Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Summary
The De Beers Airship Gravity Project
David Hatch 1 and Brad Pitts 2
1 De Beers Group Services, Johannesburg, South Africa
(Now with Gedex Inc, Toronto Canada) (David.Hatch@Gedex.ca)
2 De Beers Prospecting Botswana (Pty) Ltd, Gaborone, Botswana
(Now with Simplicity Projects Botswana (Pty) Ltd, Gaborone Botswana)
(brad@simplicity.co.bw)
To successfully execute a diamond exploration program in Botswana, De Beers Group Exploration
required that a large, high-quality gravity data set be acquired. The 4000 km 2 area could not be
surveyed on the ground within the allotted time, and the noise and resolution levels of existing airborne
gravity gradiometer systems were considered to be inadequate to confidently explore the area.
To rapidly acquire high-quality gravity data, the solution of mounting an existing instrument on a
radically more stable airborne platform was investigated. A Zeppelin airship provided two significant
advantages over a fixed wing aircraft platform for gravity surveying, i.e., it represented a benign
acceleration environment arising from the increased stability, and it could be operated as a slow
moving platform Tests of the Bell Geospace Air-FTG® system mounted in a Zeppelin airship
demonstrated that the data quality was sufficient to achieve the goals of the project.
The challenges of an airship program in Botswana were formidable. The hot summer daytime
temperatures, high elevation, and harsh desert conditions of the survey location combine to produce a
very difficult operating environment. The largest impact on productivity was airship downtime which
was mainly attributed to the deployment of a prototype airship where it was exposed at all times during
maintenance, mooring and surveying to the harsh desert conditions. The weather also accounted for a
significant amount of downtime, although this was not unexpected given the severe summer weather
and strong seasonal winds.
The high-quality airship gravity data allowed kimberlite targets to be confidently selected without the
large number of false targets associated with an equivalent fixed-wing survey (i.e., Type I errors). This
greatly reduced associated follow-up costs and the risk of missing potentially economic bodies (i.e.,
Type II errors). The discovery of a new kimberlite in an area that had been heavily prospected utilising
other exploration techniques is described.
Introduction
The Jwaneng Mine in South-Central Botswana is the richest diamond deposit in the world. Since the
mine opened 25 years ago, the surrounding region has been covered by successive waves of
diamond exploration utilizing both heavy mineral sampling and geophysics. The goal of the exploration
program between 2005 and 2008 was to discover all kimberlites which had the potential to be
economic that had not been detected in these earlier phases.
High resolution magnetic data had been collected over the area, but as is commonly found, some
pipes do not possess a magnetic response. Electromagnetic techniques are often used to assist in the
discovery of non-magnetic kimberlites. However, the area is covered by younger Kalahari Group
sediments up to 100 m thick which can be impregnated with saline ground water. This greatly reduces
the effectiveness of electromagnetic surveys, and a time-domain survey carried out in the area
confirmed that the response of kimberlites would be masked.
Orientation surveys conducted over kimberlites in the Jwaneng cluster showed the majority of these
possessed a significant gravity anomaly. Given the previous exploration experience over the area,
acquisition of airborne gravity data was thought to be the most cost-effective and timely route to make
new discoveries over the roughly 4000 km 2 exploration area. The use of the Bell Geospace full tensor
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