Aquifers are easy once you get below them. Find the first cavern, build a drain, and dig up into the aquifer however you want.
This, IMHO, is the key to generalized aquifer penetration.
Everybody wants to end up with nice wide passages through the aquifer layers to avoid traffic bottlenecks. I like to separate that problem from the more pressing chore of just getting any passage at all through the aquifer. Once a drain (to cavern or through map edge) can be established, it's simple to open arbitrarily large spaces in aquifer layers. Then I can put vertical access wherever I want, as wide as I want, without any aquifer complications.
All aquifer layers (true aquifer, not the damp stone below) but the bottom can be drained handily into the aquifer layer below by digging down (or up+down) stairs into the upper layer over up (or up+down) stairs into the lower layer. When there's another aquifer layer below, you need only one pumping operation from the upper layer to enable access to dig the first down-over-up stairs combination. The pump can then be dismantled while the rest of the upper layer is dug out with down-over-up stairs, as widely as you require. Small areas of the upper layer can be simply mined out to leave flat floors (leaving pristine aquifer tiles beneath), to simplify pump placement to start working the next layer down... and securing an initial stairwell through the bottom aquifer layer.
Digging the down stair from the layer above reveals the composition of the tile below, and probably indirectly whether that layer is aquifer or merely damp from aquifer proximity. If it's a permeable soil or rock tile that can support an aquifer, the next layer is also aquifer; if it's clay or some non-permeable stone, it's (probably) not (I had one multi-biome embark with puddingstone that carried the aquifer way, way down, which I abandoned without complete understanding).
So...you need only one pump (moved layer to layer) and zero wall/floor/stair construction+deconstructions to get down to the bottom aquifer layer, and recognize it as such by the first up+down stair dug in it. That bottom aquifer layer can swallow all the water leaking from the boundary of all shallower layers, but until you establish a drain there's no place for it to go. Whatever you dig out in the bottom aquifer layer will fill with 7/7 water. Non-aquifer rock (or clay) below won't absorb water, so the up-over-down stairs combo won't dry out the bottom aquifer layer.
Fortunately, a single pump placement can drain two tiles enough to let you build isolating walls
if the directly or diagonally adjacent tiles aren't full of water. This lets you punch a single (1x1) up+down stair through the last layer, with a little care not to dig too close to that point with the upstairs draining the layer above.
To clear an arbitrarily large area (at least 5x5 [7x7 counting boundary walls]) area of an aquifer layer
that is NOT the bottom aquifer layer, start with an up+down stair dug from the (now) dry layer above. This will reveal the composition of the layer below, and whether it's another aquifer layer (if not, skip this part and jump to ). Dig a ramp in the third tile away for intake of a pump to discharge into your stairwell. While pumping, dig down-over-up stair combinations in the upper and lower aquifer layers next to the ramp:
X up+down stair
^ up ramp
d down or up+down stair (up or up+down stair in layer below)
. original tile, unmined
Aquifer layer *overlying*another*aquifer*layer*
.......
.......
.......
...X...
.......
.......
..d^d..
Once you have a couple down-over-up stairs dug, you can stop and dismantle the pump and extend the down-over-up stairs as far as you like; include stairs to the layer above for access, and then dig out the ramp with down-over-up stairs to clear out the aquifer tile below. Don't designate too large an area at at a time; if too many down stairs get dug too far away from up stairs below, water flowing to distant up stairs will keep your miners from digging closer up stairs drains. At best, job cancellations will greatly impede progress and designation calculations can leave unmined aquifer tiles below to muck up final drainage; you might even put yourself in an unrecoverable situation. Before moving on to the next layer, check to be sure every intended aquifer tile actually got mined away; a single straggler left by a job cancellation is easy to fix now, but can bite you hard later.
With water falling from upper aquifer layers into lower aquifer layers, there's no hurry to construct sealing walls. Yeah, the falling water might impose a slight FPS burden, but in early days with low population and little junk there's FPS to spare. I defer sealing until after everything is drained, busy work for dorfs with nothing better to do.
