Magnetotelluric Method Research Applied In Zhongliang Mountain Tunnel Geologic Survey

The area through which the Zhongliangshan tunnel will pass is located in the middle of the Zhongliangshan Mountains. Zhongliangshan Mountains belong to the hilly areas, the developed valleys which are eroded by the Changjiang River and the Jialing River in the southeast of the Sichuan Basin. Its terrain is controlled by conformation and lithology. Soluble rock, sandstone and mudstone alternatively distribute in the both wings of the Zhongliangshan Mountains's back and form the parallel ridge-valley landscape called"penholder shape". It is described as three parallel ridges which extend along the N-NE alternate with two karst valley. The resistivity discrepancy between soluble rock, fractured zone, karst cave, underground river, weak zone and watery zone with wall rock in the study area affects the attenuation characteristics of electromagnetic induction. It causes the resistivity anomaly. As the advanced steepness of the Zhongliangshan Mountains, dense vegetation and shallow buried depth of the tunnel, it will be more advanced by using the EH4 magnetotelluric method because of the portability of the complete set of apparatus and short observation time in the Zhongliangshan Mountains tunnel's geological survey.We adopted a new wiring to solve the issues that the traditional wiring can't work in the specific landscape conditions such as everglade, irrigated land, pond, etc. According to the EH4 theory, we preprocess the data by IMAGEM software. Analyzing and studying the interference patterns of the time series curve and the impacts on EH4 measurement which came from the various types of disturbances and using the appropriate solution to resolve the problems. Utilizing the proper software to deal with the EH4 data and then get earth-electricity profiles. Comprehensively interpreting the earth-electricity profiles combined with the field records, geological data and drilling data, and then revealing the underground geological structure to achieve the purpose of the geological survey in this area.We got a certain valuable effect in the practice such as the article infra.1. We studied the interference phenomenon in the collection process of the EH4 information caused by the sources such as the high voltage wire with 220,000 volt, the civil high voltage wire with 20,000 volt, field cable, military cable, rock crusher, iron shelf of the high voltage wire, and so on. And we also studied the corresponding solutions to ensure the reliability of the data collection, processing and interpretation.2. We adopted a new wiring to solve the issues that the traditional wiring can't work in the specific landscape conditions.3. Researching the relationship between the changes of the EH4 resistivity with the development level of fractured zone, weak zone, watery zone and karst cave, then we can achieve the earth-electricity profiles and reveal the underground geological structure to realize the purpose of the geological survey in this area.This paper makes some geological surveys about the geological condition of the Zhongliangshan Mountains both in theory and practice, improving the success rates of geologic survey in railway tunnelling. It is supposed that the research of applying EH4 magnetotelluric method to do geologic survey in Zhongliangshan Mountains will arouse more focus among scholars in the field of geophysical exploration.