the wave packet to take one round-trip between the two boundaries of the cavity in which the waves are enclosed. High-frequency oscillations (above the chromospheric acoustic cutoff frequency of approximately 5 mHz) are believed to be reflected by the corona-chromosphere boundary, and their signals are measured in the region where they propagate. Travel time computed by correlation techniques indicates the time the wave packet takes to return to the observing plane the second time after it encounters both the upper and lower turning points. Correlation techniques do not directly provide the shortest travel time, which would be the time to return to the observing plane after an encounter with either the upper or lower turning points. Inversions of travel time at high frequencies should include the path of the wave packet through the chromosphere between the observing plane and the corona-chromosphere boundary where travel time can be significantly affected by the local thermal, magnetic, and flow properties of the chromosphere.