Direct Detection of Buried Orebodies Using Gradiometry

摘要

The routine use of gravity gradiometry for the detection of orebodies with no outcrop is nowrnbeing undertaken. The commodities and orebody styles range from banded iron ore formationsrnto volcanogenic massive sulfi de (VMS) style polymetallic deposits. This recent innovation of fl yingrngravity gradiometry was pioneered by BHP Billiton and Lockheed Martin with the Falcon technologyrn(Lee, 2001), and now two additional systems, also using Lockheed Martin instruments, are deployed.rnIn South Africa, magnetic gradiometry is being developed and deployed with the same objectives inrnmind. In this case, a system from Institute of Photonic Technology (IPHT) is being used.rnThe detectable wavelengths from such systems have decreased to less than 200 m in the last year,rnand it is expected in 2011 that wavelengths to less than 100 m, with anomaly signal strengths of lessrnthan 2 Eotvos for gravity gradiometry will be viable.rnThe key to these advances lies in the improvements of processing software and geological modelling.rnIn particular, methods using all the tensor components are starting to be routinely deployed to fi lter,rnlevel and grid the observed signal. These methods do, however, require full insight and understandingrnof the reference coordinate systems used by the various instrument designers and survey operators.rnThe digital terrain models need also to be observed to an accuracy of better than 20 m cell sizes, usingrnsuch techniques as Lidar. Then the terrain effects can be modelled.rnThe topographic surface represents the largest and most proximal density contrast encountered inrnan airborne survey. Hence terrain effects can have signifi cant impact on airborne gravity gradiometryrndata. A decorrelation of the terrain effect with the observed signal requires a method of best fi ttingrnwith a regional surface density. A novel technique has been developed for this purpose. This techniquernuses a tensor algebra to estimate a density between 1 - 4 g/cc that explains the average observedrnsignal. Armed with this result, the anomalous buried bodies can be revealed by subtraction.