The Cross-well Seismic CT Technology And Its Application In Railway Karst Survey

The karst information provided by traditional drilling and geophysical methods isoften incomplete in the railway survey. carring out seismic exploration in borehole andapplying the seismic computerized tomography, a clear velocity image is abtained. Thenthe situation of karst is accurately infered by geology interpretation to meet therequirements in high-speed railway survey and design.The paper first reviewed the progress about Cross-well Seismic and introducedresearch status in the field of the ray tracing and tomographic inversion methods. andbased on Fermat’s principle and Snell Snell’s Law,the Linear Traveltime Interpolation(LTI) ray tracing method was analysed in this paper.then the formula of the minimaltraveling time and ray tracing trail was derived in each grid cell.After computing the ray traveling time through the layerd media model, Such alinear inversion equation as AX=B was established. inversion problem was boiled downto strike a large sparse matrix equations. Simultaneous Iterative ReconstructionTechnique (SIRT) Algorithm was elaborated and was chose to solve the equation.The karst in the underground was simulated to abnormal seismic velocitycompared with the normal. therefore, the different mathematical models were constructed.Applying LTI to obtain the ray traveling time and making use of SIRT inversion to getthe slowness of each unit body, images of velocity distribution was built. Throughwhich, the feasibility of applying seismic tomography in the karst exploration wasqualitatively analysed in this paper.Relying on railway karst exploration projects, the physical tests betweenCross-well Seismic CT and well logging was carried out. CT imaging results was verifiedreliable and accurate.because the acoustic and resistivity curve are consistently changed.Through the widely applying Cross-well Seismic CT on Changsha to Kunming railwaykarst survey, the paper proposed a geophysical principle of interpretation about karstexploration and summaried the factors affecting the quality in all aspects from fieldsurvey to data processing. Cross-well Seismic CT technology has good promotion andapplication value in the field of karst and mined investigation in the future.