| China is one of the countries with the most developed karst in the world,especially in Southwest China,including parts of Sichuan,Hubei and Hunan.The development and distribution of karst are mainly controlled by rock properties,geological structure and hydrodynamic conditions.In the construction of underground engineering,the karst often appears the danger of water inrush,mud inrush and collapse,so it is of great significance for human society to find out the location of karst development and take preventive measures.Due to the concealment of karst development and the complexity of detection objects,there are often misjudgments and omissions in a single method,which can be reduced by two or more geophysical methods.In this paper,high-density electrical method,audio frequency magnetotelluric method and ground penetrating radar method are selected to study and apply in combination with the geological conditions of the two work areas,the geophysical properties of karst and the application characteristics of geophysical methods.The development of karst can not be separated from water,so karst caves often show the characteristics of low resistance,and sometimes dry caves also show the characteristics of high resistance.The changes of karst cave fillings make it have obvious characteristics of wave impedance difference with surrounding rock.These physical differences lay a foundation for the application of high density electrical method,audio frequency magnetotelluric method and ground penetrating radar.There are many types of high-density electrical method devices.Combined with a large number of previous engineering examples,it is proved that in the condition of complex background interference and noise,the Wenner device has strong antiinterference and higher reliability of the collected data.Therefore,the numerical simulation of high-density electrical method in this paper is mainly carried out under the Wenner(?,?,? arrangement)device.According to the characteristics of low resistance and high resistance in Karst and the fact that the karst is not a single one in development,the high-density electrical method is obtained through numerical simulation,which has intuitionistic and clear distribution characteristics of underground resistivity.The apparent resistivity profile of karst shows obvious interpretation signs of low resistance trap,high resistance trap and half trap anomaly.At the same time,due to the different arrangement of temperature equipment,there are different Due to the characteristics of exploration,the high-density electrical method has a strong adaptability in Karst Exploration in the face of complex geological conditions in the field.Audio frequency magnetotelluric method(EH-4)is widely used in tunnel karst investigation under complex geological conditions due to its characteristics of large depth,wide range,economy and high efficiency.According to the high and low resistance properties of karst and its development often accompanied by some geological structures,it is obtained by numerical simulation that karst is in the form of trap in TE mode and high and low in TM mode The stopband is abnormal.Because TE mode is suitable for horizontal structure imaging,TM mode is suitable for vertical structure imaging.When facing complex geological objects during karst exploration,comprehensive consideration is often needed to select the best result map to display.In the numerical simulation of GPR method,taking the filling material of karst cave as the model,according to the characteristics of radar wave image,it is concluded that the in-phase axis at the top of karst cave is always horizontal and hyperbolic on both sides.According to the change of wave polarity,the interface between air and water medium in the half filled karst cave is analyzed and identified,which can be used to determine the water content of karst cave qualitatively to some extent;in the dissolution of filling water medium The attenuation of radar wave in the tunnel is serious,which is not conducive to deep imaging.Because the high-density electrical method is suitable for shallow karst exploration,it is intuitive and accurately reflects the distribution characteristics of underground resistivity,and the audio-frequency magnetotelluric method is suitable for the investigation of karst in a wider range and a greater depth.According to the audiofrequency magnetotelluric method,the detection resolution of shallow karst is often low,so the high-density electrical method is often combined to solve the karst exploration with large burial depth and complex geological conditions,such as tunnel karst Measurement.Compared with high-density electrical method,GPR has higher resolution,which can judge qualitatively the filling of karst cave and the water content of karst cave to a certain extent.Generally,the combination of the two methods can improve the recognition rate of karst,that is to say,it solves the problems of inaccurate recognition of high-density electrical method and insufficient radar detection depth.Single geophysical method has its limitations.Using two or more geophysical methods,combined with the advantages and disadvantages or complementary relationship between each method,can improve the accuracy of karst detection.Finally,combined with two typical engineering examples,the comprehensive exploration of high-density resistivity method,GPR method,high-density electrical method and audio frequency magnetotelluric method are carried out,and the comprehensive interpretation of geological geophysics is carried out in combination with geological data.Through the karst exploration in different background,the collection,processing and interpretation process of the relevant method set data are summarized;how to complement the various geophysical exploration methods in the karst exploration in different engineering background is summarized,which has certain reference value for the selection and comprehensive interpretation of the concealed karst exploration geophysical exploration methods. |
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