Elastic Wave CT Technology In Dection Of Karst Grouting Effect

Elastic wave CT method is known as seismic CT method.It computes the distribution rules of velocity in the rock and soil body using much speed info- rmation of seismic wave by special methods.Elastic wave CT can be used to detect the distrIbution and the formation of karst.It is more efficient and accurate than other conventional exploration methods.The combined applicat- ion of drilling and elastic wave CT method may eliminate the potential disaster during engineering survey in karst region. Thereby it can reduce the entire cost of the project.The paper introduces the theoretical basis of elastic wave CT,that is Randon transformation and image reconstruction.Image reconstruction consis- ts of continuous and discrete image reconstruction.Discrete image reconstruct- ion converts into the problem of solving linear equations.Generally,the solving methods of equations adopt iterative algorithms.It concludes back projection technique ( BPT ) , algebraic reconstruction technique(ART), simultaneous iterative reconstruction technique(SIRT) and LSQR technique.The theoretical basis of ray tracing is showed,especially straight ray tracing and linear interpolation ray tracing.The ray tracing has many methods.Different algorithms has differences in applicable conditions, computational efficiency and applicable effect.We can adopt straight ray tracing method when the measured profile is uniform.Or we adopt bending ray tracing method in order to calculate head wave traveltime and path rapidly.While bending ray tracing method is realized by Fermat's principle.In this paper,we adopt simultaneous iterative reconstruction technique(SIRT).Through installing mathematical model, we can get the first break time according to initial velocity and appropriate formula. Using inversion we can get a series of velocity imaging maps. For imaging results,the impact of iterative number, medium speed and ray density are emphasis discussed.Conclusions are drawn as follows.1)SIRT's convergence is good.The larger difference in speed between high-speed and low-speed regions,the faster its convergence.2)In the process of speed imaging inversion, whenΔv =1000m/s,number of iterations should be 300.WhenΔv =2000m/s,it should be 200.WhenΔv =3000m/s,it should be 150. The larger difference in speed between high-speed and low-speed regions,the smaller optimal iteration-number.In practical projects,we can select proper number of iterations according to the above conclusions.3)When ray density is 77 strips per m 2,optimal iteration-number is 150.When ray density is 47 strips per m 2,optimal iteration-number is 200.This shows the convergence is slow when ray density is small.That is to say,number of iteration need be increased to achieve the best results.However,when ray density is 33 strips per m 2,the image can't be the same with actual speed model.This shows ray density should not be too small.4)In velocity imaging maps,when ray density is 77 strips per m 2,first iteration error gradually decreases with iteration number increases.But after optimal iteration number,some increase and some decrease before increasing.This shows error is not certainly minimal when iteration number is optimal iteration number.So we can't infer optimal number based on the iterative error.Finally,according to a project ,the paper introduces working methods, data acquisition system.We design observing system according to above conclusions,and select proper iterative number to image.Then the test results are interpretated.We can see that the application effect of elastic wave CT in karst grouting diction is good.