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

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.

Water footprint in gold extraction: A case-study in Suárez, Cauca, Colombia

“…A 2016 study [4] used satellite images and other remote sensing tools to quantify the number of hectares affected by mining in Colombia, determining that the department of Cauca has the least impact among the seven gold producing departments, according to the authors this may be due to a lack of accountability for primary ore mining in the remote sensing method used. Recent remote sensing studies in Africa and South America have successfully account for the footprint of small-scale artisanal gold mining [33,34].…”

Structure, composition and diversity of restored forest ecosystems on mine-spoils in South-Western Ghana

“…In addition, redressing land degradation through integrated landscape management is government of Ghana priority with several projects being initiated and implemented including the multi-sectoral mining integrated project [2]. Although the contribution of mining to land degradation in Africa is largely unknown because numbers and areas of abandoned mine sites are not well documented [3], it is estimated that land area under surface mining in Ghana may have more than doubled within the last decade due to proliferation in galamsey and main mining operations [4][5][6]. Compliance and adherence to these mine closure regulations have been somewhat positive among mining companies in the country and restoration or reclamation efforts are gradually rising.…”

Mapping Artisanal and Small‐Scale Gold Mining in Senegal Using Sentinel 2 Data

“…New field observations would be necessary to adapt the current method to these sites, but an overall similar methodology could be applied. Recently, Forkuor et al (2020) used Sentinel 1 data (radar) for mapping and monitoring small-scale mining activities in Ghana (2015-2019). An approach based on detection of surface properties modifications has allowed the detection of new illegal sites on an annual basis at the beginning and end of the rainy season.…”