There was a gas explosion early today at a vacant home on Richmond Street in New Bedford.  Now police are looking for a man seen in the area shortly before the explosion.  They think he may have stolen copper piping from the house, which then triggered the explosion.   The man is described as white, about 5-6 to 5-8, in his late 30's or early 40's, and wearing a dark jacket, dark jeans and a back pack.  Witnesses said he was riding a bike that was very old and beat-up.   Police are asking for anyone with information to call them at 508-991-6300, extension 137.  No one was hurt in the explosion.