Airborne magnetometry in the years immediately following the Second World War was dominated by modified variants of the fluxgate magnetometers developed for detecting submarines, but research had already begun that would lead to a completely different way of measuring magnetic fields. Proton precession magnetometers, based on the magnetic properties of the isolated protons that constitute the nuclei of hydrogen atoms, had the advantage, from an airborne survey point of view, of measuring total field regardless of sensor orientation, making feasible the installation of the sensors in ‘birds’ towed behind and below the aircraft and well away from the varying magnetic fields associated with it.
The first airborne surveys to use a proton magnetometer in Australia were made in the BMR’s Cessna 180, VH-GEO, in 1963, in central NSW. Following that ‘proof of concept’, the aircraft was taken to Mt Isa in western Queensland, with Bevan Dockery as party chief, and in early 1964 he also took the aircraft to Kalgoorlie. After the work there had been completed, the aircraft was transferred to western Tasmania to fly survey over the Renison Bell tin field. It was my first stint as party chief, but reading through my report, and viewing it as now a historical document, I realised there was much about the survey that was too routine at the time to even mention but which would now be incomprehensible to a modern reader.
Renison posed some challenges not encountered in the previous surveys. The pyrrhotite-rich ores that were beginning to be exploited there, and the abundant mafic and ultramafic rocks that were also present, produced anomalies of hundreds or even thousands of nanotesla, much larger than any encountered in earlier surveys, and although the Mt Isa area is not flat, its topography is nowhere as extreme as that typical of western Tasmania.
Here, then, is a memoir of how things were done back in the days when paper records were the only option, navigation was visual and computer processing of digitally recorded data was an unthinkable dream.
Colebrook Hill. Photo P. Brown, 2019. The steep slopes challenged the pilot, and the dense forest cover challenged the navigator.
The aircraft
VH-GEO was a single engine Cessna 180 with a fixed ‘tail-dragger’ undercarriage, two seats forward, and space for baggage or another two seats behind. The high wing allowed the pilot and navigator/operator excellent sight of the ground. In survey mode the left-hand and right-hand seats were occupied by, respectively, the pilot and the operator, and the instruments, recorders and ancillary equipment were located behind the seats.
Navigation
The night before each flight, the operator would prepare the photo-mosaics of the area to be flown by drawing on them the proposed flight lines, using a chinagraph crayon. Because of the low level at which the survey was carried out and the generally short lines, and in contrast to the higher level, much longer lines of the DC-3 surveys, the operator’s navigation function was reduced to guiding the pilot to the start of the line while pointing in the right direction. From that point onwards, the aircraft would be flown on a constant heading until the pilot was informed he had reached the end of the line. In front of the operator was a ‘fiducial’ counter, counting seconds, and the counts at the starts and ends of the line would be noted by the operator in a flight log.
A special feature of the Renison survey was that the major topographic features in the area were defined by slopes that rose more steeply than the aircraft could climb. The solution was to fly all lines in the same direction, i.e. downhill. This had ‘knock on’ consequences for data processing.
Flight-path recovery.
During the flight, images were recorded at 4-second intervals on 35mm strip film by a modified Vinten camera with a 186 ° wide-angle lens. The images produced were circular and were, of course, highly distorted away from their centres, with the aircraft main wheels visible at the edge of each image. This form of imagery was necessary because of the low level at which the surveys were carried out; the more conventional images obtained by the camera in the DC3, in which the film tracked across the image plane at roughly the speed at which the image itself was moving, relied on the much greater separation from the ground to provide coverage over a sufficiently wide swathe either side of track with a more normal lens.
A second fiducial counter was incorporated in the camera, and was photographed with every image. It was one of the vital roles of the observer to ensure that this counter was synchronised with the counter on the instrument panel, since this was the only way of relating the photographic images, and hence the aircraft position, to the data being collected.
The installation placed physical demands on the observer, since the camera had to be lowered into position in a mount above a hole in the floor of the baggage compartment once the aircraft was airborne and had to be removed before landing, to eliminate the possibility of the (very expensive) lens being damaged by stones thrown up from the gravel airstrip. This required him to release his seat belt, kneel on his seat and lean over its back while maneuvering the heavy camera into and out of position. This was by no means a simple operation in the high-turbulence conditions often encountered in the survey area.
