The Application Of Electric Sounding Method In Detecting The Paleochannel And Fresh-salt Water Interface In Ningbo Plain

Ningbo Plain is located in the south wing of the Yangtze River Delta.With the continuous development of cities,the hydrogeological environment conditions in the study area change accordingly.The study area is located in the northwest of Ningbo city.There are many ancient rivers in the area and a large fresh water body surrounded by saline water.It is an important part of the hydrogeological environment in the study area.At present,the distribution pattern of ancient rivers and the interface between saline and fresh water bodies in the study area are relatively vague,and the investigation degree of integrated hydrogeological environment is relatively low,which restricts the development of land resources and the management of geological environment in the study area.Therefore,the detection of the distribution pattern of paleochannel and the interface between saltwater and freshwater in the study area has become an important basic geological work to be studied and solved urgently.In view of the above problems,this paper puts forward a solution idea based on the geophysical method of symmetrical quadrupole sounding.By means of physical model test,the paleochannel morphology and geophysical characteristics of saltwater-freshwater interface under similar strata in the study area are studied,and the identification of the distribution pattern of paleochannel and the interface between saltwater and freshwater in the study area is proposed.Relevant research results have been validated in ascertaining the spatial distribution of paleochannel in the fault zone and the interface between saltwater and freshwater in the study area.The main research work and achievements of this paper are as follows:(1)Starting from collecting and sorting out the geological data and geophysical characteristics of Ningbo Plain,combined with the analysis of the borehole sounding curve of known boreholes,the general distribution of paleochannel and interface between saltwater and freshwater bodies and the electrical parameters of rock and soil layers in the study area are obtained.(2)Based on the principle of similarity,the paleochannel model of fault zone and the interface model of saltwater and freshwater are established,which are similar to the stratigraphic conditions in the study area.By analyzing the model's one-dimensional sounding curve and two-dimensional inversion resistivity section,the identification characteristics of paleochannel in the fault zone and the interface between saltwater and freshwater are obtained.(3)In order to verify the feasibility and validity of electrical sounding method in detecting the paleochannel in fault zone and the fresh-salt water interface under actual geological conditions,the study area is divided into two survey areas.Through the analysis and processing of the symmetrical quadrupole sounding data collected in the study area,the one-dimensional processing and two-dimensional inversion of the apparent resistivity data of the sounding point are carried out in combination with the GeoElectro data processing system developed by Jilin University.At the same time,a preliminary judgment and explanation are made for the anomalies appearing in one-dimensional sounding curve and two-dimensional inversion resistivity profile.Then combined with the results of physical model tests and regional geological data,it is determined that the ancient rivers in the study area were formed between the last glacial maximum and the early Holocene.The main stream line of the ancient rivers distributes along the line of Laoshifang-Cuijiabu-Dongqianfang-Zhulin Bridge-Linbao,and the interface of saltwater and fresh water distributes along the line of Tangjiacao-Xiaoan-Lengshuicao-Zhoujia-Lujia-Ejiucun.Finally,the feasibility of the symmetrical quadrupole electrical sounding method in detecting the interface between the ancient channel and the saltwater is verified by revealing the strata through boreholes,which lays a foundation for the exploration of the ancient channel in Ningbo Plain and the delineation of the interface between saltwater and freshwater.(4)Based on the analysis of the isopolar apparent resistivity plane map of the electrical sounding data in the area I,the buried depth of the ancient channel is determined to be between 35 m and 75 m,and the inversion depth of the electrical sounding method is determined to be AB/4.The research results provide an important reference for the determination of the inversion depth of the electrical sounding work in Ningbo Plain.