Improvement of the traditional transient electromagnetic method and its application to advanced geological forecast of tunnel

In order to ensure the safety of construction personnel and equipment during tunnel excavation, it is necessary to master the development of water-bearing poor geological bodies in front of the tunnel face. The most commonly used geophysical exploration method to detect water-bearing poor geological bodies is the traditional transient electromagnetic method. Since no attention has been paid to the coupling relationship between the detection devices and the geological bodies, the traditional transient electromagnetic method for data acquisition along a single survey line can only launch a magnetic field from a single angle to obtain the detection results from a single angle. Therefore, problems such as inaccurate forecast and missed forecast have always existed. In order to overcome the above problems, improve the accuracy of forecast, the traditional transient electromagnetic method is improved in this paper, based on the well-strong coupling relationship between the detection devices and the geological bodies, a coaxial dipole method with multi-angle detection from horizontal and vertical directions is proposed. By rotating the detection devices to emit magnetic field from multiple angles, the geological bodies that cannot be detected from a single angle can also achieve a well-strong coupling effect with the magnetic field, thus reducing the possibility of inaccurate forecast and missed forecast. The coaxial dipole method has more accuracy than the traditional method because of its multi-angle detection results. The traditional method and the coaxial dipole method were used to forecast in Tianchi tunnel in Jiumian expressway from YK223+135 to YK223+035. The excavation verification shows that the accuracy of the coaxial dipole method for water-bearing poor geological bodies such as water-bearing joints and karst fractures is higher than that of the traditional method. This method can provide some references for similar tunnel advanced geological forecast.