| The geological condition under which modern tunnel construction is carried out is more and more complex. In order to ensure the safety of construction, in the tunnel excavation process, the method of combining advanced drilling with geophysical prospecting is often adopted to make advanced geological prediction towards the front part of the heading. Nevertheless, due to limited space of tunnel heading, the prospecting process is often impacted by the rig, trolley and other interference near the heading. As a result of this, the electromagnetic geophysical method often leads to inaccurate detection results during the prospecting process. What’s more, the existing geophysical methods can hardly determine the fixed position of geological anomalous bodies in the radial direction of the front of the tunnel heading. Therefore, similar to ground-well transient electromagnetic method, heading-drilling transient electromagnetic method receives the electromagnetic wave signal at a certain depth in front of the heading through the advanced holes that were drilled during the construction process of digging drilling ahead. If the receiving probe is near the caves, underground rivers and other geological anomalous bodies, the lateral resolution of the prospecting can be improved, and the distribution characteristics of geological anomalous bodies around the drilling holes can be prospected more effectively; but if the interference caused by the rear part of the heading disappears, signals with higher signal-ratio ratio can be achieved, and at the same time the efficiency and profit of the drilling holes can be largely enhanced. In other words, the drilling cost can be saved. Besides, as a new field of the ground-well transient electromagnetic application, heading-drilling transient electromagnetic method can also extend the application of transient electromagnetic method in the engineering geophysical prospecting.The general idea of this paper is to introduce the ground-transient electromagnetic method into the advanced tunnel geological prediction. By making analogy to ground-well transient electromagnetic principle and method, and comparing the prospecting environments of the tunnel heading prospecting and the ground-well transient electromagnetic method, the basic theory of tunnel heading-drilling transient electromagnetic method will be studied, and a comprehensive exposition of the technical characteristics and applicable device type of the receiving transient electromagnetic method during the drilling process will be made. To study the response of the tunnel face-drilling transient electromagnetic method towards geological anomalous bodies, finite difference time domain differential equation will be used for calculating reasonable time sampling intervals and spatial discretization intervals. Moreover, the 3D Yee cell will be used for making grid subdivision towards the forward model, through which 3D forward modeling of water bearing structure with different size, different shape, as well as different position will be carried out. In this way, heading-drilling transient electromagnetic response characteristics and the basic rules under the effect of different geological anomalous bodies will be summarized and analyzed, and the data processing methods will be presented in this paper. On the basis of numerical simulation, large tunnel advanced geological prediction physical experiment system will be made use of to simulate the response of the tunnel face-drilling transient electromagnetic method towards the abnormal body under real construction environment. Through comparing the difference of the signals achieved by the receiving probe at different depth with different transmitting coils as well as determined by whether there is water bearing structure or not, more true conclusion can be drawn, which can validate each other together with the numerical simulation results. On the basis of the theoretical research and model test, this method will be applied to Cenxi Junchang tunnel heading grouting inspection work, and will be compared with the result of the non-center receiving transient electromagnetic method, through which the reliability and practicability of this method will be proved. Finally, the paper will put forward the conclusion, summarize the innovation, and propose some opinions towards the development direction of heading-drilling transient electromagnetic method and even the underground engineering transient electromagnetic method. |
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