Spectral remote sensing data as
applied to mineral exploration traditionally involves the use of satellite imagery,
typically Landsat TM data, as a means of providing a regional geological and structural
overview in support of area selection, with, in some cases, the added bonus of target
selection where clay-iron enrichment is indicated by the use of standard TM band ratios.
Satellite data, although being low cost with substantial coverage, is limited in both
spatial and spectral resolution and in the relatively flat lying, easily accessible
Archaean terrain of Western Australia, is not necessarily effective even as a
reconnaissance exploration tool.
Airborne spectral remote sensing has been around
now for over 12 years but has only recently come of age as far as its ability to provide
true value so far as the mineral explorer is concerned. This has come about largely as a
result of the advent of high powered PCs and sophisticated image processing software. The
spatial and spectral resolution of airborne instruments has long been known to offer
significant advantages over satellite systems. However, the difficulty in georeferencing
the distorted airborne data and reducing its large volumes to produce scaled maps of
geological or mineralogical information capable of integration with other datasets has
held back the widespread application of the technology to mineral exploration.
Furthermore, a perceived high cost has prejudiced the technology as an effective
alternative to more traditional exploration techniques.
However, the availability of archival Geoscan
data in Australia is now being used in an innovative way to provide both reconnaissance
and ongoing advanced exploration support at costs which are more than competitive with
alternative approaches to an inherent deep weathering problem which has hitherto
restricted the application of any form of remote sensing. Western Australia is an arid
terrain, which would be ideally suited to geological remote sensing were it not that
bedrock geology has been modified by prolonged tropical weathering to produce a deep
lateritic profile which is now itself in various stages of erosion or burial. Thus, even
Landsat TM data has little impact in trying to map bedrock geology and exploration has
traditionally resorted to geochemical methods of outlining prospective target areas.
In recent years however, detailed research into
weathering processes has developed an understanding of metal ion movements within the deep
weathering profile and has focussed attention upon the need for regolith mapping as a
framework upon which to base exploration geochemistry. Spectral remote sensing has a
significant role to play in regolith mapping and Landsat TM has begun to be widely used.
However, it is limited to providing interpretive regolith maps at 1:50 000 scale at best
in areas where 1:10 000 is needed.
Archival Geoscan AMSS data flown in the late
1980s and early 1990s offers a high resolution alternative that is finding
increasing use not only as a tool for regolith mapping, but also in exploration targeting.
The data are processed by Australian Geological and Remote Sensing Services (AGARSS) to
produce indices of iron oxide, clay, carbonate and silica content which are then draped
either individually or as RGB algorithms over grey scale albedo or DEMs for regolith
and landform analysis. The same indices can also be used to identify hydrothermal
alteration in erosional terrains and residual alteration minerals in transported
overburden to generate exploration targets. Currently, the Leonora and Menzies 1 : 250 000
sheet areas are already available, and archival data over other areas will continue to be
processed by AGARSS upon request.
In 1997, AGARSS entered into a joint venture arrangement with
Mackay and Schnellmann to promote this product. Standard data packages comprise indices of
iron oxide, clay, carbonate and silica content on CD ROM in ER Mapper TM format
and also a standard lithological discriminator image in .gif format. Specific project
packages may be negotiated to include a full prospectivity analysis, follow up ground
truthing and/or planned geochemical sampling programs, carried out by Mackay and