Abstract
In areas where weathering has hydrogeological significance, geophysical methods can assist to map the subsurface characteristics for groundwater occurrence. In this study, electrical resistivity tomography (ERT) survey in combination with joint profile method (JPM), magnetic method and borehole data was conducted to investigate the aquifer potential in strongly weathered volcanic rocks. The aim was to assess the geological units related to the water-bearing formation of aquifer systems in South Guangdong, China. The resistivities were measured along four profiles each with a total of 81 electrodes, a spread length of 400 m and an electrode spacing of 5 m insuring continuous coverage. The data from a borehole survey revealed three different layers i.e., highly weathered layer, partly weathered layer and fresh basement rock, whose respective thickness were integrated into ERT images to get more useful results about the real resistivity ranges of the these layers (i.e., 22 Ωm–345 Ωm for highly weathered layer, 324 Ωm–926 Ωm for partly weathered and 913 Ωm–2579 Ωm for fresh bedrock). The electrical resistivity imaging including the surface topography provides spatial variations in electrical properties of the weathered/unweathered layers since resistivity depends on the properties of a material rather than its thickness. ERT sections were integrated with JPM and magnetic method to delineate the main faults (F1, F2 and F3). ERT sections show a geometric relationship between different layered boundaries, particularly those of the aquifers with fresh basement and surface topographies. These layers comprise an overburden of 50 m thickness revealed by ERT sections. The results show that weathered and partly weathered layers between the topographic surface and bed rock yield maximum aquifer potential in the study area. ERT imaging method provides promising input to groundwater evaluation in the areas of weathered environment with complex geology.
Highlights
Electrical resistivity tomography (ERT) is an effective tool to investigate the hydrogeological characteristics of the subsurface materials
Weathered rock is the second layer revealed underneath the top layer with an average thickness of 10 m and resistivity ranging from 324 Ωm to 926 Ωm
The electrical imaging is more successful technique when it is compared with other geophysical methods to get a more detailed view of the subsurface features for better understanding of the groundwater occurrence in the weathered conditions
Summary
Electrical resistivity tomography (ERT) is an effective tool to investigate the hydrogeological characteristics of the subsurface materials When using this technique, the target is the groundwater itself and the geological structures and materials that are capable of storing and transmitting the groundwater. Since the 1990s, the applications of this method have increased due to the advancement in data acquisition techniques, interpretation techniques and computer technology [1,2,3,4,5]. As a result, it has become more precise and efficient to map the complex and small-scale geological features [6,7].
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