In order to study the sub-surface structure variations associated with the formation and evolution of major active regions producing large, long-lived sunspot groups and intense flare activity, we analyze the localized power spectra at the sites of selected active regions from well before their emergence through their disappearance, comparing the helioseismic data with those for quiet regions of the same size and at the same latitude during similar time ranges. Because of the need for continuous data through disc passages of the selected regions over several Carrington rotations, we have analyzed GONG Doppler data, for which nearly continuous observations are always available, whereas continuous MDI data are usually limited to two rotations or less. The studied active regions were selected from among those that attained the greatest sunspot group area during the years for which there is reasonably complete GONG+ data coverage, from the middle of 2001 through 2005