If the next aquifer layer might be the bottom of the aquifer, don't dig anything too close to where you intend to put the final piercing up+down stair. That tile and the tiles where you'll put its sealing walls needs unmined neighbors for protection from all the water coming from the rest of your diggings; mine the penultimate layer in this pattern:
X aquifer-penetrating up+down stair location
^ up ramp
d down or up+down stair (up or up+down stair in layer below)
_ any mined, walkable (floor or stair) *NOT* mined below
Aquifer layer *overlying*bottom*aquifer*layer*
ddddddd
dd___dd
d_____d
d__X__d
d_____d
dd___dd
ddddddd
Dig the outer down-over-up drains before the undrained interior area, to minimize cancellation delays from water produced by the last interior aquifer tiles.
Channel out the tile and set the pump to drain the final penetrating stair location. While pumping:
THREE times: Dig ONE up ramp orthogonally adjacent to stair location, and construct a sealing wall there
Dig up ramp at the fourth orthogonally adjacent location. Construct the central up+down stair first, then the fourth sealing wall.
There's no need to construct walls in the diagonally adjacent tiles (aquifers bleed only orthogonally), but DO NOT MINE THOSE TILES until the penetrating stairwell connects to the drain system. If you want to put walls there (belt and suspenders?), you'll need to leave more tiles unmined (or allow for more pump relocations) to avoid bulk drainage spoiling the construction.
Construct down (or up+down) stairs in the layer above, and dismantle the pump. You can now dig up+down stairs to get below the aquifer (and the damp layer just under it).
Dig stairs straight down to the depth of your drain channel layer, at least three levels (preferably more) below the bottom aquifer layer. At least at drain channel depth and at the second layer below the aquifer, mine laterally to the boundary of the aquifer area you want drained. Dig up+down stair drains around the periphery, from the second layer below the aquifer to your drain channel level. Dig your drain channel to the caverns (or map edge; DON'T FORGET TO SMOOTH AND FORTIFY MAP EDGE TILES TO ENABLE DRAINAGE OFF MAP!).
To minimize job cancellations and FPS drag, up in the aquifer I find it best to dig out all but the outside two tiles of the area to be sealed (one buffer plus where the sealing wall will be constructed) and dig the peripheral drain under the sealing wall position.
First dig out the complete drain system below the damp sub-aquifer layer. Then dig up-stairs from the drain upwards, all around the periphery, one layer at a time, all the way to the top aquifer layer. Working up one layer at a time avoids job cancellations when an upper tile gets dug first and its water impedes access to dig the tile adjacent or below.
Once the peripheral drain reaches the top of the aquifer, dig out the one-tile buffer zone in each aquifer layer, again working layerwise but top-down through the bottom aquifer layer. Simple mining would probably work, with a bit more cancellations, but I stick with the proven down-over-up stairs method.
Work strictly top-down when walling off the periphery; a wall on a lower level can obstruct the drain enough to make sealing the level above, or even removing the errant wall construction, impossible (I think). Before building any walls on a level, double check to be very certain that every interior aquifer tile has been mined away; as walls increasingly inhibit drainage flow, even a single aquifer tile can flood a level. Don't forget to dig out the "buffer" tiles around the initial penetration stairwell when the bottom aquifer layer has drained enough; some water will flow into the initial stairwell, but you did connect its bottom to your drain, right?
Once everything's drained away, your miners can remove up stairs from the bottom aquifer layer to leave you flat floors. Alas, upper layers will be riddled with down stairs until you construct floors. I haven't yet had the need, but you can probably dig out small areas to leave floors suitable for clay or sand collection (not possible from constructed floors nor down stairs...not sure about up+down stairs).
This procedure takes about a game year to clear a 30x60 (or so) region through a two or three layer aquifer (longer to finish sealing walls, but there's no real rush if the drain's out of the way); first breakthrough takes only a few weeks. A nice benefit is safe access to the large magnetite cluster and platinum that always seems to lie in the damp stone right at my fort. Digging the drains gives a bit of stone for early buildings and mechanisms. I can put stairwells wherever I want through the drained area without hassle. All that soil mining trains legendary miners, ready to be deadly killers in my first elite infantry squad.
Maybe I'm doing it wrong, but a single double-slit operation takes several months per penetration and still focuses traffic.
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Topic: !!Science!! of Aquifer Piercing: Leo's 5x5 Procedure (Read 13267 times)