Brice Sevin

Regolith-geology mapping with support vector machine: A case study over weathered Ni-bearing peridotites, New Caledonia

Abstract Accurate maps of Earth’s geology, especially its regolith, are required for managing the sustainable exploration and development of mineral resources. This paper shows how airborne imaging hyperspectral data collected over weathered peridotite rocks in vegetated, mountainous terrane in New Caledonia were processed using a combination of methods to generate a regolith-geology map that could be used for more efficiently targeting Ni exploration. The image processing combined two usual methods, which are spectral feature extraction and support vector machine (SVM). This rationale being the spectral features extraction can rapidly reduce data complexity by both targeting only the diagnostic mineral absorptions and masking those pixels complicated by vegetation, cloud and deep shade. SVM is a supervised classification method able to generate an optimal non-linear classifier with these features that generalises well even with limited training data. Key minerals targeted are serpentine, which is considered as an indicator for hydrolysed peridotitic rock, and iron oxy-hydroxides (hematite and goethite), which are considered as diagnostic of laterite development. The final classified regolith map was assessed against interpreted regolith field sites, which yielded approximately 70% similarity for all unit types, as well as against a regolith-geology map interpreted using traditional datasets (not hyperspectral imagery). Importantly, the hyperspectral derived mineral map provided much greater detail enabling a more precise understanding of the regolith-geological architecture where there are exposed soils and rocks.

Neogene terrestrial sediments: a record of the post-obduction history of New Caledonia

The poorly studied iron-rich terrestrial sediments, referred to as the Fluvio-lacustrine Formation, that crop out in the southern part of the Grande Terre of New Caledonia document the last 25 Ma of the geological history of the island. The age of this formation, which is mainly derived from the erosion of an ultramafic regolith, is not tightly constrained yet; however, it has recorded several episodes of post-obduction erosion and sedimentary infill preceded and followed by weathering and reactivated erosion. A correlation with early Miocene slab break-off, which may have triggered a first stage of erosion marked by coarse conglomerate, is suggested. Thereafter, sediments filled topographic lows and were in turn weathered during an interval of tectonic quiescence. Finally, Holocene sea-level drop and southward tilt of southern New Caledonia, owing to the involvement of eastern Australian Plate in the New Hebrides (Vanuatu) subduction zone, locally changed the drainage pattern and deeply eroded the sediments.

Mapping of the Regolith of New Caledonia with Airborne Hyperspectral Remote Sensing

Recently, regolith mapping based on hyperspectral remote sensing has stirred up a growing interest, in particular for mining exploration purposes. For an island like New Caledonia, which nickel resources are one of the most abundant on Earth, and which economy is mainly based on nickel exploitation, regolith mapping is of first interest. In 2010 airborne hyperspectral remote sensing data were acquired for the first time over several mining sites of New Caledonia. At the same time, field spectrometradiometric measurements were made on the same sites. One site was selected to evaluate the potential of airborne hyperspectral remote sensing for the mapping of the regolith. Combining the analysis of the field measurements and the processing of the airborne data, we managed to map the regolith with great results. In the following we present broadly the geological context of New Caledonia, the dataset acquired, the method developped and the results.