Abstract Electromagnetic measurement-while-drilling (EM-MWD) is a real-time monitoring technology for bottom-hole engineering parameters, and applied to underground in-seam horizontal drilling to obtain high accuracy borehole trajectories for enhancing the output of coal bed methane. However, the received signal intensity of EM-MWD is affected significantly by many factors. In this study, a finite-element model for simulating the signal transmission was proposed based on the quasi-static electric field theory, and the reliability of the model was proved by performing a sensitivity analysis and scale-model experiment. Then, factors including full-dimension signal channel, coal seam resistivity and thickness, surrounding rock resistivity, borehole location, electrode location, and bit drilling into surrounding rock were investigated. Results indicate that higher resistivities of coal seam and surrounding rock imply stronger intensity of the received signal, and the coal seam resistivity is the primary influencing factor. When the resistivity of coal seam is less than that of surrounding rock, the intensity of received signal increases with rising the receiving distance and decreasing the distance between borehole and surrounding rock. The received signal intensity of electrode in surrounding rock is higher than that in coal seam under the above condition. Moreover, a sudden change in the received signal intensity will be observed as the bit drills from coal seam into surrounding rock, which is an indicator for determining whether the bit has drilled into surrounding rock. The finite-element model and the findings in this research are helpful for the design, feasibility evaluating and application of the EM-MWD in coal mines.

中文翻译：

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地下煤层水平钻井中电磁随钻测量接收信号强度的影响因素

摘要 随钻电磁测量（EM-MWD）是一种井底工程参数实时监测技术，应用于地下煤层水平钻井，获得高精度井眼轨迹，提高煤层气产量。然而，EM-MWD的接收信号强度受许多因素的影响很大。本研究基于准静态电场理论提出了一种模拟信号传输的有限元模型，并通过灵敏度分析和比例模型实验证明了该模型的可靠性。然后，研究了全维信号通道、煤层电阻率和厚度、围岩电阻率、钻孔位置、电极位置、钻头钻入围岩等因素。结果表明，煤层及围岩电阻率越高，接收信号强度越大，煤层电阻率是主要影响因素。当煤层电阻率小于围岩电阻率时，接收信号强度随着接收距离的增加和钻孔与围岩距离的减小而增加。在上述条件下，围岩电极的接收信号强度高于煤层。此外，当钻头从煤层钻入围岩时，会观察到接收信号强度的突然变化，这是判断钻头是否钻入围岩的指标。本研究中的有限元模型和发现有助于设计，