Page 128 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

Catchment Basin Analysis of Stream Sediment Anomalies 127

explanations of such algorithms can be found in Jenson and Domingue (1988), Martz
and Garbrecht (1993) and Jones (2002). Sample catchment basins are labeled with
unique IDs corresponding to the stream sediment samples they represent. The area of
each sample catchment basin can then be incorporated in the stream sediment
geochemical database.

Measuring areas and areal proportions of lithologic units in catchment basins
Calculation of areas and areal proportions of lithologic units in stream sediment
sample catchment basins involve map (cross or intersect) overlay and table (join)
operations as illustrated and explained schematically in Fig. 5-6. Maps used in the map
overlay operation can be either in vector or in raster format and the measured areas can
be either in units of, usually, square metres or in number of pixels. The table operation
uses simple arithmetic calculations. The operations may vary slightly from one GIS
software package to another; however, the overall procedure is straightforward.

Estimation of local background uni-element concentrations
Estimation of weighted mean uni-element concentrations due to lithology (see above)
can be readily performed in the attribute table of the stream sediment geochemical data,
in which areas of individual lithologic units per sample catchment basin have been
appended to the stream sediment geochemical database. Estimation of local background
uni-element concentrations in every sample catchment basin via the multiple regression
analysis explained above may not be readily performed in a GIS. That is because many
GIS software packages do not have algorithms for multivariate data analysis, such as
regression analysis. However, an attribute table of the stream sediment geochemical
data, which contains the areal proportions of lithologic units in sample catchment basins,
can be exported to file formats supported by statistical software packages in which the
multiple regression analysis can be performed. Estimates of local background uni-
element concentrations per stream sediment sample catchment basin can then be
exported back to the GIS for further analysis.

Correction for downstream dilution
Application of either equation (5.8) or equation (5.9) to correct for downstream
dilution can be readily implemented in an attribute table of stream sediment geochemical
data containing areas of sample catchment basins and estimates of local background uni-
element concentrations per sample catchment basin. It is also possible to calculate
productivity per sample catchment basin (Polikarpochkin, 1971; Moon, 1999), which
also represents dilution-corrected geochemical residuals. However, unlike in equation
(5.8) or (5.9), where A i is incremental catchment area between a sample site and the next
sample site upstream, the A i used for calculation of productivity is total catchment area
upstream of a sample location. Explanations for automatic calculation of total catchment
areas from points along drainage networks can be found in Martz and Garbrecht (1993).
Alternatively, it has been shown that, to estimate productivity, stream order (based on