Method and apparatus for actuating a differential pressure firing head

A method of actuating a differential pressure firing head to actuate a perforating gun at a downhole location in a subterranean wellbore adjacent a formation, the method comprising:

• positioning the perforating gun and the differential pressure firing head at a downhole location on a tubing string;
• communicating an applied fluid pressure to a wellbore annulus, a first chamber, and a sealed second fluid chamber, wherein the wellbore annulus is defined between the perforating gun and firing head and the wellbore, wherein the first chamber communicates the applied fluid pressure to a low-pressure side of the firing head, and wherein the sealed second fluid chamber communicates the applied fluid pressure to a high-pressure side of the firing head;
• trapping the applied fluid pressure within the second fluid chamber;
• and
• decreasing the applied pressure in the first chamber and the wellbore annulus, thereby creating a differential pressure across the firing head sufficient to actuate the firing head.