Resolution Of Loop-source Transient Electromagnetic Method

Because of its sensitivity to low resistivity,loop source transient electromagnetic method has become the preferred method for coal mine hydrogeology survey.It is used to divide water-bearing strata and detect abnormal water-bearing bodies.However,at present,the resolution of the method is not clear,which leads to the lack of theoretical basis for early construction design and the lack of reliability for later data interpretation.In view of this,the induction electromotive force responses of different geoelectric models and different loop lengths are calculated by numerical simulation.With 5% measurement error as the resolution threshold,the relationship between the longitudinal and lateral resolution of TEM and loop lengths is analyzed.The main research results are as follows:(1)For the same geoelectric model,when the stratum parameters of the target layer remain unchanged,when the depth of the thin layer is shallow,the side length of the transmitting loop has a certain influence on the response result of the induced electromotive force.But when the thin layer is deep or thin layer with high resistivity,the sensitivity of the induced electromotive force is basically independent of the side length of the transmitting loop,indicating that the vertical resolution is also related to the side length of the transmitting loop at this time.Length has nothing to do with it.(2)When the side length of the launch loop is fixed,the vertical resolution is mainly related to the formation parameters of the target layer.For a transmission loop with a side length of 100 m,the longitudinal resolution satisfies the relationship min 0 1(z/ h)(28)16.10590809?(r/ r)-16.74251672.For low resistivity thin beds with buried depth of 100 m and electrical property difference of 1:2 from surrounding rocks,it can be distinguished when the thickness of thin beds is greater than 6.44m;for low resistivity thin beds with buried depth of 100 m and electrical property difference of 1:4 from surrounding rocks,it can be distinguished when the thickness of thin beds is greater than 2.1m.(3)The lateral resolution of three-dimensional anomalous body is related not only to the side length of the emission loop,but also to the model stratigraphic parameters of the anomalous body.When the side length of the transmitting loop is fixed at 200 m and the burial depth is fixed at 100 m,the lateral resolution of the transmitting loop satisfies the relationship:0.04520 1 min(/)623.4732Lr re-(28)?.For low resistivity anomalous bodies with a ratio of 1:5 to wall rock electrical properties,it can be distinguished when the size of anomalous bodies is greater than 107m×107m×107m;for low resistivity anomalous bodies with a ratio of 1:10 to wall rock electrical properties,it can be distinguished when the size of anomalous bodies is greater than 91m×91m×91m;for low resistivity anomalous bodies with a ratio of 1:50 to wall rock electrical properties,it can be distinguished when the size of anomalous bodies is greater than 56m×56m×56m.(4)For two anomalous bodies with different intervals of 50m×50m×50m at the same depth,the ultimate resolution is 80 m,and the interval is 60 M.For two anomalous bodies with the same vertical size and resistivity,the result of anomaly resolution is a whole.(5)According to the electromagnetic response characteristics of thin layers with different formation parameters,eight attribute parameters representing the burial depth of thin layers are proposed as independent variables.Sample banks under different parameters are calculated.A total of 3253 sample models are built.The stepwise regression equation of the burial depth of thin layers is established by using SPSS software and stepwise multiple linear regression method.The relative error of the buried depth of the model is within the precision range of the relative error of the buried depth of the model.