interesting... i visited Israel a few years back, and the IDF is still clearing land mines from archaeological sites and tourist areas that were planted back in the 50's and 60's.
a mine finding robot isn't a new idea, but if you can get the thing to sniff for nitrates, or more complex explosive compounds, it may be able to detect non-ferrous mines. such sensor technology is not out of reach, as a lot of R&D is going into "chem labs on a chip", and some of these are already available. the trick would be to distinguish between nitrates naturally occurring in soil, and explosive compounds.