Groundwater Detection Using Geophysical Methods in Different Overburden Types in the Karst Region of Southwest China

Abstract The overburden has a decisive influence on geophysical groundwater detection. (1) Exposed type area: The soluble rock aquifer is directly exposed at the surface. For conventional direct current (DC) electrical methods, this layer exhibits high resistivity, which hinders the application of the DC resistivity method. Non-ground-contact geophysical methods such as ground-penetrating radar (GPR) and transient electromagnetic (TEM) methods are often used for groundwater detection in such areas. (2) Covered type area: The soluble rock aquifer is overlain by clay, sand, gravel, and other materials, which forms an electrical layer with resistivity ranging from tens to hundreds of Ω·m. When the overburden thickness is less than 30 m, the conventional resistivity method is suitable for groundwater detection; when the overburden thickness exceeds 30 m, methods such as audio-frequency magnetotellurics (AMT), controlled-source audio-frequency magnetotellurics (CSAMT), or TEM are recommended. (3) Buried type area: The soluble rock is overlain by clastic rocks, which generally have resistivity ranging from tens to hundreds of Ω·m. For relatively intact, slightly weathered basalt, the resistivity can be on the order of n×10 3 –n×10 4 Ω·m. When the groundwater level is shallow, electromagnetic methods are preferable. When the groundwater level is deep, the conventional resistivity method can be used within the target detection depth range. However, under confined site conditions and with deep target bodies, methods such as AMT, CSAMT, or TEM should be used for groundwater detection. Other methods may be integrated when geophysical anomalies are ambiguous or multiple interpretations are available to ensure comprehensive interpretation.