Potential Oil Tank - How to Approach - North NJ

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I'm under contract for a house in a great area, but I'm worried about the potential for an underground oil tank. Seller inherited the house and they say they are not aware of any oil tanks underground. 
While I did order a sweep with GPR, I later realized there was a deck, 16x16 in the backyard, attached to the house. The most likely place where  a tank would be. The company reassured they would do as much as they can to find one, but that doesn't help me. I believe clearance is just at 20" and they said they may not be able to crawl under.


Not a big fan of the deck.  I'm torn between this:
1. Ask the seller to remove the deck so we do a proper scan and some credit to put it back if no storage tank is found. The seller agreed to remove ust if found.
2. Proceed with tank, if something suspicious found do nr 1. If nothing found, leave as is. If we can't find something, a future potential buyer is not going to either as long as the deck is there, right  ?


Which brings me to this:
Couldn't more people just build low clearance decks and use that to hide USTs ?

I appreciate your thoughts!