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?