Study On The Response And Decay Characteristics Of Transient Electromagnetic Field In Layered Whole-space Medium

The layered mine model can approximate the horizontal stratified structure of coalfield,and its response characteristics can directly reflect the full-space effect.And on that basis,the formula of transient electromagnetic response in whole-space of loop source is deduced,and the numerical filtering algorithm is used to calculate the response in whole-space of layer.The response characteristics of half space,4? space of superposition method and typical whole-space and their relations are analyzed,and the influence of device parameters and geoelectric model parameters on transient electromagnetic induced voltage is discussed.The results show that,the accuracy of the whole-space response calculated by combining Hankel filtering algorithm and cosine numerical filtering method is high;the half space can be approximated as the case of high resistance in the upper half space of the whole-space,and the greater the difference between the resistivity in the upper and lower space,the closer the whole-space response is to the half space response;the whole-space response values are greater than the half space response values composed of them,which are obtained by superposition method The 4? space response of is not equal to the whole-space response.For the response of central loop source and coincident loop source,they have the same attenuation law in the late stage,but there is a big difference in the early stage.The early response intensity of the former is between that of the nearest upper and lower level of the loop source,and the early response of the latter does not change with the change of electrical parameters.The electromagnetic attenuation factor and apparent resistivity are used as the interpretation parameters of the whole-space transient electromagnetic field diffusion.On this basis,the characteristics of the whole-space relative attenuation factor and the half space relative attenuation factor after deformation are studied.The correctness of the calculation method of the whole-space response is analyzed according to the theoretical and practical data considering the situation of the roadway.The results indicate that,the whole-space transient electromagnetic is sensitive to the low resistivity body.When the outer layer resistivity of the lower half space is small,the electromagnetic attenuation factor and the relative attenuation factor of the whole-space change violently,which can be divided into two branches: the first and the last.When the delay time is long,the relative attenuation factor of the half space tends to a fixed value on the last branch with the delay time,which indicates that the whole-space medium and the half space medium exist In a multiple relationship.In the interpretation and analysis of the whole-space transient electromagnetic method,the all-time apparent resistivity can not only reflect the horizon information in the early period,but also can be applied in the late period;the roadway has little influence on the whole-space transient response,and mainly in the early period.Based on the relative error of late-time apparent resistivity,a method for calculating the whole-space delay parameters of coincident loop source is proposed.According to the sequence of relative error of late-time apparent resistivity,relative error of late induced voltage and relative error of early induced voltage,the range of early and late delay is obtained.The results show that,the late range is determined by 10% of the relative error of the late-time apparent resistivity,and the time constant value is more than 9.And the relative error of the late induction voltage is 13%.The early range is determined by 13% of the relative error of the early induction voltage,and the time constant value is less than 2.After the single turn loop with equal magnetic moment is equivalent to the multi turn loop,the early and late division rules will not be changed.Applying Duhamel integral,the whole-space transient electromagnetic response under the ramp current is calculated,and the influence of turn off time corresponding to different side length and turn number on the whole-space response is analyzed by using equal magnetic moment.The results indicate that,the transient electromagnetic response in the early,late and whole region of the ramp current is distorted by the influence of the turn off time of the coil;the more turns of the coil with the equivalent small side length is,the longer the turn off time is,the greater the influence on the oblique step response is;the late-time delay correction is conducive to solving the distortion problem caused by the turn off time,but it is not suitable for the long turn off time.