A father and his 13-year-old son were badly burned in a fire in the basement of a Lawrence home where they were working Saturday afternoon, the Lawrence Fire Department said. Firefighters responded to a call for “an explosion with injuries’’ around 2:20 p.m. at 71 Woodland St. to find the boy and his father still conscious outside the residence severely injured, said Lawrence fire Deputy Chief Brian Murphy. Both victims were taken to Lawrence General Hospital, and then flown by medical helicopter to Massachusetts General Hospital for their wounds, said Murphy. Their conditions were unknown as of Saturday night. The blaze, which was contained to the basement, is believed to be accidental and did not involve illegal activity, said Murphy. The fire, as well as the cause, is under investigation.