Discriminating Weathering Degree by Integrating Optical Sensor and SAR Satellite Images for Potential Mapping of Groundwater Resources in Basement Aquifers of Semiarid Regions
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Unlike in coastal and sedimentary basins, regional-scale exploration of groundwater resources using only geophysical methods is costlier in consolidated rocks such as volcanic rocks and crystalline basement complexes in Africa because of the highly heterogeneous structure of aquifers. Therefore, advanced analysis of remotely sensed images and an accurate assessment of groundwater resources are crucial before carrying out a geophysical prospecting survey. This study proposed a joint analysis of satellite images from optical sensors and synthetic aperture radar (SAR) which aimed to enhance potential mapping accuracy of groundwater resources in crystalline rock areas in a semiarid region. The backscattering coefficient of the SAR data analysis effectively detected the zones of relatively high weathering degree and thus having thick permeable regolith. In addition, a modified clay index calculated from the four band reflectances of the optical sensor image—red, near infrared, and two shortwave infrared bands—was applied to discriminate clay-rich zones from high vegetation activity zones. The clay-rich zones detected corresponded with the highly weathered zones estimated from the small SAR backscattering coefficients. The zones also corresponded with a large density of faults and lineaments and furthermore were verified by high potential yields from groundwater wells. The thickness of weathered zones was likely to increase with a decreasing backscattering coefficient and higher modified clay index values. Conversely, large backscattering coefficients in the narrow zones along the major lineaments from large volumetric scattering because of high vegetation activity, as confirmed by the large vegetation index values, suggested that high moisture content was retained in the soils. In fact, the potential yields of the groundwater wells tended to increase near the lineaments. Accordingly, shallow groundwater occurrence is plausible in those zones.
KeywordsRegolith Backscattering coefficient Vegetation index Clay index Lineament Mozambique
We thank the Japan International Cooperation Agency (JICA) for supporting this study and the Water and Sanitation Division of Tete Province (DAS-Tete) in Mozambique for providing the groundwater well data. This work was partially supported by JSPS KAKENHI (Grant Number 18H01924). Sincere thanks are extended to two anonymous reviewers and Editor-in-Chief Dr. John Carranza for their valuable comments and suggestions that helped improve the clarity of the manuscript.
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