Study On Attenuation Law And Influencing Factors Of Transient Electromagnetic Field In Multi-turn Small Circuit

Transient electromagnetic method(TEM)has been widely used in recent years.It plays an immeasurable role in guiding coal mine production safety.However,the problem that the apparent resistivity readings of multi-turn and small-loop transient electromagnetic field are several orders smaller than the normal values has not been solved,which not only directly affects the reliability of qualitative and quantitative interpretation of mine transient electromagnetic data,but also relates to the scientific code of conduct and safety production responsibility of practitioners.Firstly,the transient characteristics of multi-turn small loop as transmitter and receiver and the transient characteristics of induced electromagnetic field in medium and their coupling relationship are systematically studied.The analytical formulas of transient process of primary field,secondary field and total field are derived,and the laws of primary field,secondary field and total field are analyzed respectively.Then,based on Simulink simulation software,the equivalent circuit of TEM is established.The equivalent circuit of TEM is divided into square wave signal equivalent circuit,transmitting loop equivalent circuit,geological body equivalent circuit and receiving loop equivalent circuit.The equivalent circuit of induction module is established through the actual induction relationship of transmitting,receiving and geological body.After the equivalent circuit is established,by changing the values of inductance,resistance and capacitance of different modules in the circuit,the influence of Nongeological factors such as loop side length,number of transmitting loop turns,series damping of transmitting loop and distributed capacitance on transient electromagnetic field is studied,and the influence of geological factors such as geological body resistance and inductance on the field is also studied.The concept of field translation is put forward to analyze the feasibility of multi-turn small circuit transient electromagnetic method.This paper draws the following conclusions: the influence of loop side length on transient electromagnetic field is mainly affected by the change of time constant.The influence of loop turns on transient electromagnetic field mainly depends on the attenuation law of transient electromagnetic field through the different magnetic moments.With the increase of turns,the magnetic moment and the amplitude of induced electromotive force,the attenuation time becomes longer.Essentially,the values of R,C and L play a role in the equivalent circuit.Series damp of transmitting loop has little effect on transient electromagnetic field.The main reason is that the transmitting loop circuit is under-damped,which makes the tail of signal oscillate and affects signal recognition.The existence of distributed capacitance will increase the amplitude of induction electromotive force,and the time for large capacitance to attenuate to zero will also increase.The increase of loop turns and side length has little effect on the normalized magnitude of secondary field current,but only on its attenuation time.In the case of multi-turn and small circuit,it is insensitive to the change of resistivity of geological body,while in the case of large circuit,it has better response to low resistivity body.The change of inductance of geological body can only get better response in the secondary field under the condition of small loop or large loop.The transient electromagnetic fields with different resistivity values are simulated by Maxwell forward modeling.The attenuation curve of induced electromotive force can well reflect the variation of resistivity of geological bodies,and the calculated apparent resistivity values can accurately reflect the relative multiple relationship of apparent resistivity values.The reason why the apparent resistivity value of multi-turn small circuit device calculated by late apparent resistivity formula is small is put forward.First,the total field value in late period is larger than the secondary field value.The apparent resistivity value calculated by the total field signal can not reflect the real formation information,which results in the apparent resistivity value being smaller.The smaller the degree is related to the ratio of the total field to the secondary field.The larger the ratio of the total field to the secondary field,the more the calculated apparent resistivity value is.The smaller the order of magnitude of resistance value.The second is that the effective area is too small because the length of 2m edge is too small.The signal of induction electromotive force is not enough to compensate for the difference caused by the effective area,so the apparent resistivity value calculated is low.