套管折射波对弹性波CT反演结果的影响探讨

Influence of Casing Refraction Wave on Elastic Wave CT Inversion Result

在弹性波CT测试过程中,为保护钻孔孔壁及发射、接收探头,常常在钻孔中放置套管。根据理论,当弹性波在套管中的速度大于在周围介质中的速度时,会产生折射波,如果把折射波走时参与反演计算,则影响反演结果的准确性。通过在地铁线路勘探中利用钻孔间弹性波CT方法勘察孤石的实例,分析了在低速介质(黏土)中寻找高速异常体(孤石)的复杂情形。实例研究结果表明:当钻孔存在套管,且弹性波在套管中的传播速度大于在套管周围介质中的传播速度时,将会在套管中产生折射波,并且折射波会早于直达波到达检波器;如果以折射波初至时刻进行反演计算,会得到异常体移位、虚假异常体等结果。为此,依据初至起跳相位与正常直达波相反、初至走时为线性这2点来判定折射波,提取正确的直达波并进行反演计算。通过研究可知,该方法能准确判定异常体的位置、范围,效果良好。

In elastic wave computerized tomography(CT)detection,PVC cases are always arranged in boreholes to protect the borehole wall as well as the transmitting and receiving transducers.Refracted wave would be generated when the elastic wave velocity in casing is larger than that in surrounding medium,hence reducing the accuracy of inversion result(dislocated or even false anomalies).In this article,we took a subway line investigation as a research case,and analyzed the complicated detection of high-velocity anomalies(boulders)in low-velocity medium(clay).According to two principles of identifying refracted wave,namely,first arrival travelling time being linear and take-off phase being positive,we could differentiate between refracted wave and direct wave,and pick up the correct direct wave for inversion.The proposed method is efficient and accurate in identifying the location and distribution of anomalies.