They have penetrating radar   Yes.  used to find density differences in any solid mass.  Absolutely.     That’s how they often find oil.  No. Right concept, but no.    Sorry, going to be a bit pedantic. Hope you can forgive.  GPR only gets you a couple hundred feet, best case. Except for a few exceptions, like the Ghawar field, most O&G deposits are FAR deeper.  30K+ feet in some cases.    From the surface, O&G guys mostly use seismic detection tools. It's like GPR in concept, but it uses low frequency acoustic waves vs high frequency radio waves.    Ref.