If I recall correctly, according to another thread a while back, when a geo survey team actually scores a 'hit' on the check where they actually do something, the game selects a mineral (equal chances except for Duranium, which is twice as likely), adds a random(?) amount, and then re-rolls the accessibility as if it was generating the stuff for the first time. If it's higher than the current accessibility, it goes up, and if it's lower it remains the same. I'm not sure how many additional tons are found is determined, but I most often seem to see an additional 100k or so.
It's also worth noting that this means they can find minerals that weren't previously on the body - so say if Mars had Duranium, Sorium, Gallicite, they could find 100k tons of Corundium. This includes bodies that were surveyed as barren, as well, so if there's a convenient planet with no minerals you can drop a geo team and see if they get lucky. They'll keep attempting to make finds until they make a certain roll that makes them decide there's nothing left (which can be after one time if you're unlucky).
The 'magic number' for maximizing success rates is 140 team skill, if I recall correctly; anything less makes it more likely they'll declare the survey over and anything more just makes it occur more often. If I recall correctly, the gravity of a body also affects speed, so Venus will survey faster than Mars will survey faster than a random asteroid.
So the short version is, yes, they can do both, even on a body that your geological ships said had no minerals at all.
A note: This can take a lot of time, especially on small bodies. I've had a team on Venus for a couple of years and they've only made a couple of discoveries.
