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A machine learning approach to geochemical mapping

Kirkwood, Charlie; Cave, Mark; Beamish, David; Grebby, Stephen; Ferreira, Antonio

Authors

Charlie Kirkwood

Mark Cave

David Beamish

Antonio Ferreira



Abstract

Geochemical maps provide invaluable evidence to guide decisions on issues of mineral exploration, agriculture, and environmental health. However, the high cost of chemical analysis means that the ground sampling density will always be limited. Traditionally, geochemical maps have been produced through the interpolation of measured element concentrations between sample sites using models based on the spatial autocorrelation of data (e.g. semivariogram models for ordinary kriging). In their simplest form such models fail to consider potentially useful auxiliary information about the region and the accuracy of the maps may suffer as a result. In contrast, this study uses quantile regression forests (an elaboration of random forest) to investigate the potential of high resolution auxiliary information alone to support the generation of accurate and interpretable geochemical maps. This paper presents a summary of the performance of quantile regression forests in predicting element concentrations, loss on ignition and pH in the soils of south west England using high resolution remote sensing and geophysical survey data.

Through stratified 10-fold cross validation we find the accuracy of quantile regression forests in predicting soil geochemistry in south west England to be a general improvement over that offered by ordinary kriging. Concentrations of immobile elements whose distributions are most tightly controlled by bedrock lithology are predicted with the greatest accuracy (e.g. Al with a cross-validated R2 of 0.79), while concentrations of more mobile elements prove harder to predict. In addition to providing a high level of prediction accuracy, models built on high resolution auxiliary variables allow for informative, process based, interpretations to be made. In conclusion, this study has highlighted the ability to map and understand the surface environment with greater accuracy and detail than previously possible by combining information from multiple datasets. As the quality and coverage of remote sensing and geophysical surveys continue to improve, machine learning methods will provide a means to interpret the otherwise-uninterpretable.

Citation

Kirkwood, C., Cave, M., Beamish, D., Grebby, S., & Ferreira, A. (2016). A machine learning approach to geochemical mapping. Journal of Geochemical Exploration, 167, https://doi.org/10.1016/j.gexplo.2016.05.003

Journal Article Type Article
Acceptance Date May 7, 2016
Online Publication Date May 10, 2016
Publication Date Aug 1, 2016
Deposit Date Jun 10, 2016
Publicly Available Date Jun 10, 2016
Journal Journal of Geochemical Exploration
Print ISSN 0375-6742
Electronic ISSN 1879-1689
Publisher Elsevier
Peer Reviewed Peer Reviewed
Volume 167
DOI https://doi.org/10.1016/j.gexplo.2016.05.003
Keywords Uncertainty, Modelling, Soil geochemistry, Quantile regression, Random forest, South west England
Public URL http://eprints.nottingham.ac.uk/id/eprint/33879
Publisher URL http://www.sciencedirect.com/science/article/pii/S037567421630098X
Copyright Statement Copyright information regarding this work can be found at the following address: http://creativecommons.org/licenses/by-nc-nd/4.0

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Copyright Statement
Copyright information regarding this work can be found at the following address: http://creativecommons.org/licenses/by-nc-nd/4.0





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