To me it sounds a lot like the so called shimmer effect credited to Daniel Lanois and Brian Eno, and it is actually not a synthesizer sound (in this case).
What I am hearing is the effect responding to the plucked guitar notes you can also hear in the soundscape.
The effect is used (although more sparingly) by The Edge on a number of U2 songs. Most notably on the intro to "City of blinding lights". And it can also be heard on many Daniel Lanois records.
The effect itself is basically a combination of a delay chain that has a 1 octave pitch shift per delay slap as well as a long tailed reverb effect to smooth things out. Depending on the decay and feedback settings on these effects, you can achieve many different sounds.
Depending on your DAW environment there are different ways of achieving this. I am a Reaper user, and the free patching there makes it real easy, but I am sure it can be done in every DAW.
The basic chain would be either:
having a feedback loop between delay and pitchshift (be careful with feedback settings on the delay)
__________
↓ ↑
input → delay → pitchshift → reverb
just building a chain of multiple delays and shifters (usually 4-5 iterations is enough). If you can link the delay time and feedback controls on all delays, it makes it a lot easier, but you can also manually adjust them to be the same (or maybe try having them different if the original effects doesn't do it for you):
input → DL → PS → DL → PS →DL → PS ... → reverb
There is no rule that says you absolutely HAVE to input a guitar - you can use synths (or your voice) just as well. I'd imagine sound with shorter decay would work best, but there could be other routes to explore.
There is aa nice gathering of information at the link above. It is connected to a plug-in they have built to recreate it, but as described above, you can easily do it yourself as well.
http://valhalladsp.wordpress.com/2010/05/11/enolanois-shimmer-sound-how-it-is-made/
