[Users] unsupervised NN - why should I only use phase-independent attributes?

[Colin Hawke]

I have a set of 3D horizons picked as part of a sequence stratigraphic study 
and these nicely define stratigraphic "intervals" for volume based seismic 
facies analysis. I notice that it is recommended to use phase-independent 
attributes as input to the unsupervised NN. What is the reasoning behind 
this? It seems intuitive to me that attributes that contain polarity 
information should help with the clustering of soft vs hard lithology and 
avo attributes might help for soft shale vs soft sand vs coal for example. 
Is it simply that we want attributes that define "layers" rather than 
"interfaces"? I know there is a good reason but am unsure what it is.

Thanks - Colin.