Getting the initial reading can take a variable amount of rustroot, and it's most likely to be a large number. But once you get a sniff of metal, you can pinpoint it's position with just 3 uses of extract, and then initiate the construction with one more use of extract. Basically, you triangulate the mine's position. If you don't know how, I wrote a short tutorial using a simple but not very elegant method, it's in this thread.
You can also use this method to pinpoint the tile with best clay or soil in an area where these resources are above 10 quality.