VSP横波速度反演实用性研究

利用VSP转换波旅行时,通过迭代可得到较为精确的横波层速度。本文从理论模型出发,研究不同初值方法、不同旅行时拾取误差及纵波速度误差对横波反演结果的影响。理论模型计算表明:用纵波速度和纵横波速度比估算横波速度值,且以假定地层为均匀介质时的横波速度为初值,对迭代结果不会产生不利影响;转换波旅行时拾取误差对反演结果会造成较大误差;当纵波速度出现误差时,对反演结果的影响仅限于最后一层的计算,对其他地层横波速度的计算影响较小。应用实例说明,地下倾角较小时,横向速度变化不大,同一口井反演出的横波速度相似性较好。

Study of practicality of VSP S-wave velocity inversion

The more precise S-wave 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 P-wave velocities on results of S-wave inversion. The theoretical computation shows that,using P-wave velocity and P-wave-to-S-wave velocity ratio to evaluate the S-wave velocity and supposing the S-wave 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 travail-times produce larger errors in inverted results; when errors appear in P-wave velocities, the influence on inverted results only occurred on the computation of last layer and there is a little influence on the computation of S-wave velocities in other layers. The applied real cases show the similarity of S-wave velocities inverted for same a well is better under the condition of small underground dip and gentle variation of lateral velocities.