Transient Electromagnetic Field Migration Imaging Based On Differential Pulse

Transient electromagnetic method is widely used in the survey and detection of mineral resources.As the exploration and development of mineral resources gradually turn to the deep,it is required that the transient electromagnetic method can achieve better results in the deep exploration of mineral resources.At present,the transmitting waveforms commonly used in TEM are monopulse and wide-band waveforms,and the detection resolution is difficult to meet the increasing exploration demand.Therefore,in order to improve the detection ability of transient electromagnetic emission sources,this paper studies the use of differential pulse as the emission waveform.Its advantages lie in that the waveform has a small band range,main frequency energy concentration,no interference from low frequency harmonics,and higher detection resolution for underground media.At the same time,the differential pulse transient electromagnetic diffusion field is transformed into the wave field,and the virtual wave field obtained from the transformation is used for migration imaging processing,so as to improve the resolution of the transient electromagnetic on the medium structure and physical interface.Firstly,the differential pulse waveform is constructed by the step function,then the differential pulse transient electromagnetic forward modeling can be derived on the basis of the electromagnetic theory of the step wave.Through the spectrum of the analog wave and the differential pulse,it is found that the radiation energy of the differential pulse concentrates near the main frequency and is higher than that of the square wave.According to the skin depth,the detection ability of the differential pulse with more concentrated main frequency is more prominent.Under the same parameters of the layered model and the same pulse width,the transient electromagnetic forward modeling results show that the detection ability of the differential pulse is obviously better than that of the square wave,and the same results are obtained in the simulation experiment of the three-dimensional model.In addition,it can also be found from the differential pulse forward modeling results that each differential pulse width corresponds to a certain depth space detection capability.The differential pulse with different pulse width can realize the shallow to deep scanning detection of the formation,and the detection resolution is higher than that of the traditional square wave.After the differential pulse transient electromagnetic field data is obtained through forward simulation,the electromagnetic field data is converted to virtual wave field by wave field transformation technology.Then,this paper analyzes the migration imaging of differential pulse transient electromagnetic virtual wave field based on Kirchhoff integral method.First,the differential pulse transient electromagnetic forward modeling of the model is completed,and the differential pulse virtual wave field imaging technology is verified by the forward modeling data.Then,a five-layer model containing two low-resistivity layers is established,and the scanning detection of the model is carried out by emitting differential pulses of different pulse widths,and the imaging results at different depth interfaces are obtained.When all the imaging results are correlated,the top and bottom interfaces of the two low-resistivity layers can be presented in the same imaging profile after superposition,and the resolution of each interface is higher than that of the imaging results of a single pulse width.In the three-dimensional simulation experiment,the imaging results with the same effect are obtained,and 3%-5% random noise test is added.The final results show that the transient electromagnetic virtual wave field imaging method based on differential pulse scanning emission can accurately distinguish the location and structure information of the abnormal body,and the method can effectively improve the detection resolution of the transient electromagnetic abnormal body at different depths underground.