The bulge in the tail boom of the Prakla Seismos seismic survey 212s in Iran was not a counterweight to the MAD boom; it was a ground wave radiation detector. We had a sky wave radiation detector immediately behind the centre console, just forward of the electronics operator’s station and the ground wave detector was in a box which bulged out of the starboard side of the baggage bay. They both had to be calibrated to check background radiation at the start of every survey flight.