The Numerical Calculation And Data Correction For Electromagnetic Response Under Ramp-step Excitation Of Curved Surface Target

The time-domain transient electromagnetic method has the characteristics of wide detection range and high detection efficiency,and been rapidly developed both domestic and international due to its wide application in fields such as mine exploration,petroleum detection,and groundwater detection.At the same time,the refinement of transient electromagnetic detection and the precise interpretation of data have also become research hotspots.On the one hand,though the finite difference method is common used in the three-dimensional numerical simulation of transient electromagnetic,it often results in step approximation errors owing to the limitations of meshing when simulating curved targets.On the other hand,the conventional processing and interpretation of the electromagnetic data are mostly based on step waveform excitation,and field measured data is affected by the ramp-step effect,which overlapping the primary and secondary induced voltages affected by the transition process.What's more,there is a certain difference between the ramp-step waveform and step waveform response,resulting in the data cannot be processed and interpreted directly.In view of the above problems,this paper proposes a numerical simulation method for 3D curved target objects based on conformal mesh technique.In addition,the ramp-step effect in electromagnetic data is corrected based on the deconvolution algorithm,and the ramp-step waveform excitation response is reverted to the step waveform excitation response.The main research contents are as follows:1)Based on time-domain three-dimensional finite difference method under the step waveform excitation,the equivalent conductivity of the electric field at the edge of the conformal mesh is re-derived combining with conformal mesh technique,and then the iterative equation of the electric field is updated.As a result,the stepped gridapproximation error of the traditional finite difference method is reduced,which realized the time-domain three-dimensional finite difference method of the curved target.2)Proposed a method for determining conformal parameters and an automatic generation method for conformal meshes based on triangular facet modeling.For the triangular prism target model and sphere model,the effectiveness and accuracy of the conformal mesh technique are verified respectively.Based on triangular surface element modeling method,the electromagnetic response calculation of arbitrary curved target is realized.3)Derived the convolution relationship between the ramp-step waveform excitation response and the step waveform excitation response;The SVD method is used to fit the electromagnetic response under the step wave excitation into exponential form,the relative error of the fit is less than 0.04%,and the magnetic field response under the step waveform excitation is obtained by integration;Obtained the ramp-step waveform excitation response by convolution algorithm;And gradually reduces the off time of the emission current,compared with the step waveform excitation response,the correctness of the convolution algorithm is verified.4)Analyzed the influence of the ramp-step effect of the transient electromagnetic signal,including the difference between the response of the ramp-step waveform and the step waveform excitation,and the distortion effect of the transition process of the receiving coil under the ramp-step waveform excitation;A frequency domain deconvolution method based on IFFT is proposed to correct the distortion of the receiver coil transition process;Based on the SVD method,the difference between the ramp-step waveform excitation and the step waveform response is eliminated,and the ramp-step waveform excitation response is reverted to the step waveform excitation response;Based on the numerical simulation of the surface target,construct the simulation data,summarize the correction process of the ramp-step effect,and correct the ramp-step effect of the constructed simulation data.Compared with the original step waveform electromagnetic response,the relative error is less than 2%.The correctness of the ramp-step effect correction method is verified.5)The correction method of ramp-step effect is applied to the measured data in the campuses and the suburbs of Changchun City.The pulsed magnetic field fitting error of the two sets of measured data is less than 3%.The amplitude is greater than the magnitude of the ramp-step electromagnetic response,which can verify the validity of the correction method of the ramp-step effect in the measured data.The research results in this paper can effectively complete the numerical calculation and data correction for electromagnetic response under ramp-step excitation of curved surface target,which provides the basis for the improvement of the accuracy of both the three-dimensional numerical simulation and data interpretation of transient electromagnetic.It provides a foundation for the shallow information detection of underground resources and structures.