| With the achievements in the 13th Five-Year Plan,the tunneling and underground engineering construction in China has reached a world-class top-level in engineering amount,construction difficulty,and development speed.During the"14th Five-Year Plan" period,plenty of upcoming engineering construction will be conducted in this country.Among the projects that have been and will be built,there is a large scale of demands on tunneling and underground engineering construction,facing multiple technical and geological challenges and risks,at the present and in the future.Under these circumstances,geological hazards such as water inrush are likely to occur during the constriction,resulting in significant losses of personnel and equipment.Therefore,it is a vital demand and of vital importance to prospect the subsurface source of water inrush,predict the possible water inrush hazard,and protect the engineering construction from danger.It is not only a great need to maintain the safety of people's lives and property,but also an inevitable requirement to face the main battlefield of the national economy.The transient electromagnetic method(TEM)plays an important part in the water inrush prediction method system,which is an effective tool to detect the conductive target and is sensitive to the regional response of groundwater enrichment.The transient electromagnetic field is generated by an artificial pulse in a very short time.The measured TEM signal comes from the subsurface eddy current generated by this short pulse.It is difficult to extract the exact position or boundary geometry of the water-bearing structures from the TEM field because of its rapid diffusion and fast variation.The induced eddy current dissipates in the earth with time,and the signal describing the shape of water-bearing structure gradually decays with time,which makes the TEM response signal "good at"revealing the distribution of water-abundant areas,but "poor" in describing the shape and structure of the water-bearing structure.Therefore,improving the detection resolution of TEM for water-bearing structures has always been the key problem concerned by the majority of scholars.To further separate,extract,highlight and amplify the detection information related to water-bearing structure,and improve the resolution of transient electromagnetic detection,the theory of transient electromagnetic wavefield transform uses mathematical transform to convert the observed data,changes the shape of the signal,and highlights the abnormal information related to the shape and boundary of water-bearing structures.It is an optimal solution to improve the detection accuracy of water-bearing structures,and the research of related theories and methods has important scientific significance and practical engineering value.In this regard,the corresponding research has been carried out at home and abroad,but it is not perfect:? the rapid diffusion of TEM signal is not conducive to fully extract the structural and morphological information of water-bearing structures,so it is urgent to reduce the velocity of fictitious wavefield in wavefield transform,to reduce the wavelength of fictitious wavefield and improve the resolution of fictitious wavefield to the target.? From the perspective of forwarding wavefield transform,the research on the property of wavefield transform is not in-depth,and the effect of forward and inverse wavefield transform on signal needs to be further clarified.? The TEM response signal is good at sensing the internal electrical distribution of the water body,and the extracted fictitious wavefield is good at describing the external shape of the water body.To solve the above problems,this thesis focuses on locating,mapping,and imaging the source of water inrush hazards,by using mathematical derivation,theoretical analysis,numerical calculation,and field test methods.The method that slows down the velocity of the fictitious wavefield,the corresponding wavefield transform method,the forward and inverse wavefield transform method,and their response characteristic were studied.The main research work and achievements of this thesis are as follows.(1)The method of fictitious wavefield velocity reduction and the relationship of wavefield transform in them are introduced.Based on the governing equation of electromagnetic field,the method to slow down the velocity of the fictitious wavefield is given.The relationship between the fictitious wavefield of velocity change and the electromagnetic field in the time domain is established by mathematical derivation.Through theoretical analysis,the main properties of wavefield transform relation and its effect on the signal are studied and summarized,the key parameters to control the velocity of fictitious wavefield are defined,and how to reduce the velocity of fictitious wavefield is answered.(2)The forward wavefield transform method of the TEM and its response characteristics.Through mathematical derivation,the wavefield transform formula from fictitious wavefield to TEM signal is established.Through theoretical analysis,the effect of forward wavefield transform on the fictitious wavefield signal is summarized.Starting from the integral expression of the transformation,the three-step optimization idea is adopted to study and give the range and method of parameters in the process of forward wavefield transform.By analyzing the response of forward wavefield transform,its response characteristics are summarized.It is recognized that the result of wavefield forward wavefield transform has nothing to do with the waveform and frequency of fictitious wavefield,and the influence of fictitious wavefield velocity on the result of forward wavefield transform is clarified;Finally,from the perspective of forward wavefield transform,it is proved that reducing the velocity of fictitious wavefield can improve the resolution of TEM detection.(3)Inverse wavefield transform method and response characteristics of TEM based on fictitious wavefield deceleration.Aiming at the ill-posed problem of inverse wavefield transform,starting from the integral expression of the transform,this paper adopts the idea of three-step optimization and studies the range and method of parameters in the extraction of the fictitious wavefield.Under the condition of ensuring the accuracy,the interval length of integral of kernel function of wavefield transform is reduced,and the accuracy and efficiency of the inverse wavefield transform are improved.The effect of reducing the velocity of fictitious wavefield on the accuracy of fictitious wavefield extraction is studied and verified.The theoretical assumption of improving the resolution of transient electromagnetic detection by reducing the velocity of fictitious wavefield is realized.(4)Water-bearing structure imaging method based on TEM fictitious wavefield extraction.Aiming at the transient electromagnetic signal and the extracted fictitious wavefield,it combines the global apparent resistivity and Born approximation imaging methods,so that the transient electromagnetic signal and the fictitious wavefield can develop each other's advantages and avoid disadvantages,forming a set of imaging method and data processing flow for underground engineering water-bearing structure detection and imaging.(5)Based on the above research,field tests were carried out in the Hanjiang-to-Weihe River Valley Water Diversion Project of Shaanxi Province and Weijiadi Coal Mine in Gansu Province.The detection results are consistent with the actual geological conditions,which verifies the effectiveness and reliability of the method. |
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