Numerical Simulation Study On Cross-hole Elastic Wave CT Technology For Detecting Goaf

In recent years,various parts of China have entered a period of rapid development in the construction of transportation facilities such as high-speed railways,highways,and subways.When promoting these engineering projects,unfavorable geological bodies such as fault fracture zones,karst caves,goaves,or weak strata are often encountered.Therefore,how to accurately and efficiently detect geological structures is particularly important for ensuring project quality,schedule,and safety of construction personnel.As a high-reliability and high-precision geophysical exploration method,cross-hole seismic CT technology can accurately divide the stratigraphic structure of the target area to meet the needs of early exploration and design.The cross-hole seismic CT numerical simulation test can help us to understand the response of underground undesirable geological bodies in the actual cross-hole seismic CT data through the simulation of seismic wave propagation and numerical calculation.Through the simulation analysis of a variety of complex geological models,it can effectively guide the practical application of engineering.This article systematically summarizes the research status and basic principles of cross-hole elastic wave CT imaging technology,and takes the goaf as an example to conduct detailed numerical simulation research based on various geological conditions that may be encountered in actual exploration work.Multiple geophysical models were established based on variables such as the size,shape,spatial location,and cave filling of the goaf in the formation,including a single completely water-filled abnormal body,a single incomplete water-filled abnormal body,multiple completely water-filled abnormal bodies,simultaneous inversion of porous data,and double-layer complex background velocity.Finally,through the REFLEXW forward and inverse software,the ray tracing numerical simulation experiment based on the Eikonal equation is carried out for the established model.In addition,Tesseral 2D software and Geogiga Seismic Pro software were used to conduct the finite difference wave equation numerical simulation for the complex model,compared and analyzed the inversion results of different software(different methods),and summarized their advantages and disadvantages.The results of numerical simulation show that:(1)The ray tracing numerical simulation based on the Eikonal equation has a high resolution for single or multiple abnormal bodies in the uniform background velocity model,but is not sensitive to the slope and internal filling of the abnormal body.The results of simultaneous inversion of porous data is slightly worse than those of single pair drilling data,but the observation system has been improved through the arrangement of surface reception,nd the imaging resolution has been significantly improved.For the double-layer complex background velocity model,the stratigraphic interface recovers well,but the recovery of local anomalous bodies is poor.(2)The numerical simulation based on the finite difference wave equation shows that the resolution of the single abnormal body in the uniform background velocity model is slightly lower than the numerical simulation of ray tracing based on the Eikonal equation,but the resolution of the abnormal body in the double-layer complex background velocity model is higher.In practical application,the imaging results obtained by the two methods can be comprehensively referred to reduce the non-uniqueness of the solution.