Artist and electronics hound Jon Cohrs made this tool for urban prospecting. He writes:
The project, called the Urban Prospector, is basically a modified metal detector outfitted with a combustible gas sensor that can be built for under 100 dollars. By scanning the surface of your neighborhood, you will be able to determine pockets of oil and other toxins. Until recently, oil prospecting has been a field left to the professionals because it requires sophisticated tools for detection. But in much the same way gold prospecting empowered people to find small nuggets of profit, urban prospectors now have the potential to find small nuggets of oil near oil spills, abandoned gas stations, and industrial sites. Given the current high cost of oil, these urban spills or potential gold mines are waiting to be tapped.
Check out his Instructable on converting an old metal detector to sense combustible gas.