Locations of small-scale diamond and gold dredges detected using Synthetic Aperture Radar on the Kadéï (Sangha) River, Central African Republic

Publication Date

2023-01-23

Start Date

2015-01-01

End Date

2019-12-31

Alessi, M.A., 2023, Locations of small-scale diamond and gold dredges detected using Synthetic Aperture Radar on the Kadéï (Sangha) River, Central African Republic: U.S. Geological Survey data release, https://doi.org/10.5066/P9FWFC7R.

Diamond and gold mining has been practiced by artisanal miners in the Central African Republic (CAR) for decades. The recent introduction of riverine dredges enables artisanal miners to access in-channel diamond deposits that have been concentrated by fluvial processes, and indicates a transition from artisanal/manual digging techniques to small-scale mining methods. Remote sensing analysis of Synthetic Aperture Radar (SAR) data has been implemented to map gold and diamond dredges operating on the Kadéï (Sangha) river in the CAR. Riverine vessels are identified in Sentinel-1 SAR data between 2015 and 2019 and their activity levels are mapped over time. Riverine vessels on the Kadéï produce a significant SAR backscatter signature compared to surrounding surface water and are clearly visible operating due to their size, structure, and material which return strong and prominent corner reflections. These strong reflection points are manually identified and digitized as point locations. The use of SAR technology, with its ability to see through cloud cover, broad coverage, and available wavelength combinations, allows for higher temporal resolution and improved vessel detection in the monitoring of small-scale mining dredges. There is virtually no boat traffic on the Kadéï apart from dredge mining, a ferry barge in Nola, and limited tender boat activity in support of the dredges. Minimal non-dredge boat activity on the river means that a vessel identified by SAR data is likely to be a mining dredge. Any vessel identified near the path of the ferry crossing was assumed to be the ferry, and any that appeared in the same location in all the SAR scenes for that year was labeled ‘idle.’ Accuracy of a selection of identified vessels was assessed through visual interpretation of very high-resolution imagery collected in the same time-frame. In this optical imagery, visual identification of dredges operating on the river improved and verified interpretation of their corresponding backscatter signal in the SAR data. While the signal of vessels in SAR imagery is fairly distinct, some situations such as spoil piles and riverside docks may mimic their backscatter. When available, high-resolution imagery was used to verify the accuracy of the vessel identifications from SAR data.

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