Magnetometer sensor
Placing and removing the camera was not the only operation that required the observer to kneel on his seat and face backwards. The magnetometer sensor consisted of a copper coil wound around a bottle of hydrocarbon fluid about six inches in diameter, the whole being enclosed in an aerodynamic ‘bird’ that sat in a cradle under the fuselage. A reading cycle lasted half a second; at the start of which a strong electric ‘polarising’ current was passed to the coil down a cable linking it to the onboard electronics. After about a quarter of a second, current flow was terminated, there was a short pause for cut-off transients to dissipate, and the coil then acted as an aerial to detect the weak signal generated by the reorientating protons in the bottle.
Clearly, a fixed-wing aircraft could not either take-off or land with the cable extended. In the baggage area it wound on to a manual winch, which the observer used to deploy, and, at the end of the survey, retrieve, the bird. The bird could be seen by him in a strategically-placed mirror and some skill was needed to bring it safely back to the cradle, because the cable oscillated more and more violently as it shortened. Timing was all.
Magnetometer
The frequency of the sensor signal was converted in the electronics of the MNS1 nuclear magnetometer into a reading of total absolute magnetic field, and the result was displayed on a Moseley six-inch rectilinear chart running at four inches or eight inches per minute. Fiducial pulses produced markers every ten seconds and it was part of the job of the observer not only to ensure that the ink was flowing smoothly to the pens, so that a record was actually being obtained, but also to annotate the marks manually with the fiducial number at least once on every line, and to check as often as possible that the control panel and camera fiducial counts were in step.
If the two counters lost synchronization due to some random pulse within the system, the survey could continue if the chart was suitably annotated, but although constant monitoring could ensure that errors of one or more seconds did not occur, this was not enough to ensure that there was no displacement between recorded magnetic value and plotted position. In part, such ‘parallax errors were inherent in the system, because in survey mode the ‘bird’ was several tens of metres behind the camera, but there were also electronic delays which had to be determined by direct observation. In normal surveys with alternate lines flown in opposite directions these would be apparent in the development of ‘herringbone patterns’ in the final contours, but in the unidirectional Renison survey the measurement of parallax error had to be made at the beginning and end of each survey flight, by flying in both directions a special baseline over ground replete with identifiable features and including a sharp magnetic anomaly. The results from this line would also be used to correct for diurnal changes, assumed linear, in magnetic field.
The chart was normally set to a full-scale deflection of 100 nT, and the pen would ‘back off’ across the chart if it reached the chart edge, but a problem introduced by the discontinuous nature of the record was that if there were changes of the order of 100 nT between consecutive readings, it was impossible to know how many hundreds of nT were involved. This was only a minor problem in the western (Renison Bell Hill-Dreadnought Hill) part of the main survey area, where the anomaly range was about 1000 nT, but for the eastern, Colebrook, part, where the range was several thousands of nT, the only solution, given that the aircraft could carry only one recorder, was to re-fly some of the affected lines at 1000 nT full-scale deflection.
Altimeter
Ground separation was monitored by an AN/APN-1 radio altimeter with outputs to a dial on the pilot’s instrument panel, and also to a limit light system that notified him of significant deviations from nominal survey height. In principle the aircraft was to be flown at a constant 280 ft above the ground surface, which would supposedly place the sensor in the towed bird at 250 ft above ground, but this was never completely possible, and in the Renison survey wide variations in ground separation were inevitable.
A continuous record of ground separation was preserved on a TIC (Texas Instruments?) six-inch curvilinear pen and ink chart recorder. It was yet another of the operator’s tasks to ensure that the ink was flowing freely to provide a clear and continuous record, and to annotate this chart also with fiducial numbers. However, no attempt was made to use the information to correct the magnetic readings.
Post-flight
Having successfully de-installed the camera and restored the ‘bird’ to its cradle in flight, all the observer then had to do was remove the paper charts from the chart recorder and the film from the camera once the aircraft had landed and then, usually, develop the film and pass it and the mosaics over to the draughtsman and geophysicist, who would plot the true positions of the flight lines and make the magnetic maps. He also had to write a flight log, which the most frequent occupant of the observer’s role, the electronics technician, would often enliven with comments such as ‘this is suicidal. I do not go again’. It thus became one of the more difficult jobs required of the party chief to persuade him to ‘go again’, or else go himself.
There was, in fact, no part in the entire operation that the party chief would not, on occasion, be expected to undertake himself, apart from actually flying the aircraft. From a long-term professional perspective, that provided an invaluable store of experience in the nuts and bolts of airborne geophysical operations that remained a valuable resource even when acting as a consultant up to fifty years later.