Research And Application Of Shallow Buried Abnormal Body Virtual Source Refraction Tomography

Karst,soil cave,boulder and other shallow abnormal bodies are the main problems affecting urban infrastructure construction.Urban road collapse and high-rise railway pile foundation settlement will seriously endanger life safety.The detection of urban abnormal body is the key and difficult point of urban construction and development.Geophysical exploration is an effective method to detect abnormal body,and the surface wave exploration is the research hotspot in recent years.Combined seismic transverse and longitudinal wave can more comprehensively and stereoscopically invert the velocity structure of abnormal body.Surface wave exploration inversion obtains the velocity structure of stratum transverse wave.Shallow seismic refraction wave is a very effective method to collect shallow seismic longitudinal wave.Due to the influence of low velocity energy absorption and surface noise,shallow refraction is often far offset with low signal-to-noise ratio.The virtual source refraction method based on seismic interferometry reconstructs the virtual source refraction data through cross-correlation and convolution of seismic refraction data,which can effectively improve the signal-to-noise ratio,increase the ray density and improve the inversion accuracy.In this thesis,noise is added to the original data of horizontal interface and undulating interface model,and the virtual source refraction method and conventional denoising method are used for processing and comparative analysis;The forward models of horizontal interface,undulating interface,platform abnormal body and karst abnormal body are established.The model data are processed by virtual source refraction methods such as correlation,superposition and convolution to obtain virtual source data.The tomographic inversion results of virtual source data and original data are compared and analyzed.By comparing the data before and after the virtual source refraction method and the data tomography inversion results,and applying the method to the actual data,the following conclusions are obtained:(1)The seismic data reconstructed by the virtual source refraction method is consistent with the original data.The virtual source refraction method is feasible and effective,which can improve the accuracy of refraction tomography inversion.The results show that the virtual source refraction method is suitable for both horizontal interface and abnormal body.(2)The results show that the effect of virtual source refraction method is better than that of conventional denoising method,and virtual source refraction method can effectively improve the signal-to-noise ratio of data.(3)The actual data are processed by virtual source refraction method,and the results before and after processing are compared with those of tomography inversion.It is proved that the virtual source refraction method has good inversion effect on the actual data.This thesis has 65 pictures,9 tables and 94 references.