Study on spatial structure characteristics of epikarst zone interpreted by integrated geophysical method: Taking the slope runoff field of Guohua town ecological experiment base in Pingguo City, Guangxi as an example

The epikarst zone is widely distributed in the karst area of Southwest China, and it is the intersection zone of four circles (lithosphere, atmosphere, biosphere and hydrosphere). It is sensitive to environmental changes, in which rapid chemical reactions and strong karst actions are likely to take place. It records much information on short-term environmental change. It also has the function of regulating and storing karst water, and thus can effectively provide water resources for residents in karst mountainous areas. Therefore, the exploration and interpretation of the structure of epikarst zone is of great significance to the study of water cycle and the storage function of epikarst zone. In this study, comprehensive geophysical methods such as high-density resistivity method, spontaneous potential method and ground penetrating radar method have been used to explore and interpret the spatial structure characteristics of epikarst zone in slope runoff field of Guohua town ecological experiment base in Pingguo City. The high-density resistivity method can obtain the electrical structure section of slope runoff field by data inversion, and divide karst development areas by electrical structure. As an important method to interpret the development position of strong runoff belt, the spontaneous potential method can obtain the spatial position of polarization source through data inversion, and the generation of polarization source is closely related to water flow and electrochemical reaction between water and surrounding rock. With high resolution, the ground penetrating radar method is effective to interpret the development thickness of epikarst zone. The high-density resistivity method and the spontaneous potential method can be used to detect and comprehensively interpret the spatial position of the strong runoff belt in slope runoff field, and the ground penetrating radar method can be used to detect the thickness of the epikarst zone in the slope runoff field. In this study, according to the resistivity inversion cross-section and polarization source inversion results, the spatial positions of three strong runoff zones have been comprehensively explained, and their development depths vary between 5 m and 10 m. The development depth of the epikarst zone has been interpretated by the ground penetrating radar image. According to the development characteristics of epikarst zone in this area, the antenna frequency of the ground penetrating radar has been set as 400 MHz, and the radar wave reflection time has been converted into depth with the combination of the relevant geotechnical dielectric parameters of rock and soil. The development depth of epikarst zone in the slope runoff field is about 2.5 m (about 50 ns, with the speed of 10 cm·ns⁻¹) The lateral variation of development thickness is significant. The accurate depth needs to be calibrated by the results of velocity measurement through drilling. In addition, there are deep karst fractures in the northwest of the slope runoff field, which gradually descend to the southeast and gradually expand to the surface. The research results show that the location of polarization source obtained by the inversion of spontaneous potential data can directly indicate the location of groundwater; therefore, the spontaneous potential method is effective to detect karst water-bearing structures. Combined with the results of high-density resistivity inversion section, the development location and spatial characteristics of strong runoff belt can be accurately interpreted, and the ground penetrating radar can detect the development thickness of epikarst zone and the development of shallow karst fracture structure with high resolution. In short, the integration of ground penetrating radar method, high-density resistivity method and spontaneous potential method can effectively detect the structural characteristics of epikarst zone, address the multi-solution problem, and hence an effective means to interpret the thickness of epikarst zone and to detect the spatial distribution of strong runoff belt.