Study On The Response Characteristics Of Transient Electromagnetic Fields Of Multi-Turn Small Loops

The multi-turn small loop transient electromagnetic method(TEM),is the main geophysical exploration method for detecting water-conducting structures in Chinese coal mining faces,and it is also widely used in the fields of tunnel advance exploration,geological exploration of urban environmental engineering,and urban underground exploration and development.Compared with the large-loop device,the multi-turn small-loop transient electromagnetic method has its particularity.The most prominent problem is that the actually measured induced voltage attenuates more slowly and lasts longer than the magnetic dipole transient electromagnetic field,which leads to a serious low calculation result based on the late apparent resistivity formula of the magnetic dipole transient electromagnetic field.At present,the research of domestic scholars on this problem is mostly concentrated in terms of time domain induced voltage value.It is believed that the increase in the amplitude of the induced voltage in primary field or other additional fields is the reason for the lower calculated value.The amplitude of induced voltage is reduced by corresponding methods,so that the apparent resistivity is corrected to a level close to the intrinsic resistivity of the strata.Those correction methods,however,brings new problems.The apparent resistivity value is greatly increased while the signal duration remains unchanged,which leads to the inverted depth greatly increased.The measured signal of multi-turn small loop TEM often lasts for tens of milliseconds,and the depth calculated by this kind of method can reach hundreds or even thousands of meters,which is obviously inconsistent with the actual detection distance of multi-turn small loop device.In this thesis,theoretical research,numerical simulation,time-frequency analysis,ground and underground tests are used to systematically study the transient electromagnetic field response law of multi-turn small loop devices from two dimensions of time domain and frequency domain.The characteristics and mechanisms of the influence of the side length,turns,inductance,capacitance and internal resistance of transmitting and receiving coils on the measured induced voltage and apparent resistivity calculation results are revealed.The research results show that the transmitting coil and receiving coil have different influences on the transient electromagnetic field observation signals of multi-turn small loop device.The main results are as follows:(1)The smaller the side length of the transmitting coil,the earlier it reaches the late-time condition,the influence on the induced voltage and apparent resistivity is lower,and the influence time will be shorter.When it comes to the frequency domain,the smaller transmitting coil is,the frequency band of its secondary field signal is wider,which is closer to the frequency band of the signal excited by the magnetic dipole.The frequency band range of the secondary field signal excited by a small loop with a side length of 2 meters is about 10 times that of a transmitting loop with a side length of 100 meters.(2)The influence of turn-off time on induced voltage and apparent resistivity is far greater than that of side length.When sampling is started at the end of shutdown,the early response value of induced voltage decreases with the increase of turn-off time,and the calculated value of apparent resistivity is larger.When sampling at the moment when the current starts to turn off,the early response value of induced voltage increases with the increase of turn-off time,and the calculated value of apparent resistivity is smaller.The influence of turn-off time on the calculation of induced voltage and apparent resistivity decreases exponentially with time.The effect duration is about14～20 times the turn-off time.The longer the turn-off time is,the narrower the frequency band of the secondary field signal will be.When the turn-off time is longer,the high frequency components in the signal will be greatly reduced,which is not conducive to the exploration of the shallow target.Therefore,in engineering practice,the number of turns of transmitting coil should be reduced as much as possible to shorten the turn-off time,so as to reduce the measurement blind area and improve the resolution.There are obvious differences in response characteristics of frequency domain between the measured signal of multi-turn small-loop TEM and the numerical simulation results.The measured one has narrower frequency band and larger lowfrequency response amplitude.(3)The influence of the transient process of the receiving coil on the primary field is related to the damping factor of the receiving coil.The larger the damping factor,the shorter the duration of the primary field after the shutdown,and the smaller the influence on the secondary field.For multi-turn small loop devices commonly used in coal mines,the primary field can be attenuated below the sensitivity level of the instrument about 1ms after the shutdown,and the primary field can be reduced or eliminated from the source by adopting similar conical field source or equivalent counter-magnetic flux method.(4)The transition process of the receiving coil reduces the frequency band of the secondary field induced electromotive force,resulting in the early signal amplitude weakening and the late signal delay.By comparing the calculated results of superposition theory with the measured signals,it is found that the transient process of the receiving coil is the main reason for the slow attenuation,long duration and low calculated value of apparent resistivity of the induced voltage of the multi-turn small loop device.(5)According to the above knowledge,a frequency domain calibration method based on the principle of scale transformation is proposed to eliminate the influence of the transition process of the receiving coil.The principle of scale transformation describes the objective fact that the frequency domain compression multiple is the same as the time domain expansion multiple,and reveals the mathematical and physical mechanism of the influence of the transition process of the receiving coil on the secondary field.By calculating the ratio of the output signal of the receiving coil to the theoretical input signal bandwidth by the frequency domain calibration method,the time domain expansion multiple can be determined,and the measured induced voltage can be corrected on the time scale.This method avoids the defect that the measurement of electrical parameters of receiving coil is influenced by environmental factors,and reduces the influence of subjective factors on data processing results.Engineering examples show that the corrected apparent resistivity calculation and time-depth conversion results are more accurate and objective.