Modern airborne gravity relies largely on measurements of gravity gradients, defined as gravity changes over very short distances, rather than gravity field. Given how very small those changes in field will inevitably be, it is clear that measuring them is bound to be very, very difficult. Nevertheless, it was being done in Australia in the late 1920s with some success. For quite a number of years, I had custody of a torsion balance that had been one of the instruments used to do it. And, partly but not entirely because of my having at least handled it, its users have my unstinted admiration.
How I came by the instrument is soon told. At some point in the mid to late 1980s, I was contacted by a colleague at the Royal School of Mines in Imperial College to say that there was there a torsion balance that was destined for the rubbish dump unless a home could be found for it. Without quite knowing what I was letting myself in for, I said that, of course I would take it in. A few days later a very large and very heavy wooden box arrived. It was more than five feet long and about two feet square, and it took two of our stronger technicians to manoeuvre it into position in an obscure corner of the geophysics lab. And there it sat until I retired. I made some attempts to investigate it but was discouraged by the discovery that the essential tripod stand on which it would have been mounted was missing. Presumably that had made it to the dump.
I retired and left it where it was. Some time later it was moved to a store on the far side of London’s Euston Road, but that became a target for demolition when High Speed Rail required a swathe of destruction in order to fail to reach Euston station. Again I had a phone call, and this time I had to collect the machine myself. It took me two years to persuade the powers that be at the Science Museum that they ought to have it, and when they took it it was only because it had a story to tell. Although hard to prove, Its provenance was obvious. J McG Bruckshaw had been a senior member of the team in the 1928-1930 Imperial Geophysical Experimental Survey in Australia, and J McG Bruckshaw subsequently became Professor of Geophysics at the RSM. He must have been responsible for it being there.
Figures 103 and 104 from the IGES report. Left: Torsion balance schematic. The lower weight is at the lower end of the vertical tube on the right. The upper weight is in the tube on the left at the level of the movable platform, the tube itself being there to maintain balance The signal is generated by the (very) slight difference in the gravity fields acting on the two weights. Right. The enclosure for the torsion balance, essential to shield the system from air movements.
Perhaps for the Prof it had sentimental value, but it had clearly been a horrendous thing to use. That fact is well documented in the IGES report The Principles & Practice of Geophysical Prospecting, from which the quotations below are all taken. Even getting it halfway round the world must have been something of a challenge, but using it in conditions typical of the Australian bush even more so. For anyone brought up with the simplicities of gravity survey using gravity meters, the entire operation sounds a nightmare – and, as the IGES report recorded, with the emphasis on the night !
Observations taken at night are always more reliable than those made during the day and, in the experience of the I.G.E.S. and of McLintock and Phemister, the Oertling torsion balance, even with the extra protecting hut, is useless in the day time. (p.150)
The people who planned things back in London had, however, retained some shreds of humanity and recognised that : –
with a small party, on an extensive survey, it is very necessary from the point of view of the observer’s efficiency that he does not have to stay up for the greater part of the night making observations.(p.150)
The solution to that problem was to opt for entirely photographic recording, so that after the instrument had been set up for the night, the field crew (consisting of a party chief, an observer, a surveyor and two or three labourers) could get some sleep.
Did that not mean that there would be only one or perhaps two readings made in a night? Yes, it did, but that was inevitable anyway, because the observations themselves took several hours and :
The I.G.E.S. party found that the time taken for the complete move to, and setting up on a new site about 400 ft. away is normally 1-1½ hours for the torsion balance (including the preparation for photographic registration). (p.151)
Any sort of survey was, it was clear, was going to take a long time, even though
The transport of the Oertling torsion balance is very much facilitated by the method in which it is clamped to its metal cover and the whole carried on the trolley. (p.151)
Perhaps only one labourer would be needed for that, but the others were likely to have been kept very busy, because
The site for the station should be selected so that there are no marked irregularities in the ground, and no feature such as a large tree, ditch, etc. should lie within about 10 ft. of it. Then, for a radius of 3 ft., the station is cleared of scrub, grass, etc., and minor irregularities on the surface are smoothed out. Usually, but not always, it is necessary to drive pegs into the ground for the instrument to rest on.(p.151)
In the end the IGES classed the gravity work as a success, but that was, perhaps, due as much as anything to one other factor. They had another type of instrument as well.
For use in Australia and other thinly populated countries, small, readily portable instruments of the gradiometer type are to be preferred to the full-sized torsion balance. They are more readily transported to the site of work, are more rapidly moved from one site to the next, and measure the gradient in a shorter time than does the torsion balance. (p.150)
But that is yet another story.