非对称声源多极子随钻声波测井实验室测量研究

摘要

Currently, acoustic logging while drilling (LWD) mainly generates and employs three measurement modes separately by excitation of a pure monopole, dipole or quadrupole source correspondingly. Due to the complication of the LWD operating environment, we often can not get expected signal by traditional single mode measurement. We propose in this paper an approach to trigger all the modes simultaneously in the borehole using an asymmetric point source. Four receivers are azimuthally distributed around the LWD tool to receive the signals of all modes propagating in the borehole. By combining the signals from the four receivers in different ways we can get monopole, dipole and quadrupole waveforms in every single measurement In this way, we can evaluate formation velocities using multiple modes simultaneously. We tested this method with a scaled borehole and LWD tool model of 1 : 12 in laboratory. And the lab results confirm that we can get effective monopole, dipole and quadrupole signals by anasymmetric source and measure these modes simultaneously. Finally, based on the theoreticalanalysis of the lab data in time and frequency domain, we demonstrated that the formation shearvelocity can be determined by a joint inversion of the monopole, dipole and quadrupole dispersioncurves.%现有的随钻声波测井通常采用三种测量方式:单极子、偶极子及四极子测量模式.分别采用单极子源、偶极子源和四极子源激发单极子、偶极子和四极子信号对地层速度信息进行测量.由于随钻测井环境比较复杂,钻杆在井孔中占据大部分空间,钻杆速度高于地层速度,导致接收到的信号中仪器波能量占主导,无法有效地提取地层的纵横波速度信息.本文探讨一种采用非对称偏心的点声源,一次激发,利用方位分布的四个接收器接收,经过合成后同时分别得到单极子、偶极子和四极子模式波信号的测量方法,为建立同时利用多极模式波信号测量地层纵横波速度的随钻声波测井实用技术提供基础.本文利用1 ∶12的缩比模型在实验室对该方法进行了考察,实验结果证明,可以通过非对称点声源得到有效的地层单极子、偶极子和四极子模式波信号.通过分析和对比实测数据和理论模拟的频散曲线,本文进一步提出利用三种模式波的频散曲线联合反演确定地层横波速度的方法.