Research On Coil Excitation Transient Electromagnetic Response Characteristics And Multi-channel Acquisition System

Transient electromagnetic exploration between wells is to place a transmitting coil in one well and an array receiving coil in another well.By controlling the on and off of the current in the transmitting coil,the transient electromagnetic field is excited in the formation,and the receiving coil response is used to identify the distribution of the electrical parameters of the formation.It is necessary to study the transient electromagnetic field distribution and response waveform in the whole space.In this thesis,the one-dimensional transient electromagnetic field response is deduced from the Maxwell equation--both attenuation and phase shift,and its diffusion speed changes with frequency.The transfer matrix method is used to calculate the reflection and transmission waveform in the one-dimensional three-layer medium.Using the analytical solution of the excitation response of the coil in an infinitely uniform medium,the response waveform and the spatial distribution of responses at different times of the near-source distance and the far-source distance are calculated,with three kinds of excitation unit pulse,step,on and off.At the moment of excitation,the amplitude of the response waveform rises rapidly,and then slowly decreases after reaching the extreme value.The response peak time delay increases with increasing distance from the transmitting coil.The attenuation of near-source electromagnetic energy is low,high frequency components are abundant,propagation speed is fast,and the delay of peak time is less.The attenuation of far-source electromagnetic energy is high,high frequency components are few,propagation speed is slow,and the delay of peak time is much.The spatial distribution shape of the transient electromagnetic response at different moments is a radial dipole.With the increase of time,the spatial distribution area gradually expands,and the electromagnetic field diffuses outward.Afterwards,it contracts,and the amplitude of the electromagnetic field gradually decreases during the diffusion process,and the amplitude at the far position is small,the main amplitude of the transient electromagnetic field is distributed near the excitation source.These results provide a reference for understanding the transient electromagnetic response waveform and analyzing the measured data of the instrument.Experimental tests were carried out,and the received waveform of coil was in the same shape as the theoretically calculated waveform,which verified the correctness of the response calculation method.The real-axis integral method was used to calculate the electromagnetic response waveform of different positions,and calculate the spatial distribution of the response of casing well model under unit pulse excitation at different times.Compared with the spatial distribution in infinitely uniform medium,it is found that the existence of casing mainly changes the distribution shape of the formation around the well.However,the distribution in the formation away from the casing is still in the shape of a radial dipole.The results clearly show that the transient electromagnetic field effectively penetrated the thin casing wall,diffused in the formation,and carried the formation conductivity information.A multi-channel acquisition system was developed.The developed acquisition system has a sampling frequency of 4 k SPS,and can carry out 64-channel synchronous acquisition with an accuracy of 24 bits.It can automatically complete the acquisition of transient electromagnetic signals from multiple deep formations.