Study On Cross-well Seismic Computerized Tomography And Its Applications In Covered Karst Prospecting

Expressway construction in karst developing area, we need exact information of karst. Cross-well seismic computerized tomography is a geophysical exploration method, which use seismic wave to explore the media between wells. It can provide high resolution image of the target.LTI and SIRT are widely used in computerized tomography, as the travel-time tomography between wells is a nonlinear inverse problem which is solved by a series of linearization iterative steps, this problem is usually ill-posed. To achieve a better inversion result, we can start from two aspects:first, giving a better initial model; second, giving a better distribution method for residual errors. The first arrival time which measured in the receive well should be related to slowness field, so we can establish a better initial model after making full use of the first arrival time. When seeking the correction value of the grid in SIRT method, we get the average value of the correction values of the rays which passing through the grid, the average distribution of residual errors makes the convergence of the SIRT method is easy from a mathematical point of view, but the problem of average is usually covered up details, because the same grid has different effect on each ray actually, conversely, the correction value of each ray should not be averaged distributed to each grid. Improved algorithm proposed in this paper is better than the original algorithm by the numerical modeling experiment, the different values of threshold,magnification,weight coefficient in improved algorithm have great influence on the result of CT. Generally, the threshold is relatively large, the magnification value is relatively small, the weight coefficient is also relatively small when the model is complex.Improved algorithm proposed in this paper is used in the exploration of bridge in GUGANG exchange for DA-LIU Expressway, we achieve better results.