Simultaneous use of geological, geophysical, and LANDSAT digital data in uranium exploration

Abstract

In southern Libya, in an 850-km 2 area of uranium-bearing crystalline rocks, which had been mapped geologically in 1974 and over which concurrent low-altitude gamma-ray spectrometry (total count) and magnetics were also flown, a fourteen man-month pilot study was made using multivariate statistical methods of merged datasets from quantified geological, geophysical, and LANDSAT data. Key to the analysis were, 1. 1. The use of a new grid by which to resample the datasets, oriented with the specific azimuth of a LANDSAT-2 pass, with spacings defined by the average pixel sizes of that coverage. 2. 2. Scaled, 1:1 aspect-ratio computer printouts, which were used as the base maps for the grid and which were compared to semi-controlled photomosaics, geological, and geophysical maps, all at 1:50,000 scale. 3. 3. A “defocused” LANDSAT cell, of almost 480×458 m, achieved by averaging 8 pixels (West—East) by 6 pixels (North—South). In being still clamped to the geometry of LANDSAT, this facilitated the extraction of the radiance datasets from the satellite coverage. 4. 4. Thirty-one parameters were extracted from the 1:50,000 scale geological maps, and two geophysical parameters obtained by resampling the contoured geophysical maps. 5. 5. Appreciating that LANDSAT data are also quantified geophysical matricies and that from them 6 nonredundant ratios could be obtained. A total of ten LANDSAT variables were thereby extracted. 6. 6. Multivariate analysis of 43 variables for the 3364 cells was performed. Transformations of the variables to approximate the normal distribution were necessary to effectively apply the discriminant function analysis. The results included contoured probability maps of uranium prediction, with analysis of the role and the relative contribution of each of the 43 parameters. By far, the key parameters for the geological set were contact relationships (lengths/cell) and for LANDSAT were the average brightness of channel 6 and ratio 7/4. Adding the LANDSAT data significantly increased the definition and spatial coherence of the anomalies, retaining some of the key contact lengths but adding more information from the geological maps (rock percentages/cell, etc.) with which LANDSAT (here) shows high agreement. The extraction of the multivariate signatures (geological and geophysical and LANDSAT for this study area, would allow extension to adjoining areas, as the geological framework of this test site extends an appreciable (500 km) distance to the east and to the west, as well as south into the N. Chad uranium belts. Automation of some of the data extraction and data-handling steps has been completed recently. Considerable further development of the methodology is underway, particularly with respect to the statistical treatment of the data, as well as the continued parametric analysis of this excellent data set, especially the use of linear regression against the radiometric data.