| In the mining of mineral resources today,deep prospecting technology attracts more and more attention from academics.The detection capabilities of current equipment and multiresolution data interpretation techniques are,however,severely challenged as the exploration depth increases and the detection environment becomes more complex.Transient Electromagnetic(TEM)has been widely used in various engineering fields due to its advantages of high construction efficiency and sensitivity to low-resistance abnormal objects.It is known that the traditional transmission waveform uses a square wave,but the waveform has a wide frequency band,and the detection effect is greatly affected by low-frequency harmonics.The traditional data interpretation method is apparent resistivity definition,but this imaging technology can only reflect the electrical properties of the underground medium.However,the transient electromagnetic field is diffuse field,and it is not easy to image.According to previous research,on the basis of improving the resolution of the transient electromagnetic diffusion field data,this paper proposes a multi-resolution full-field apparent resistivity imaging method and a multi-resolution pseudo-seismic fast imaging method,which can extract multi-resolution information of the underground targets and realize interpretation quantitatively and qualitatively.Therefore,this paper has contributed to three scientific fields,including transient electromagnetic emission waveforms,transient electromagnetic multiresolution apparent resistivity definition method,and multi-resolution pseudo-seismic fast imaging.The following are the paper’s primary innovative accomplishments:(1)This paper realizes the multi-resolution three-dimensional fast forward modeling method of electrical source transient electromagnetic.First,by comparing the spectrum diagrams of the square wave and the differential pulse,it is clear that the square wave has a wider frequency range and that low-frequency harmonics have a significant impact on the detection effect.In contrast,the energy of the differential pulse is primarily concentrated near the main frequency and contains fewer low-frequency components,which would be easier to identify the underground targets.Thus,the resolution capability of the transient electromagnetic diffusion field is improved by converting the conventional square wave emission waveform into the differential pulse during power supply.In order to balance the relationship between calculation efficiency and accuracy,this paper utilizes a hybrid algorithm of the backward Euler method and the SAI-Krylov subspace technology in the transient electromagnetic threedimensional forward calculation.By calculating the typical layered model,it can be seen that the transient electromagnetic detection technology based on the differential pulse emission waveform has a higher resolution.(2)Based on the transient electromagnetic differential pulse emission waveform,this paper suggests a multi-resolution full-field apparent resistivity definition method.First,the time shift method was adopted to realize the transient electromagnetic one-dimensional forward calculation of the differential pulse emission waveform.The characteristics of the electromagnetic response at various pulse widths,positions,and component levels were investigated as a function of the uniform half-space resistivity,and it was discovered that the curve of the magnetic induction intensity vertical component has a straightforward shape that is suitable for the calculation of the full-field apparent resistivity.Then,according to the inverse function theorem,the iterative formula for the full-field apparent resistivity definition is obtained,and the exploration of the best parameter values follows.In addition,the impact of flying height and receiving point offset on the full-field apparent resistivity definition is investigated.Finally,the five-layer model,the three-dimensional model with low-resistivity blocks,and the thrust fault model were calculated.The results demonstrate that the full-field apparent resistivity definition method based on the differential pulse emission waveform can reflect the resistivity distribution characteristics of the underground medium,and its imaging resolution is better.(3)This paper proposes a transient electromagnetic multi-resolution Born approximation pseudo-seismic fast imaging method based on pseudo wavefield.First,the mathematical relationship between the transient electromagnetic diffusion field and the pseudo wavefield is established.Next,the ill-conditioned equation of the wave field inverse transformation is transformed into a process of integration using a precise integration transform algorithm,meanwhile,the multi-window time-sweeping wave field inverse transformation is applied to obtain the multi-scale and multi-resolution pseudo wavefield data.Following the acquisition of the pseudo wavefield,the wave equation in the time domain is first transformed into a wave equation in the frequency domain.The total wave field is subsequently divided into background field and scattering field,and the total wave velocity is split into background velocity and disturbance velocity.In order to linearize the integral equation,the Born approximation algorithm is adopted after using Green’s theorem,which would obtain the expression between the velocity disturbance and the scattering field.Finally,The Fourier transform also makes it possible to quickly determine the velocity disturbance of the electrical interface.Through the dialectical analysis of the advantages and disadvantages of the Born approximation algorithm,it can be seen that although this method can achieve linearizing the integral equation,the imaging accuracy needs to be further improved in the geological model with large velocity disturbance.Born approximation algorithm only retains the first item in the scattering sequence and loses some useful information,as indicated by the scattering sequence expression.As a result,the first two terms of the scattering sequence are kept in consideration of the recursive relationship between the items,and the expression for the second-order Born approximation is derived.By examining how resistivity and layer thickness changes affect imaging accuracy,it can be seen that the second-order Born approximation algorithm has better imaging accuracy than the traditional Born approximation algorithm.Finally,through the calculation of the twodimensional model,the three-dimensional low-resistance thin plate model,and the thrust fault model,it clearly indicated that the second-order Born approximation algorithm is more accurate than the traditional Born approximation algorithm in locating the underground electrical interface and describing the shape of the abnormal body.(4)This paper shows an electrical source transient electromagnetic multi-component Born approximation pseudo-seismic fast imaging method.First,the sensitivity to resistivity,the offset of the receiving point,and the signal strength of the diffuse field of the magnetic induction intensity vertical component and the electric field horizontal component which parallel to the field source are compared.The detection capacity under various circumstances can reflect the information about the geological structure from various perspectives.While the magnetic induction intensity vertical component is relatively anomalously larger in the target layer with low resistivity and the receiving point under the condition of short offset,the horizontal component of the electric field’s signal strength is stronger and more sensitive to the highresistance layer and long offset.Finally,by calculating the 3D model of underground targets with different electrical characteristics,it can be shown that the electrical source transient electromagnetic multi-component Born approximation pseudo-seismic fast imaging method can reflect the geological structure information more comprehensively.(5)This paper verifies the practicability of full-field apparent resistivity imaging method and second-order Born approximation pseudo-seismic fast imaging method by processing the electromagnetic filed data of Weijiadi coal mine goaf in Gansu Province.First,the general situation of the work area,coal seam characteristics,working methods and layout are introduced,and then the corresponding three-dimensional theoretical model is designed according to the electrical characteristics of the survey area for verification.The calculation results show that the joint interpretation of the two imaging methods can not only obtain the electrical information of the underground medium,and can better describe the geological structure features.Finally,after preprocessing the field data,the joint imaging method can reflect the distribution characteristics of the underground resistivity,the overall trend of the formation and the location of the gob.The imaging outcomes are in accordance with the available drilling data,demonstrating the effectiveness of full-field apparent resistivity imaging and Born approximation pseudo-seismic fast imaging. |
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