Frequency Domian Induced Polarization Based Tunnel Ahead Prospecting Method For Water-bearing Structures

China has become the country with the largest scale,the largest number,and the highest difficulty in tunnel construction.However,there are often major geological disasters such as water gushing during construction,which will bring huge economic losses and even casualties to the tunnel construction.In process of tunnel construction,it is an effective way to ensure the safety of the tunnel construction to detect possible water inrush disaster sources in the direction of tunneling.Therefore,it is very important to carry out research on ahead prospecting methods suitable for tunnel environments.The induced polarization method is sensitive to the response of underground water-bearing structures.This method realizes the exploration of the water body by measuring the IP effect of underground water-bearing medium,which is divided into two types:time domain and frequency domain induced polarization.Among them,the frequency domain induced polarization method(FDIP)has obtained good application in the field water search because of its strong anti-interference performance,and has attracted the attention of many scholars.At present,the FDIP method has not been applied to the tunnel advance detection.Limited by the narrow and complex observation environment of the tunnel,the FDIP method still has several key problems in the aspects of observation configuration,interference removal,and inversion imaging,including:(1)It is urgent to design an observation mode that is suitable for the narrow observation space of the tunnel and can obtain effective information about the water body in front of the tunnel face;(2)Aiming at the electromagnetic and metal interference in the complex tunnel environment,it is urgent to solve the problem of interference removal and effective signal extraction;(3)In the aspect of water-bearing structure inversion imaging,problems such as single-frequency multi-parameter inversion and multi-frequency data simultaneous inversion need to be further studied and solved.Therefore,it is urgent to carry out research on FDIP advance water exploration method for tunnel environment,so as to realize effective detection and imaging of water-bearing structures in front of the tunnel face,and promote its application in practical projects.In view of the above problems,this paper adopts theoretical analysis,numerical calculation,and field test verification methods,with the goal of accurately detecting the waterbearing structure in front of the tunnel face,and systematically carrying out research on the FDIP observation configuration,complex environmental interference removal,and singlefrequency and multi-frequency induced polarization multi-scale and multi-parameter inversion algorithm suitable for the narrow space of the tunnel.A set of FDIP advance prediction methods for tunnel water inrush disaster sources has been formed,and its effectiveness was verified by field engineering tests.The main research work and the results of this paper are as follows:(1)Design of tunnel FDIP observation configuration and analysis of response characteristics of the front anomalies.The FDIP multi-electrode sources array suitable for tunnel narrow space is optimized and designed.The response characteristics of typical waterbearing structures are revealed through numerical tests,which lays the foundation for advanced observation of tunnel FDIP.(2)Interference characteristics and removal method of induced polarization data in a complex tunnel environment.Aiming at the two problems of electromagnetic interference and metal structure interference suppression,an innovative electromagnetic interference removal method based on adaptive optimization variational mode decomposition was proposed,and a metal interference removal method based on the spatial distribution law of single point source electric field was proposed,which realized effective information extraction and provided highquality data for the subsequent inversion research.(3)Tunnel FDIP single-frequency multi-scale multi-parameter inversion method.In this paper,from three perspectives of multi-parameter sensitivity balance,water location,and boundary characterization,a single-frequency and multi-parameter inversion method of tunnel FDIP based on parameter weighting and convolution wavelet multi-scale is proposed,and multi-parameter imaging(direct current resistivity,chargeability,exponent constant and time constant)and boundary characterization of the water-bearing structures are realized effectively.(4)Tunnel FDIP multi-frequency simultaneous inversion method.In view of the difficulties and poor results of multi-frequency data fusion inversion,a multi-frequency data simultaneous inversion method based on multi-grid subdivision and scale weighting is proposed on the basis of single-frequency multi-parameter inversion,and the data processing software of tunnel FDIP advanced detection is developed independently,which provides technical support for the application of this method in practical projects.Based on the above research,a field test of tunnel FDIP advanced detection was carried out by relying on the Central Yunnan Water Diversion Project in Yunan Province.The effectiveness and feasibility of the proposed method were verified by comparing the detection results with excavation exposure results in this paper.