Gravity Anomaly

During late 2009 I was involved with a diamond core drilling operation testing a gravity and magnetic anomaly under sediments of the Canning Basin on De Grey pastoral station east of Port Hedland in the Pilbara district of Western Australia. The target was a geophysical magnetic anomaly that was interpreted by the geophysicists as a buried volume of hematite iron mineralisation. The magnetic anomaly was also associated with a deeper gravity anomaly. The local stratigraphy comprised flat lying, Cretaceous sediments of the Canning Basin, about 60 metres thick, overlying folded band iron formation and sediments (BIF).

The first drill hole was inclined at a nominal -60 degree dip, downwards, but experience resulted in the start dip to be -56 degrees, (slightly shallower) to compensate for down-hole deflection while rock rolling through the soft cover to reach fresh rock. The hole is then cased with steel pipe (PQ casing) and diamond coring commences at the start of fresh rock.

Down-hole surveys are conducted at 30 metre intervals using a sophisticated device that measures hole dip, magnetic azimuth, xyz components of both the magnetic and gravity fields, and temperature. The survey instrument contains a gravimeter and magnetometer and operates via an electronic timer set by the driller. Down hole surveys are crucial in establishing down-hole deviation in order to work out where the drill hole ultimately finishes. It is a fact that bore holes diverge from the planned direction due to drilling technique and more importantly structural anisotropies in the rock being drilled. In general bore holes “spiral” downwards like a low angle shaped corkscrew. The deeper the hole bore, the greater the change in hole direction due to inherent flexibility in the drilling pipe. Good drillers, with appropriate technology and equipment, can perform directional drilling with great precision.

GWD001 was planned to test a magnetic target to a depth of 400 metres at a nominal dip of -60 degrees.

Notice the two anomalous positive dips in red. The driller and I replicated the survey three times, so the readings are accurate.

So what is at fault? The instrument and hence the data, or the theory?

Serial Time Date Depth Dip Azimuth Roll Mag_TF GTotal MTotal
32155202 15:08 27/10/2009 90 36 250.6 44 O/B 3365 74324
32155202 16:43 29/10/2009 120 36 324.9 44 O/B 3360 105496
32155202 13:54 31/10/2009 150 -56.5 315.3 98.9 O/B 1002 163683
32155202 14:07 1/11/2009 180 -56.4 3 110 O/B 1002 115735
32155202 16:05 2/11/2009 210 -57.2 17.1 -5.2 O/B 996 63214
32155202 3:03 4/11/2009 239 -57.2 302.6 -15.8 O/B 996 49221
32155202 21:52 5/11/2009 270 -57.2 317.1 -122.4 O/B 998 17694
32155202 14:09 6/11/2009 300 -57.4 282.1 -49.4 O/B 995 145132
32155202 20:47 8/11/2009 330 -57.1 325.9 -160.7 O/B 999 105545
32155202 23:00 9/11/2009 360 -57.1 232.1 -151.6 O/B 999 96795
32155202 14:32 10/11/2009 387 -56.9 285.4 46.8 O/B 999 116599

About Louis Hissink

Retired diamond exploration geologist. I spent my professional life looking for mineral deposits, found some, and also located a number of kimberlites in NSW and Western Australia. Exploration geology is the closest one can get to practicing the scientific method, mineral exploration always being concerned with finding anomalous geophysical or geochemical data, framing a model and explanation for the anomaly and then testing it with drilling or excavation. All scientific theories are ultimately false since they invariably involved explaining something with incomplete extant knowledge. Since no one is omniscient or knows everything, so too scientific theories which are solely limited to existing knowledge. Because the future always yields new data, scientific theories must change to be compatible with the new data. Thus a true scientist is never in love with any particular theory, always knowing that when the facts change, so too must he/she change their minds.
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2 Responses to Gravity Anomaly

  1. fabio says:

    I am glad to read your site again after weeks it was off. By the way, the little I read about plasma theory, it concedes – at least theoretically – to “neutralize” gravity, like sci-fi claim. Is this correct?

    Like

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