One of life’s little joys is sorting out one’s digital file archives collected over the years and I chanced on a backup of a thumb drive I used when I was working in 2009 for Brumby Resources NL and supervising the drilling operations in the Pilbara and Gulf country near Borroloola in the Northern Territory.
The thumb drive contained some CSV text files labelled GWD01_etc and caused me a heads up. Heads up indeed for I archived all the downhole survey data of the hole that produced the gravity anomaly described at the Malaga Bay Blog of Tim Cullen.
I had missed the first measurement at 35 meters downhole, and its addition clarifies the situation, that there was no instrument malfunction.
Clearly the start of the hole and the first survey done while open hole drilling with a PQ tricone bit showed no unusual behaviour for the instrument. It was only when the drilling passed through the flat-lying Cretaceous to recent West Canning Basin sediments, that the survey instrument started to produce anomalous data.
Notice that the driller, Holger Hotzi of DrillWest, and I replicated the data with two separate surveys at 14.45 and 14.53 hours, as well as an undocumented test out of the hole with the instrument held vertically. This data emphatically causes the rejection of the idea that the anomalous data were due to instrument failure, and confirms that gravity is electrical in nature.
The measurement at 35 meters downhole is easily explained. Being in the middle of the water saturated artesian sediment, local g, little g, is isotropic and thus cancels out the vertical gravity vectors leaving G, big G, as the dominant force acting on the accelerometer.
Once past the artesian sediment but still within the local electric field of the contained EZ water, little g remains >>> G and points to a mass attracting from above. This was duplicated at 120 metres down hole, and it is only when the drilling reached 150 metres downhole that G >> g. This means that the local electric field reaches zero between 120 and 150 metres down hole.
(The magnetic data should be completely ignored as the hole was spudded into magnetic banded iron formation, BIF).
Gravity is thus electrical as described by Wal Thornhill.
It has to be emphasized that this data is quite rare for it is not normal practice to bore diamond core holes into deep magnetite targets under artesian basins. In addition we usually don’t survey vertical bore holes either, especially if these are shallow, say 150 metres depth. Usually these are drilled with rotary airblast or percussion techniques, and deflection is usually not a problem. (Deflection can be a problem in shallow RC holes as I discovered during the drilling of some mine service holes for the Gellatly Nickel mine at Kambalda). You could say that I was the right geologist at the right time with the right background to realise the significance of the survey results. Anyone else would have dismissed it as instrument error, as they did when I questioned them.
Update: And the next step is to explain why plumb-bobs are deflected by the presence of a mountain range, the Isostasy problem.
Update: The survey measurement at 60 metres does not exist because the metal PQ casing was bedded in at 60 meters depth. All down hole survey measurements are performed in bare rock, as it were, and not in steel casing or pipe. The usual procedure is to retreat the pipe by 3 metres out of the whole, and then deploying the instrument into the uncased open hole in rock.