  We study intensity variations, as measured by the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory (SDO), in a solar
coronal arcade using a newly developed analysis procedure that employs
spatio-temporal auto-correlations. We test our new procedure by studying
large-amplitude oscillations excited by nearby flaring activity within a
complex arcade and detect a dominant periodicity of 12.5 minutes. We compute
this period in two ways: from the traditional time-distance fitting method and
using our new auto-correlation procedure. The two analyses yield consistent
results. The auto-correlation procedure is then implemented on time series for
which the traditional method would fail due to the complexity of overlapping
loops and a poor contrast between the loops and the background. Using this new
procedure, we discover the presence of small-amplitude oscillations within the
same arcade with 8-minute and 10-minute periods prior and subsequent to the
large-amplitude oscillations, respectively. Consequently, we identify these as
"decayless" oscillations that have only been previously observed in non-flaring
loop systems.
