  Quantitative description of the statistics of intensity fluctuations within
spectral line data cubes introduced in our earlier work is extended to the
absorbing media. A possibility of extracting 3D velocity and density statistics
from both integrated line intensity as well as from the individual channel maps
is analyzed. We find that absorption enables the velocity effects to be seen
even if the spectral line is integrated over frequencies. This regime that is
frequently employed in observations is characterized by a non-trivial relation
between the spectral index of velocities and the spectral index of intensity
fluctuations. For instance when density is dominated by fluctuations at large
scales, i.e. when correlations scale as r^{-\gamma}, \gamma<0, the intensity
fluctuations exhibit a universal spectrum of fluctuations ~K^{-3} over a range
of scales. When small scale fluctuations of density contain most of the energy,
i.e. when correlations scale as r^{-\gamma}, \gamma>0, the resulting spectrum
of the integrated lines depends on the scaling of the underlying density and
scales as K^{-3+\gamma}. We show that if we take the spectral line slices that
are sufficiently thin we recover our earlier results for thin slice data
without absorption. As the result we extend the Velocity Channel Analysis (VCA)
technique to optically thick lines enabling studies of turbulence in molecular
clouds. In addition, the developed mathematical machinery enables a
quantitative approach to solving other problems that involved statistical
description of turbulence within emitting and absorbing gas.
