By Dan Oancea

 

In 1666, Sir Isaac Newton, watching an apple fall from a tree, identified a previously unknown force that governs the laws of physics on Earth and beyond, namely gravity.

Some three hundred years later, during the Cold War years, people where not looking at the sky for falling apples but were looking up for missiles and satellites. It was then that the US Trident class atomic submarines desperately needed a new device that would enable them to "navigate through unchartered waters".

Bell Aerospace proposed the new 3D Full Tensor Gradient (3D-FTG), and promoted it as the “ultimate detection device”. Why detection? My guess is that the 3D-FTG was never intended for navigation, as it is sometimes noted, but was intended for the passive detection of gravity field changes. Such changes can indicate the presence of a silent killer submarine making its way through the 3D underwater world.

The end of the Cold War brought declassification of such information. The method was applied for commercial purposes. Bell Aerospace was swallowed by the giant Lockheed Martin, who became the patent holder and producer of the mighty device.

Some years before, based on research efforts and collaboration with Bell Aerospace, a proprietary technology had been developed by mining giant BHP Billiton: an airborne gravity gradiometer system called Falcon. BHP has 10.5 exclusive years for mineral exploration and 10 years exclusivity for hydrocarbons, beginning October 1999 from Lockheed Martin the manufacturer. It started to fly a Cessna Grand Caravan in 1997 and delivered the first results in 1999.

The Bell's FTG system was used during the same years as a marine surveying tool for acquiring data for oil exploration. In 2002-2003 the system was upgraded for airborne acquisition and it flies on the same type of aircraft.

Is the field large enough for both competitors? I think so, but for now have a look at both of their sites - Bell's and BHP's - and decide for yourself. Both sites contain a goodly amount of data and technical papers together with comparisons, specifications and case studies. I notice Bell's aggressive marketing, when they state that by using the Falcon system you end up with "Equity dilution & do not measure the Full Gravity Tensor".

That said, let us consider some other aspects of the problem. 

The benefits of using an airborne system vs. a ground system are obvious: it covers huge swaths of land in a short period allowing data acquisition over difficult terrains that are often out of the ground team's reach, thus generally reducing exploration times and costs.

The disadvantages are that the cost are sometimes too high for a junior's pocket, the method is not economical over small areas, you need a good weather for completion and you need not only a geophysical ground team - to make some data acquisition for comparison reasons and for assessing the noise level - but also a geological team, for providing adequate geoinformation.

The airborne gravimetric method also has its abilities and limitations. Both websites list much the same abilities such as the excellent detection of kimberlites, IOCG, VMS, buried paleochannels (prospective for diamonds, U) and the good detection of buried intrusive bodies - disseminated gold & porphyry-copper systems - and Ni/PGE deposits.

The next leg of my electronic journey brought up a really valuable paper, one in which a respectable third party assesses Falcon vs. Air-FTG in a head-on skydiving competition, where the parties were asked to survey the very same piece of land.

So, welcome to the ASEG-PESA Airborne Gravity 2004 Workshop's paper called "A comparison of the Falcon® and Air-FTG™ airborne gravity gradiometer systems at the Kokong Test Block, Botswana".

A short review of the paper follows:

- Only the vertical gravity gradient data has been considered for both studies:

"Although the Air-FTG system measures all five independent components of the gradient tensor, the other components are not discussed in this paper. As part of the processing carried out on the full tensor data, features in one component that are inconsistent with the response observed in the other components are rejected as noise. This creates a dependency between the data for the various components, and hence there is less new information than might be expected in the other components.” Surprising, isn’t it?

- Both systems acquired data over the same site: a 4 by 10 km area within the Kalahari Desert of southern Botswana,

- Seven kimberlite pipes have been previously discovered by Falconbridge with the pipes buried beneath 70 to 120 m of sand cover,

- Ground gravity data has been available for comparison,

- Each party has processed the data twice and there were significant differences between the first and the second version, which indicated that the area provides a relatively low signal to noise ratio for the response measured by the systems,

- Both surveys clearly identified the larger pipes but many smaller pipes generated responses with similar amplitude to the noise floor, which means that smaller targets were pretty much unidentifiable by both of the systems,

- A small anomaly identified by the Falcon survey and subsequently drilled proved to be a new kimberlite body. The very same anomaly had lower amplitude on the Air-FTG’s study and most likely wouldn’t have been picked for follow-up,

- A quantitative analysis of ground and airborne data showed no difference between the systems,

- An assessment of the detection levels of both systems concludes that:

“It would be unlikely that the anomalies for a 6 Ha kimberlite at 50 m depth and that for a 25 Ha kimberlite at 150 m depth (with the density contrasts specified) would be identified as anomalous. Kimberlite pipes which are larger, at a shallower depth, or had larger density contrasts might be expected to be detected.”

Here comes the story of another mining giant, De Beers. Since the discovery of Orapa in 1967 and Jwaneng in 1973 they have shifted their focus: exploration programs come in the second place. An absence of kimberlite discoveries prompted the company to revamp its exploration philosophy and to switch from the classical time consuming sampling programs to geophysical surveys. Knowing that the quality of the airborne data is decisively influenced by the speed of the aircraft, the ground clearance and the noise level caused by the vibration of the aircraft, De Beers signed a two year exclusivity contract with the German producer of the Zeppelin to have it flying over Botswana, South Africa and possible some other African countries. The Air-FTG system has been selected as the exploration tool and in September 2005, the huge bird started its African journey of discovery.

Bon voyage to both systems!

Note:  In September 2007, strong winds detached the high-tech aircraft from its moorings in Botswana and crashed it. There are only two Zeppelins left in the world - both being operated for tourism purposes - and it takes a year and a half to build another one.  De Beers will be forced to fly its geophysical instruments on regular planes that constitute a less desirable geophysical data acquisition platform.  Also, no word yet about the exploration system on board - was it the Air-FTG or the mighty Gedex system?