Dave,
The 'seismic lance' on VR-BGB is actually a magnetic anomaly detector boom. The boom was hexagonal in cross section AFAIR and the box about one third back was the MAD box. There were 4 of the aircraft in Iran for IHAC on contract to Prakla Seismos doing a uranium for Uriran. They also had a groundwave detector in the tail boom which bulged out of the side of the baggage compartment and a skywave detector fitted behind the pilots' seats. Much of the forward part of the cabin was taken up by the electronic bays. There was also a camera pointing down from the port chin window taking a photo every 30 seconds to cross check navigation. The crew was 3: pilot, navigator and electronics operator. The aircraft were fitted with Doppler to accurately measure drift and with Decca TANS. There was no Decca in the mountains in Iran, but the TANS took in information from the Doppler and the navigator kept a constant update of it from his map reading so the pilot normally just had to fly the drift indicator and keep his needle in the middle to stay on tracks. Tracks were typically 150 km long, maintaining 60-80 knots groundspeed (I forget now whether the survey height was 200 or 400 feet agl). At the end of the track we moved 500 metres to one side and then flew the reverse until we had covered all of the boxes the geologists wanted surveyed. Fun and interesting work which came to an end during the revolution when Prakla Seismos were one of the first companies to pull out (which was how I ended up in Nigeria).