Abstract:The more precise Swave interval velocity can be obtained by using travail times of VSP converted wave and iteration. Starting from the theoretical model,the paper studies the influence of different initial value,different pickup errors of travail times and errors of Pwave velocities on results of Swave inversion. The theoretical computation shows that,using Pwave velocity and PwavetoSwave velocity ratio to evaluate the Swave velocity and supposing the Swave velocity in the layer that is uniform medium as initial value,do not produce disadvantageous influence on iterated results;the pickup errors of converted wave travailtimes produce larger errors in inverted results;when errors appear in Pwave velocities,the influence on inverted results only occurred on the computation of last layer and there is a little influence on the computation of Swave velocities in other layers. The applied real cases show the similarity of Swave velocities inverted for same a well is better under the condition of small underground dip and gentle variation of lateral velocities.