What are the best data sources for regolith-terrain mapping?

Producing useful regolith-terrain maps for the exploration industry relies primarily on the analysis and visualization of topographic and multi-spectral data.

The two most useful datasets for regional regolith-terrain mapping are well processed Landsat satellite images (Landsat TM3) and digital elevation models (SRTM).

Extracting maximum value from these datasets, and then applying more robust mapping methodologies, will improve our understanding of the intimate relationship between landform, material, and process. This is a prerequisite for understanding three-dimensional and temporal aspects of the regolith.

Orthophotograph, radiometric, multispectral (ASTER), hyperspectral (HyMap) and radar datasets are also potentially useful for mineral exploration, but coverage of remote areas, such as the Tanami Desert in Western Australia, tends to be patchy.

West Tanami, Decorrelation Stretch Bands754,
temporal range 1994-2005 (sum of nine years); area 120x170km

DS754

Landsat TM3 - temporal merge (click to see more examples small hand)

Multi-band remotely sensed image data contain information on landscape patterns and lithological changes that are underutilized in regolith-terrain and bedrock mapping.

While most users would naturally focus on the most recent images as being of most value, identifying persistent patterns in the landscape that relate to geological materials is best accomplished by removing the short-term effects.

The most recent image may be the worst in terms of fire scars, floods, and revegetation.

However, a simple arithmetic mean or sum of data for a range of years produces images with improved colour depth and enhanced geological material discrimination.

The merging of Landsat TM data for a range of epochs for use in terrain mapping, producing what are now termed Landsat TM3 images (Landsat TM Temporal Merge Terrain Mapping), effectively tackles the challenge of producing a consistent set of images in an area dominated by seasonal vegetation changes and fire scars.

Tanami Region, north-west aspect shaded,
multi-scale resampled 90-m SRTM data; area 315x330km

Tanami slope

SRTM - multi-scale resampling (click to see more details small hand)

The NASA Shuttle Radar Topographic Mission (SRTM) has provided digital elevation data (DEMs) for over 80% of the globe. The SRTM digital elevation data are a major breakthrough in digital mapping of the world, and provide a major advance in the accessibility of high quality elevation data.

With good processing and visualisation techniques, SRTM images are a very effective tool in mineral exploration, particularly in areas of low relief. The use of multi-scale resampling and aspect shading adds significant value to the data, and these techniques have been actively promoted through publications and presentations.

The analysis and visualization of Shuttle Radar Topography Mission (SRTM) DEM data has been very effective for regional regolith-terrain mapping, for example in the Tanami desert, WA. Applying multi-scale analysis to resample the original data has produced the best visualization of what is essentially a level landscape.

Wheatbelt WA, north-east aspect shaded, multi-scale resampled 20-m
Orthophotograph DEM data; area 30x20km

Ortho DEM

DEM and Lidar

The techniques that have been developed to extract maximum benefit from SRTM data can also be applied to Orthophotograph Digital Elevation Models) DEMs and Lidar (portmanteau of "light" and "radar"). In both these cases, the level of detail in the original data is good, so multi-scale processing uses different parameters to enhance regional patterns in near level landscapes.

DEMs are costly to acquire, and Lidar is very expensive. They are usually of limited or project-based extent. However, if made available, they are a very good complement to the regional-scale SRTM data, and can be processed for the same cost as SRTM data.

Central Queensland, 10-m contours on multi-scale resampled ASTER DEM (red) and SRTM (blue); area 35x30km

SRTM ASTER

ASTER

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is an imaging instrument onboard Terra, the flagship satellite of NASA's Earth Observing System (EOS) launched in December 1999. ASTER is a cooperative effort between NASA, Japan's Ministry of Economy, Trade and Industry (METI), and Japan Space Systems (J-spacesystems).

ASTER spectral data can be used to create detailed maps of land surface spectral reflectance related to mineral composition. The data are relatively expensive, and difficult to process. Spectral images of higher resolution than Landsat, with targeted mineral information, can be created on demand.

ASTER DEMs are free to download, and of higher resolution than SRTM data. However, stereoscopic processing is used, rather than the direct radar measurement employed by SRTM. The data are not suited to low relief landscapes (see contours), although can be useful in areas of high relief.