The acoustic problem of an eccentric drill collar in a fluid-filled borehole has been of interest in the field of acoustic logging while drilling (ALWD) in recent years. To reduce the effects of tool eccentricity on ALWD measurements, studies on acoustic responses under such conditions are essential. This study therefore has developed an analytical method to investigate borehole wavefields with an off-center monopole ALWD tool in fast and slow formations. By evaluating the contributions of compressional and shear branch points, the effects of the tool eccentricity on individual formation primary and shear head waves are investigated. Results illustrate that tool eccentricity only affects the excitation properties and has almost no effect on the extracted velocities. The joint analysis of synthetic full waveforms and dispersion diagrams with varying eccentricity degrees indicates that multipole modes are excited when the tool is off center, and their excitation amplitudes gradually increase with the increasing eccentricity, especially in the direction of tool movement. Moreover, the dispersion analysis reveals that the two modes with intersections in the centered case are coupled when the tool becomes eccentric. In particular, the coupling performance between the Stoneley and flexural modes is the most prominent. Furthermore, the effects of tool eccentricity on the monopole acquisition method, that is, the sum of waveforms received at four orthogonal azimuths, are evaluated. Results show that the above summation method can effectively reduce the effects of slight or moderate eccentricity. However, for large or extreme eccentricity, reducing or eliminating the effects of eccentricity on the Stoneley wave is a challenge for this method. Based on the above analyses, measurements may not be reliable for formation evaluation in the case of extreme eccentricity, especially for Stoneley wave applications.

中文翻译：

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偏心套环钻进时单极测井的声场

近年来，在充满流体的井眼中偏心钻collar的声学问题在随钻声波测井（ALWD）领域中引起了人们的兴趣。为了减少工具偏心度对ALWD测量的影响，必须在这种条件下研究声学响应。因此，本研究开发了一种分析方法，该方法可使用偏心单极ALWD工具在快速和慢速地层中研究井眼波场。通过评估压缩分支点和剪切分支点的贡献，研究了工具偏心距对单个地层主波和剪切头波的影响。结果表明，工具偏心距仅影响激励特性，而对提取的速度几乎没有影响。对具有不同偏心度的合成全波形和色散图的联合分析表明，当刀具偏离中心时，多极模式会被激发，并且其激发幅度会随着偏心率的增加而逐渐增加，尤其是在工具运动方向上。此外，色散分析表明，当工具偏心时，居中情况下具有交点的两种模式是耦合的。特别是，斯通利模式和弯曲模式之间的耦合性能最为突出。此外，评估了工具偏心距对单极采集方法的影响，即在四个正交方位角接收到的波形的总和。结果表明，上述求和方法可以有效地减轻轻微或中等偏心率的影响。然而，对于大偏心或极端偏心，减小或消除偏心对斯通利波的影响是此方法的一大挑战。基于以上分析，在极端偏心的情况下，尤其是对于斯通利波应用，测量结果对于地层评估可能并不可靠。