I have been experimenting with the world gen parameters for cavern shape: openness and passage density. Min and max of each parameter are the same, so that there is on;y one possible value. I use a 'Smaller Island' world with no water in the caverns (to make exploring them easy). Embark happens in a 2x2 region with a group of 7 miners (see later), and I dig down and explore the top 2 cavern layers.
I tried 5 different parameter settings (O = Openness, P = Passage Density, H = sum of height of top 2 layers of caverns in z levels):
a) O 50 P 50 H = 33
b) O 0 P 50 H = 80
c) O 100 P 50 H = 99
d) O 50 P 0 H = 77
e) O 50 P 100 H = 16
a) as expected is a fairly average set of caverns, with some passages, and some open areas.
It is difficult to tell the difference between settings a) and b), but c) is different, with very large caves in each quadrant of the image, mostly open, but some with passages.
d) is also noticeable with no passages (as expected), while e) has passages filling all the cavern space.
I think openness has something to do with the shape of the caverns, while passage density controls how full of passages they are. Both settings seem to influence the number of z levels, but I have only looked at one embark for each set of parameters, so I don't have enough statistics to tell how properly.
There were a few things I noticed relating to the lack of underground water:
0 water level in the caverns => no underground plants in the world => no underground plant products available at embark. This includes ALCOHOL!. I had to give one miner herbalist, and one miner brewing skill and collect some plants to make drink out of. Not that I ran any of the embarks for more than a season, so I might have managed without any drink.
The 2nd level of caverns in c) had some underground trees, despite having no water.
I can upload the 5 sets of caverns to the DFMA next Wednesday if people are interested.