A geologist would definitely be a nice idea.
Maybe it could be a profesion like the book-keeper for which you select a dwarf with the necessary talent and who needs to work in his own office to do his research.
The things he could tell you should depend on
1. the amount of map space you already uncovered (via exploratory tunnels) for the z-level he researches
2. The amount or work he puts into the z-level
3. his talent in geology
What he can tell you would be (from lower to higher accuracy):
1. What types of stone can be found within this z-level
2. What types of ore can be found within this z-level
3. What types of gems can be found within the z-level
4. The exact percentage of the various stone types within the z-level (with ores/gems unspecified)
5. The exact percentage of stone and ore types within the z-level (with gems unspecified)
6. The exact percentages of all stones, ores and gems within the z-level
(of coruse every higher accuracy would include the informations of the lower accuracies as well, so if you, for example had an accuracy of 4. and therefore would know the exact percentages of stone types, you would as well know, what ore and gems you can find here, but not their exact percentages)
For each Accuracy you should also have a certain percentage of mapspace already uncoveredby tunnels else you wouldn´t be able to attain this accuracy no matter how long your geologist would do his research.
For example for accuracy 1 it would suffice to have already 1 % of the z-lvel uncovered, while for accuracy 6 you might need 30%
[ April 04, 2008: Message edited by: Proteus ]