With the continuous development of China's economy, the construction of highway tunnels, railway tunnels, hydropower construction, and inter basin diversion tunnels has also been accelerated, resulting in a tight time for tunnel construction, shortening the time for early exploration and design, resulting in insufficient time for detailed geotechnical engineering geological survey. However, China's current survey methods are difficult to accurately and comprehensively identify the engineering geology, hydrogeology, and other adverse geological conditions of the entire tunnel [1]. In the actual tunnel construction process, when encountering unfavorable geological areas with large changes in geological conditions such as karst, fissures, faults and complex structures, if relevant disaster prevention and control work is not carried out, geological disasters such as tunnel blockage, deformation, water inrush and collapse are very likely to occur, which will have a huge impact on people's property safety and project progress [2–4]. In order to ensure the safety of tunnel construction, it is necessary to use appropriate means to identify the geological conditions early, so as to prevent them in advance. As an advanced detection means, advanced geological prediction plays an important role in obtaining the surrounding rock information in front of the tunnel head in advance, effectively reducing the impact of adverse geological areas on tunnel construction safety [5–7].
According to different detection methods, the existing tunnel advance geological prediction technologies can be divided into two categories, they are destructive detection and non-destructive detection [8].The method of advanced drilling is often used for damage detection. This method carries out drilling work on the tunnel face, and analyzes the lithology and structure of surrounding rock in a certain range in front of the face of the tunnel face by means of drilling coring. It has the advantages of high detection accuracy and intuitive detection results. However, the detection results based on the limited number of advanced drilling information have the disadvantages of high cost, small detection scope and low representativeness. At the same time, since this method is carried out on the tunnel heading face, it will also affect the tunnel construction progress [9, 10], and it is difficult to popularize it in a large scale.There are many non-destructive detection technologies, such as the ground geological survey method based on the information of surface geological outcrops and geological structures in the study area, the geological prediction of tunnel boring machine (TBM) using the parameter information such as shield machine cutter head speed, cutter head torque, thrust and forward speed [11], and the geophysical detection method based on the physical and structural differences of surrounding rock [12–14].Among them, the tunnel advanced geological prediction technology based on geophysical exploration method is an effective means and main method to guide the tunnel construction mode, reduce tunnel geological disasters and ensure the normal construction of tunnel engineering at this stage with its advantages of fast detection speed, large detection range and low detection cost.
There are many advanced geological prediction technologies for tunnels based on geophysical methods, such as the transient electromagnetic tunnel geological prediction technology. This method is based on the electrical and magnetic differences of the abnormal body. On the tunnel wall or face, the ungrounded loop device or ground wire source is used to launch a pulse magnetic field on the surrounding rock of the tunnel, and the conductive geological body in the target area is induced to generate a secondary field under the influence of the pulse magnetic field, and the detection of water-rich area is realized by the analysis of the secondary field [15–17].The geological radar method is also a widely used advanced geological prediction technology at present. This method carries out detection work on the face and tunnel wall. It uses the high-frequency electromagnetic wave excited by the ground penetrating radar antenna to detect the broken zone, water-rich area and other poor abnormal bodies according to the dielectric constant difference of the target abnormal body. However, in practical application, the detection range of this technology is relatively shallow due to the electromagnetic wave attenuation [18–20].The infrared detection method based on the radioactive difference of abnormal body is a non-contact detection technology. It detects the water-bearing area of the mass within 30m in front of the tunnel face by identifying and analyzing the infrared field changes formed by the radiation of the surrounding rock mass of the tunnel.Among these methods, due to the interference of various metal parts such as steel pipe support and scaffold in the tunnel during the tunnel construction process, and at the same time, for the mechanical construction, the shield machine itself also contains a large amount of metal, the application of tunnel advanced geological prediction technology based on electrical and electromagnetic detection is more difficult [21], and the detection technology based on infrared field change has less impact on the tunnel construction. However, its detection accuracy and results are greatly affected by the humidity and temperature of the detection environment, and the detection results are greatly affected by the outside world.
The seismic detection technology realizes the detection of abnormal body in front of the tunnel by identifying the density difference, velocity difference and rock mass structure difference of surrounding rock. It has the advantages of high detection accuracy, large detection range and small electromagnetic interference by metal pipelines. It is the main technology of high-precision advance geological prediction of the tunnel at this stage [22–25]. However, the energy of the source wavelet generated by the cutter head cutting the rock mass is weak, the effective signal energy contained in the single shot set received on the ground is also weak, and it is difficult to identify. At the same time, compared with the shield tunnel, it is difficult to achieve the continuous excitation of the source, and the reliability of a few seismic signal processing results is poor. Secondly, in order to improve the detection resolution, in the process of seismic interferometry data processing, it is usually necessary to use the wavelet signal generated by the source to deconvolute the signal received on the ground. The imaging results of abnormal bodies are greatly affected by the source wavelet signal to noise ratio and signal phase delay.
In view of the above problems, this paper carries out the research of tunnel advance geological prediction technology based on seismic detectionConsidering the geological complexity, long line and large buried depth of the Daluoshan diversion tunnel, the comprehensive advanced geological prediction is adopted to detect the geological conditions of the surrounding rock in front of the tunnel head.The specific method is to apply TST and geological radar (GPR) two geophysical prospecting technologies to the diversion tunnel project, and use advanced blast holes and advanced drilling to verify, and finally analyze and study the effect of comprehensive advanced geological prediction.