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Contact 52 North
  


The increasing amount of multiple data sets available to earth scientists, has created a need for efficient capture, storage, management, retrieval and analysis of geo data. Today the earth scientist faces the difficult task of relating and integrating vast amounts of different data types, obtained from different sources and compiled on different scales. In order to use all these data for mapping, interpretation and modeling, the earth specialist should be able to spatially link field observations with ancillary data. This application introduces the earth scientist to digital processing techniques for the integration, visualization, enhancement, and interpretation of multiple geo data sets in a GIS environment.

The study area

The study area is located along a major fault zone on the Canadian Shield in the Northwest Territories area of Canada. Bathurst Inlet, a deep basin drowned by the sea, consists of sedimentary rocks protected by intrusive rocks that are resistant to erosion. This resulted in a topography that is in sharp contrast with the surrounding area being characterized by low hills and broad valleys due to extensively scouring of ice during the Wisconsian glaciation.
The Pistol Lake Area, located West of Bathurst Inlet, comprises the easternmost part of the Hood River Belt. The Hood River Belt is an Archean supra-crustal sequence consisting of thickly bedded turbidites with occasional iron formation beds. In some of these narrow iron formation beds within the upper sequence of this turbidite unit gold mineralizations are found. The Hood River Belt is intruded by granitiod batholiths ranging in composition from diorite to monzo-granite. At least three dyke swarms crosscut this geological unit. The oldest are EW trending dykes then the SE-NW trending dykes and the youngest are the North trending dykes.

Image enhancement and integration

The spectral response of each pixel in a Landsat TM image can, due to the limited spatial resolution, be considered as a mixture of spectral signatures from vegetation, water, soil and bedrock. To improve the spectral information of bedrock geology, pixels are masked that are likely to have pure or high proportions of water, fluvial sediments and green vegetation. Water has a low reflectance in TM band 7, the fluvial sediments have a particularly high reflectance in TM band 3 and to mask green vegetation a NDVI image is calculated from bands 3 and 4. TM bands 3, 5 and 7 are masked using MapCalc in combination with predefined thresholds. After that a color composite is created from the three masked bands and compared to the color composite of the original unmasked TM bands.

Color composite masked TM357

Color composite masked TM357

A image integration technique that has proven to give useful results for geological mapping is the merging of the multi-spectal bands of Landsat TM with the higher spatial resolution data of SPOT Panchromatic.
Enhancing the SPOT image requires high pass filtering, stretching the image and masking it with a MapCalc formula. To increase the spatial resolution of TM bands 3, 5 and 7 they are resampled to the pixel size of the SPOT Panchromatic image. Integrating TM and SPOT data was done by normalizing the TM bands by their sum and multiplying it with the enhanced SPOT Panchromatic image using MapCalc. Finally these new enhanced bands are used to create a color composite.

Color composite fusion TM and SPOT

Color composite fusion TM and SPOT

Color coding and relief shading are some basic enhancement techniques for the representation of aeromagnetic data. Color coding, as an aid to qualitative interpretation, is applied to aeromagnetic data creating a new representation with several limits and colors.

Color-coded aeromagnetic data

Color-coded aeromagnetic data

A powerful digital processing technique to enhance structural patterns in aeromagnetic data is relief shading: aeromagnetic data are filtered with user-defined gradient filters and a user-defined function is created to calculate relief shaded maps with various artificial illuminations.
IHS transformation is used to integrate aeromagnetic data and SPOT Panchrometic data. The method that is used for merging these images is to assign the variations in the total magnetic field to Hue and the digital numbers of the enhanced SPOT Panchromatic image to Intensity while keeping the Saturation at a constant level. MapCalc operations and eventually the creation of user-defined functions allow you to create different color composites using different values for Intensity, Hue and Saturation.

IHS composite

IHS composite

Spatial analysis

The Pixel Information window can be used to simultaneously retrieve map and table information. Some simple spatial analyses are performed to define areas with a high probability for gold mineralizations. In the Pistol Lake Area field information is extremely limited but it is known that gold mineralization are found in narrow iron formation beds within the upper sequence of a turbidite unit.
A first criterion to define a probability zonation of gold mineralization is to consider the areas within and in closest vicinity of the band iron formation outcrops. Distance calculation is performed to create a distance dependent probability model for gold deposits.

Distance map

Distance map

Visualization of the data allows you to select two buffer classes around the iron formation beds and to reclassify the distance map into a buffer map using the Slicing operation.
By making a cross map between the buffer map and the map with the locations and description of known mineral occurrences, dividing the number of prospects that appear in each buffer zone and counting the number of pixels in each zone, you can make a rough estimate of the probability to find gold deposits in the zone from 0 to 75 meters compared to the zone from 75 to 150 meters.
A second criterion to define a probability zonation of gold mineralization is to consider the occurrence of shear zones and faults that intersect the banded iron formations and their near vicinity as a higher priority target for exploration by assuming that those faults could have acted as channel ways to transport gold enriched fluids. We can model such a selective fluid transport by assigning relative weights in the distance calculation. To simplify the probability model for gold mineralization a relative weight of 0 (= unlimited fluid transport) is assigned for the fault zones and 1 for outside the faults.In order to model a decrease in relative probability from the banded iron formation outcrops and fault zones a function of the following type can be used:

P(gold deposit) = e-b.distance

Finally the probability map is combined with a TM band using a MapCalc statement.

Probability map combined with TM5

Probability map combined with TM5

For more information on this case study, contact:

E.M. Schetselaar
Department of Earth Systems Analysis,
International Institute for Geo-Information Science and Earth Observation (ITC),
P.O. Box 6, 7500 AA Enschede, The Netherlands.
Tel: +31 53 4874294, Fax: +31 53 4874336, e-mail: ernst@itc.nl

  
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