Maybe the alien ruins could be expanded? There would be a higher chance of finding them, but the rewards would be reduced. When your xenologist unit (or whatever form they will eventually take) finds the ruins and excavates them, they could discover something like an ancient defense network/fortifications, giving a small defense bonus to ground forces. Or perhaps the team gets attacked by local fauna, which would require them to be somewhat defended.
For geological surveys though, I think that the possibility of more minerals should be detected during the initial scan, and then require a dedicated team, with the extra cost of transporting the team there being a big pay off in terms of minerals. This could give it much more of an expeditionary feel to the mechanic, since instead of just sending teams to Mars, Venus, Europa, etc., your long range survey ships have found something far from your borders that is well worth the expense of exploring further.
However, both of these should probably be limited to large planetary bodies. Maybe instead for asteroids, there could be a chance where once the sensors finish the scan, the ship would scan the body again, with a chance to find extra minerals. Rather than sending an entirely separate expedition, the ship just does a double check and might find something it did not see on its first pass.