Mapping and Monitoring Small-Scale Mining Activities in Ghana using Sentinel-1 Time Series (2015–2019)

Gerald Forkuor,Tobias Ullmann,Mario Griesbeck

doi:10.3390/rs12060911

Gerald Forkuor, Tobias Ullmann + Show 1 more

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https://doi.org/10.3390/rs12060911

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Abstract

Illegal small-scale mining (galamsey) in South-Western Ghana has grown tremendously in the last decade and caused significant environmental degradation. Excessive cloud cover in the area has limited the use of optical remote sensing data to map and monitor the extent of these activities. This study investigated the use of annual time-series Sentinel-1 data to map and monitor illegal mining activities along major rivers in South-Western Ghana between 2015 and 2019. A change detection approach, based on three time-series features—minimum, mean, maximum—was used to compute a backscatter threshold value suitable to identify/detect mining-induced land cover changes in the study area. Compared to the mean and maximum, the minimum time-series feature (in both VH and VV polarization) was found to be more sensitive to changes in backscattering within the period of investigation. Our approach permitted the detection of new illegal mining areas on an annual basis. A backscatter threshold value of +1.65 dB was found suitable for detecting illegal mining activities in the study area. Application of this threshold revealed illegal mining area extents of 102 km2, 60 km2 and 33 km2 for periods 2015/2016–2016/2017, 2016/2017–2017/2018 and 2017/2018–2018/2019, respectively. The observed decreasing trend in new illegal mining areas suggests that efforts at stopping illegal mining yielded positive results in the period investigated. Despite the advantages of Synthetic Aperture Radar data in monitoring phenomena in cloud-prone areas, our analysis revealed that about 25% of the Sentinel-1 data, mostly acquired in March and October (beginning and end of rainy season respectively), were unusable due to atmospheric effects from high intensity rainfall events. Further investigation in other geographies and climatic regions is needed to ascertain the susceptibility of Sentinel-1 data to atmospheric conditions.

Highlights

Mining is one of the major causes of land and environmental degradation globally [1,2,3]
Our analysis revealed that a significant portion of the investigated S-1 data showed locally-restricted patches of strong local decrease in backscatter intensity in the order >−10 dB compared to the long-term mean intensity
The methodology was premised on the fact that mining-induced land cover changes can be detected based on changes in the scattering characteristics of time-series images acquired with the same acquisition geometry

Summary

Introduction

Mining (especially surface) is one of the major causes of land and environmental degradation globally [1,2,3]. Despite its contribution to the formal economy of the country, local communities and indigenes of mining areas receive minimal benefits but suffer the negative impacts of forced relocation, takeover of their farmlands by large mining companies, reduced agricultural productivity and unprecedented environmental pollution [10,11,12]. This situation has led to an abuse of the country’s small-scale mining law, where unregistered and unregulated artisanal miners (galamsey) resort to the use of heavy machinery/equipment for mining along river bodies, causing excessive destruction of the vegetative cover and widespread pollution of water bodies [13]. Artisanal mining in Ghana dates back to the 15th century [14,15], the introduction of heavy machinery and a sudden influx of foreign nationals (especially Chinese) [16] has attracted significant attention to small-scale mining and the environmental degradation perpetuated by these miners (legal or illegal)

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