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ON THE TOMOGRAPHIC INVERSE METHOD USED IN VELOCITY IMAGE RECONSTRUCTION

LIU FU-TIAN LI QIANG WU HUA HU GE LIU JIAN-HUA Institute of Geophysics, Academic, Sinica, Beijing. Institute of Geophysics, State Seismological Burcau, Beijing  
A new kind of tomographic inverse method used in velocity image reconstruction has been put forward in this paper. The authors especially notes that the ST (Seismic Tomography) of seismology and the CT (Computerized Tomography) in medicine are not identical in some respects and we try to deal properly with these difficulties. The analysis of forward problems concerned with velocity image reconstruction shows that the resolving power of the data of different frequency domains is different. Therefore, this difference should be noted in the interpretation of data from regional and distant earthquakes. The discussion here about the velocity image reconstruction involves not only the inverse theory and method telated wth tomographic inversion, but also the analysis for memory storage and operation requirements of the numerical calculation. As for the evaluation of the quality of image reconstruction the authors give the analysis for reliability of the inverse solution which provides a simple, convenient and audio-visual scheme.A comparison of the method of this paper with the chief methods ordinarily used shows that: 1. the velocity discontinuity surfaces permitted to exist in this paper is favourable to reveal the discrepancy of the crustal thickness in different tectonic provinces; 2. the inconsistency between the constant velocity blocks as presumed in the forward problem with the inconstant velocity blocks in the interpretation of inverse results used in ordinary methods has been overcome by the use of interpolation function of the grid velocity as the space function of the velocity in parametrizing the model; 3. as compared with the general methods of Aki et al. the memory storage required in this paper is about an order of magnitude smaller and the amount of computation is reduced to about one half; 4. and compared with the ART methods, which can not give the reliability analysis of the tomographic results, the method of this paper has an outstanding advantage in this respect.
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