JSM 2004 - Toronto

Abstract #301486

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Activity Number: 294
Type: Topic Contributed
Date/Time: Wednesday, August 11, 2004 : 8:30 AM to 10:20 AM
Sponsor: Section on Statistics and the Environment
Abstract - #301486
Title: Estimation of Background and Threshold in Applied Geochemistry
Author(s): Robert G. Garrett*+ and Peter Filzmoser and Clemens Reimann
Companies: Geological Survey of Canada and Vienna University of Technology and Geological Survey of Norway
Address: 601 Booth St., Ottawa, ON, K1A 0E8, Canada
Keywords: Geochemistry ; Outliers ; Background ; Multivariate ; Threshold
Abstract:

Geochemical methods are used in geological and environmental sciences for both mineral exploration and studies of anthropogenic impacts on the environment. The Earth's surface chemistry is heterogeneous: different rocks, surface environments, and various natural and anthropogenic processes lead to different statistical populations. A task is to identify these populations and estimate their properties so that individuals likely not belonging to them can be identified. Univariate approaches include graphical and formal methods for threshold, i.e., boundary of background, selection. Most geochemical data are multivariate and graphical methods involve subjective decisions and formal methods involve various assumptions. A procedure that identifies the point of greatest difference between the empirical and theoretical distributions of Mahalanobis distances has been developed that leads to the identification of a background dataset that permits the identification of outliers, i.e. individuals falling beyond the threshold. The procedure is demonstrated with data from an ecogeochemistry survey in Northern Europe.


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