For the average treasure hunter this is probably too expensive and too complicated. There is a lot of research in using high frequency GPR to locate landmines, but the hardware and software is very expensive and the results in cluttered environments aren't great. There were several sessions on high frequency GPR and landmines at the IEEE conference in Orlando this year. A lot of PhDs with fancy hardware and statistical analysis, but not great results in cluttered environments. There is a pretty good helicopter mounted system, REMIDS I think, that can locate about 90% of surface mines, but that's probably out of the price range of most treasure hunters.