Exploratory research on buried karst paleochannels by comprehensive geophysical methods：A case study of Wanhua cave system,Chenzhou,Hunan Province

The research area is located in the upstream of the Wanhuayan Scenic Area. The outcrop strata are the third to fourth member of Carboniferous Yanguanian Stage, upper member of Datangian Stage Shichengzi Formation, and Ceshui member of Datangian Stage Ceshui Formation,of which the lithologies are characterised by shallow-sea carbonate facies strata, i. e. micrite and limestone. It is structurally controlled by the northeast-south west trending compression-torsional fracture and the secondary fault of the New Neocathaysian system, and is also affected by north west-south east trending fault. In the southern part of the study area, there are large-scale intrusive fine grained-,and coarse grained granites and medium-coarse porphyry granite of the Indosinian stage. Groundwater mainly comprises karst water, pore water and a small amount of fissure water. To explore the existence and evolution history of karst ancient rivers in the upper reaches of Wanhua caves for enriching popular science knowledge of the karst caves, the researchers applied comprehensive geophysical methods of high-precision magnetic method, high-density electric method and audio frequency magnetotelluric sounding method (EH4) to explore the upper reaches of the scenic area. The results revealed several magnetic and electrical anomalies. We also have interpreted anomalies and made geological assumptions.The distribution of overburden layer thickness, geological structure and stratigraphic lithology in the study area are well presented by the results from both high density electric and audio geodetic sounding surveys. The high precision magnetic method reveals the granitic boulders deposited underground, which indirectly indicates the presence of karst ancient river channel and restores historical process of its evolution. The whole research idea is practical; and through this study, a approach to applying comprehensive geophysical methods to detect karst ancient river channels can be established.