| Due to the heterogeneity of karst development,the traditional drilling method is not only time-consuming and laborious in the process of karst investigation,but also because of the limited number of drilling holes,the control point is scarce and the information is small.It is not easy to grasp the development of karst in the surveying area.In order to make the karst detection work more efficient,accurate,economical and comprehensive,according to different physical properties of karst development,a variety of geophysical methods in the actual detection has been widely used,and achieved good detection results and economic benefits.At present,the main geophysical methods for karst detection are transient electromagnetic method(TEM),controllable source audio electromagnetic method(CSAMT),ground penetrating radar(GPR),high density resistivity method,shallow seismic wave method(refraction,reflection),electromagnetic wave CT method,seismic wave CT method,acoustic CT method and drilling digital imaging method and other methods.However,karst belongs to the hidden geological body of the disaster,and it is particularly difficult and complicated to detect.The complexity of the detection object,that is,terrain conditions and boundary conditions are complex.The development of karst along the geological structure is related to the water accumulation,recharge,circulation and excretion conditions,and the karst development intensity of different lithology is obviously different.The applicable conditions of each method are different,the purpose is different,So how to choose the survey method is particularly important.Different group of geophysical methods is choosed based on the geological survey,site condition and physical properties of Datengxia water conservancy in this paper.The foundation of the lock and the powerhouse has been excavated to the bedrock.Its terrian ups and downs is small.Ground penetrating radar method is choosed to detect the overall karst development of the two work areas,electromagnetic wave CT method is choosed as an auxiliary method to identify karst in important works parts.Drainage gate drains,plant tailrace and the lower pilot channel has a large area,large terrain ups and downs,the interference of large construction features.The method of detecting the karst development is carried out by using the transient electromagnetic method with low requirement and low detection depth in the high impedance background,and then the explored anomaly area isselected for drilling verification.Secondly,the theoretical basis and working principle of the three geophysical methods are introduced respectively,Combine the Datengxia water conservancy project to establish the appropriate model to proceed forward modeling with the ground penetrating radar method and the transient electromagnetic method,and inversion calculation of TEM simulation data.Discuss the feasibility of the geophysical method selection in the karst detection of different engineering parts.According to field work practice,the data processing flow and the criterion of anomaly judgment of the three geophysical methods,Based on the analyze of the detected data of the completion engineering parts,and confirm the thickness structure and geological structure of the construction area,and study the karst and soil development scale,direction,category and easy to collapse area,to provide the basis for karst treatment.Finally,through the work of a large number of field work in this project and the experience of other karst area survey,Establishment of the system of geophysical survey methods for different surveying area environments: in the prophase survey process of reservoir,dam area,curtain,underground cavity and other engineering parts,according to the terrain,we can choose the TEM,EH4,CSAMT and resistivity imaging method to detect the development of deep karst,such as the bedrock outcrop in work area,the ground penetrating radar can be used to detect the development of shallow karst.At the same time,the electromagnetic wave CT method,seismic wave CT method,sonic CT method and borehole TV digital imaging method can be used to detect the location,scale,extension and filling of karst in detail. |
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