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Residential service gas lines are usually buried no more than a few feet deep and are either metal or plastic with a wire run along side so that they can be easily located with the right type of metal detector.
If you smell gas or suspect a leak, call the gas company. They have probes that can locate a leak by sampling or by listening ultrasonically. This is not a D.I.Y. project. That last thing you want to do (or might ever do) is to locate a leak by hitting a rock with a shovel and creating a spark.