Practical Fotoboring (or surveying for real):In the summer of 1980 I started working as an assistant geologist with Consolidated Gold Fields at their Gairloch project in the North West of Scotland. I had just finished the practical work on my PhD at Aberdeen University and my supervisor, Dr. Clive Rice, was involved in research on the project. The Gairloch project was (and is) a stratiform copper-zinc-gold deposit in steeply dipping green schist of the Loch Maree Group. The Geologist-in-Charge was Elizabeth Jones, who had previously worked with (Exploration Ventures Ltd) EVL in the North-East of Scotland. The drilling company was Encore Drilling, owned by Richard Braithwaite. When I started working in June 1980 about 64 holes had been drilled and there was an active drilling programme to define the extent of the ore body. Having started with surface outcrop each stage of the drilling required deeper and deeper holes. Accurate borehole surveying was becoming essential to define the structure of the ore and surrounding rocks. Early surveys were carried out using a mechanical single shot instrument, this was not accurate and was affected by strong magnetism from quartz magnetite bands that ran parallel to the thin ore body. In addition, the ore itself was slightly magnetic as it contained greater or lesser quantities of Pyrrhotite. The structure was "modelled" using a Perspex table with the borehole and logged lithologies represented by painted wooden dowling. These assumed that the holes were almost straight (!!). Markedly curved holes were produced by cutting and gluing the sticks. This was, possibly, one of the earliest forms of interactive 3D modelling I had come across. Of course, plans and cross sections were also used to define structure and lithological relationships. However, with the deeper holes the inadequacy of the survey method and the modelling became increasingly apparent. Several holes which we believed should have hit the ore band, according to our model, hadn't.
At the beginning of 1981 Elizabeth Jones investigated available surveying instruments and found the
"recent" Atlas Copco RF-DDI (Reflex Fotobor - Direction and Dip Indicator. This was not affected by
magnetism and was available at a reasonable cost. Both Elizabeth Jones and myself were trained to operate the system. The actual surveying was straightforward (so long as you weren't distracted and miss noting a station). The tricky parts were:
Following the trial a number of the existing deep holes were re-opened and surveyed using the Fotobor.
Some of the Perspex model sticks were replaced by painted wires** and bent according to the new locational
data and the plans and sections were completely re-drawn. The result was a much clearer model of the structure of
the ore body and, based on the new interpretation, several of the existing holes were extended to either intersect ore
or to show conclusive pinch-outs. ** This "wire" technique was resurrected in the mid 1990's to model and demonstrate changes in Local Co-ordinates for the Maxibor borehole survey tool. The Fotobor was used in all drill holes up until the end of the project in 1984. It's use helped to re-interpret the structure of the project area. The large amount of data produced (1 measurement every 1.5 or 3metres - depending on configuration) allowed research by the author into the relationship between lithology, drill rod type and deviation that was proposed as a possible drill path prediction method. (REF) When the Gairloch project closed the Goldfields Fotobor was purchased by Encore Drilling to provide their clients with an extra (and, in my opinion, essential) service. Encore Drilling was well ahead of the game here being one of the first slimline drilling companies to offer a surveying service using a tool of the accuracy of the Fotobor.
The author left Goldfields in 1983 and set up his own company deeloping geological software (including borehole logging
software) in 1985. |