Transient electromagnetic responses in tunnels with strong interferences and the correcting method: A TBM example
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摘要: 以隧道掘进机（Tunnel Boring Machine，TBM）为例模拟了隧道强干扰环境下，瞬变电磁超前探测的响应曲线，系统分析了异常体（以直立充水断层为例）与掌子面距离、围岩电阻率差异、TBM长度、异常体规模等条件下的曲线特征和影响规律，发现TBM干扰源表现为低电阻率目标特征，其影响主要集中在早期，对于电性差异较大或目标规模较大的低电阻率异常（充水断层）模型能够明显地通过衰减曲线区分.根据电磁场叠加原理，将隧道腔体中包含TBM模型的响应减去纯隧道腔体响应可以获得TBM的响应信号，以此作为干扰背景，从实际包含TBM和充水断层的隧道模型总响应中减除，获得去除TBM干扰的响应信号.通过8组算例进行对比，发现经过校正的衰减曲线与模型计算曲线吻合较好，视电阻率曲线差异相对较小，能够表现探测区域的电性分布情况，确认该方法在不同情况下的适用性.即使在TBM响应计算时给定背景电阻率与实际电阻率差异达到100%的情况下，依然能够通过校正获得合理的响应信号和视电阻率曲线.该方法不仅仅适用于隧道环境，对于其他诸如地面、航空、半航空、海洋瞬变电磁勘探同样适用.Abstract: The transient electromagnetic method is widely used in prediction of water-bearing structures in front of a tunnel face. But its decay curves are easily polluted in the environment with strong interferences, which can cause false abnormal bodies in determination. We take the Tunnel Boring Machine (TBM) as an example to analyze the influence and response characteristics of such a large metal body in tunneling to TEM detection. We obtain the TBM influences on the TEM decay curves. In addition, we establish a method to remove these influences.#br#We use a three-dimensional finite difference time domain modeling algorithm to simulate the complex environment in tunnels. A vertical water-filled fault is designed in front of a tunnel face as the basic model. Models considering different distances between the fault and tunnel face, different resistivity values between the fault and the background, different fault sizes or thicknesses, etc. are calculated and compared with the normal model without the fault and TBM. By analyzing the simulated results, we propose a method to remove the influences from TBM according to superposition principle. The response of TBM is obtained by subtracting an only tunnel model response from a model containing tunnel and TBM responses. We consider this as the TBM influence background. In modeling or potential future field surveys, the TBM influence background can be subtracted from the total decay curves. Then, the TBM influence is removed. We design 8 groups of numerical models to test the efficiency of our method. The method is tested by comparing the decay curves and apparent resistivity between the removed data and the modeling data without TBM inside.By analyzing the modeling results of TBM, we find the following two significant results:(1) The response of TBM in a decay curve is likely a low resistivity target which will of course be a false abnormal body if not corrected in the result. Its influence is mainly in the early time. And (2) the response from the fault is focusing at relatively late time. By this, the responses of the fault and the TBM can be identified for some models with enough size or resistivity differences.We simulate the TEM responses of TBM in tunneling and analyze its characteristics in tunneling. Although the TBM responses pollute the decay curves, the abnormal body responses have different characteristics and can be distinguished from the receiver data. We show a simple method to remove the TBM influences from the total curves by subtracting the simulated responses. The method can be used to remove strong interferences of TEM detection not only in tunneling but also in other application environments including ground, airborne, semi-airborne and marine TEM.
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Key words:
- Transient electromagnetic /
- Tunneling /
- TBM /
- Correcting method /
- Strong interference
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