  A time-distance helioseismic technique, similar to the one used by Ilonidis
et al (2011), is applied to two independent numerical models of subsurface
sound-speed perturbations to determine the spatial resolution and accuracy of
phase travel time shift measurements. The technique is also used to examine
pre-emergence signatures of several active regions observed by the Michelson
Doppler Imager (MDI) and the Helioseismic Magnetic Imager (HMI). In the context
of similar measurements of quiet sun regions, three of the five studied active
regions show strong phase travel time shifts several hours prior to emergence.
These results form the basis of a discussion of noise in the derived phase
travel time maps and possible criteria to distinguish between true and false
positive detection of emerging flux.
