Study On Excitation And Response Characteristics Of Shallow Magnetic Target

With the continuous construction of urbanization,the continuous exploitation of mineral resources and the continuous improvement of the level of military weapons,the exploration technology of magnetic targets such as underground cavities,minerals,unexploded ordnance and metal pipelines has gradually become more complex.The exploration depth has begun to increase,the exploration types have increased,and the exploration accuracy has also improved,resulting in the detection of underground targets becoming more complex.Therefore,it is of great significance to analyze the characteristics of excitation response of underground magnetic targets.Aiming at the problems existing in the existing detection methods,this paper introduces the transient electromagnetic principle of uniform half space in detail based on Maxwell equations,and proposes array adaptive magnetic excitation detection combined with transient electromagnetic method.A systematic mathematical theoretical derivation is constructed from four aspects: the primary magnetic field generated by the excitation coil,the secondary magnetic field scattered by different magnetic targets,the secondary magnetic field detected by the receiving device and the inversion and positioning algorithm of different magnetic targets.In addition,this paper establishes the central loop detection model by using the lowfrequency electromagnetic finite element technology,applies different waveform excitation signals to the transmitting coil,and observes the law of changing the size of different magnetic targets and receiving coil induction signals under different waveform excitation.Then,bipolar rectangular wave signals with different frequencies,amplitudes and duty cycles are applied to the transmitting coil,and the finite element method is used to calculate the magnetic field distribution generated by the transmitting coil and the induced electromotive force of the receiving coil under different excitation signals.Finally,the relationship between the parameters of the excitation signal and the amplitude of the induced signal,the detection range and the detection depth is obtained.The frequency and amplitude of the excitation signal are directly proportional to the spectrum amplitude of the signal and the induced amplitude of the secondary field.The duty cycle of the excitation signal is nonlinearly related to the amplitude of the signal spectrum.When the duty cycle is 50%,the signal spectrum component is the least and concentrated in the low-frequency part.When the duty cycle is less than 50% or higher than 50%,the spectrum component increases and disperses to the medium and high frequency,which provides a theoretical basis for the actual detection of underground magnetic anomaly targets.Finally,in the laboratory environment,the designed sensor system is used to detect the metal iron pipe.The experimental results show that selecting the appropriate excitation signal can reduce the detection blind area to the minimum.When the diameter of excitation coil is 1m and the diameter of receiving coil is 0.2m,the detection of cylindrical iron pipe with length of1 m,wall thickness of 0.005 m and buried depth of 0 m ? 0.5 m can be successfully realized,which provides a reference for shallow small metal detection.