Validated linear mixture modelling of Landsat TM data for mapping evaporite minerals on a playa surface: methods and applications

Abstract

The Chott el Djerid is a large salt playa situated in southern Tunisia. The playa has a brine- type of Na-K-Mg-Cl-SO₄ and a stable mineral assemblage of gypsum (CaSO₄.2H₂O), halite (NaCl) and carnallite (KMgCl₃.6H₂O). From laboratory spectra of the three stable evaporite minerals it was found that gypsum and halite were the only evaporite minerals likely to be differentiated using Landsat TM imagery. Unmixing analysis was then undertaken on a 30 km by 60 km sub-scene of single-date Landsat TM data (April 1986) cover the northern half of the Chott el Djerid. Using principal components analysis (PCA) five end-member cover-types were identified from this image: (1) gypsum, (2) halite, (3) live vegetation, (4) alluvial material and (5) shade/moisture. By applying a linear mixture model to the single-date image, proportions maps for each cover-type were generated for the playa. Errors for each class were found to be (1)0·039, (2)0·059, (3)0·028, (4) 0·052, and (5) 0·061. The proportions maps for gypsum and halite were validated by using surface salt samples taken promptly after image acquisition in April 1986. Correlations between ground and image proportions were found to be (r²) 0·935 for halite and (r²) 0·835 for gypsum. Uses of the proportions maps generated in this study for the mapping of depositional environments are outlined.