"You mentioned there was a witch there, Is there a chance we should face some of her kind there ? This might be a simple question but I'm not really sure what art would be needed to interact with the bog itself ? is is mostly herbam due to the surface being mostly an accumulation of plants or Terram as the plants gradually turn into soil ?
Other than that, planning indeed seems to be the proper course of action. I have to meet with Eule or whoever will be my praefectum before I make definite statement but from my travel to this tribunal I've learned the hard way that I really need to borrow a labtext for a spell to create clothes, this is a common spell among Bjornaer so I hope I'll find a copy in the library. The course of action is a bit clearer now that we're aware of the location and the purpose of the chapter House.
In my opinion we'll have to search for spells to ease the process of collecting the virtus. From what I hear, but maybe i'm mistaken, the eruptions seems random and unless we can predict them, we'll waste a third of our seasons patrolling the bog. So We'll probably need intelligo vim spells to locate the virtus and understand the creation of eruption itself. Understanding how the source can provide virtus that is at the same time auram and ignem would probably also require its own spells .
I might be able to develop a spell to imprison the gas inside water bubbles if we can predict where the eruption occur to ease the collecting.
Investigating the sources, known and to be discovered, will probably require to dive inside the water of the bog, I already have spells to breathe and see in water but not really to move around inside water, once again there might be interesting labtext in the library.
We'll probably need to secure decent summae of intelligo, vim, aquam and probabaly rego at least...
This is from the top of my head of course I might be mistaken as I'm only reasoning on a description of the site.