Research Of Correction And Effect Of Transimitting Waveform On The Transient Electromagnetic Response

Transient Electromagnetic Method is a kind of advanced geophysics prospect methods, and is successfully applied to various fields of the deep soundings. However, this technology fails in resolving the shallow target in the vertical direction because of imperfectness of the transmitter, receiver and data-processing. After many years of research, scholars at home and abroad point out that enhancing the ability of shallow identification of this method needs to solve three key issues: reducing the turn-off time of the transmmitter current; shortening the the transition process of the receiver loop; eliminating the effect of transmmitter current wave on the sounding results.Transient response generated by ramp turn-off current is mainly studied in this paper. The transient response of several models under three commonly used measuring devices is analyzed. Based on these responses, the effect of transmitter current wave on sounding and the requirements of transmitter system for shallow measurement is further analyed. A numerical direct method of calculating the apparent resistivity without turn-off effect correction is studied. The main tasks of the thesis are presented as follows:Beginning with the principle of Transient Electromagnetic Method, the application fields, commonly used measurements devices, typical characteristics, as well as the existing problem of this method is introduced. The research status of the effect of transmitter current wave on sounding, turn-off effect correction as well as apparent resistivity calculation methods are introduced and analyed. The scope of these methods and need for improvement is also pointed out.The origin of responses generated by step turn-off transmitter current of several models with three commonly used measurements devices is introduced. Based on these responses, the responses generated by ramp turn-off transmitter current in induction stage and decay stage are derived. From two aspects—the limit and the special relationship between magnetic fields and time derivative of magnetic field,the correctness of the derivation is proved. The effect of stableness and symmetry of transmitter current wave on the second induction voltage is analyed. Taking central loop as an example, the effect of turn-off time on the induction voltage is analyed. The effect of forefront and cycle time of transmmitter current on induction voltage in induction stage and decay stage is discussed. According to the magnetic field and time derivative of magnetic field of three commonly used measuring devices, the relationship among magnetic field, induced voltage and apparent resistivity is discussed. A calculation method for all-time apparent resistivity is proposed. Finally, the practicability of the all-time apparent resistivity direct calculation algorithm is proved with theoretical models and field data experiments.