Study On Floating Coefficient Space Intersection And Equivalent Current Loop Inversion Of Downhole TEM

The accurate detection of concealed water bodies will be the key and difficult points in the future because of the hydrogeological condition of coal mine in our country is very complicated,and flood prevention and control situation is becoming increasingly grim.Water disaster occurred mainly during the period of tunneling and work-plane stoping.Although geophysical prospecting method has been used to find the abnormal geological body in advance,it has played an important role in ensuring the safe production.However,there are many shortcomings such as short detection(almost 100 m),too many false anomaly,short penetration distance in work-plane(Generally not more than 200m)and insufficient resolution.Moreover,when grouting the aquifer in the roof and floor of work-plane,there is a technical gap in detection the small volume of the water bearing collapse column,karst,fault and other water bodies accurately.Aiming at the deficiency of current technical means,a kind of transient electromagnetic method which receive the induction field in horizontal borehole in coal mine,and use ground loop to excite the primary field.It is called Downhole Time-Domain EM method(Downhole TEM).It can produce enough magnetic moment so that concealed water bodies can be inspired.The measured points in the horizontal borehole which closer to the abnormal body is useful to reduce the decay of inducted secondary field.In theory,the Downhole TEM has a high resolution for distinguish the concealed water bodies.The application of this technology in the advance of the coal mine tunnel excavation can achieve the purpose of using a single borehole to detect the concealed water bodies in front of the tunnel accurately.It also can be used to locate the concealed water bodies around the borehole,which can improve the accuracy and efficiency of exploration.The three components of the excitation field distribution and the anomalous body induced eddy current field are studied as the research object.Formula derivation,three-dimensional finite difference time domain numerical simulation and theoretical analysis are used as research methods.Response characteristics of anomalous body to Downhole TEM,spatial directivity of the three components of the induced eddy current field,space intersection algorithm of the floating coefficient and least square inversion algorithm have been studied.The purpose of these studies is to locate the abnormal body in 3D space by Downhole TEM.The main research results are as follows:(1)The distribution characteristics of the excitation field and the optimal size of the loop source based on the analytical solution of the excitation field are obtained.The analytical solution of magnetic induction intensity of circular loop source,rectangular loop source and square loop source is deduced from Biot-Savart law.The magnetic field of the current carrying loop is calculated,its distribution characteristics are analyzed and summarized.The intensity of the magnetic field generated by the loop source at different depths is calculated.Its distribution characteristics are analyzed and summarized,and the selection method of the optimal source size is formed.The spatial distribution characteristics of the excitation field of the three kinds of excitation sources are basically the same.The magnetic induction intensity is mainly affected by the distance far away from the source.Ordinarily the magnetic field near the loop is the strongest.Magnetic induction intensity decayed with the increase of the distance from the loop source.The distribution of the magnetic field intensity is characterized by strong middle and weak edge,and the contour is circular or elliptic on the plane after reaching a certain distance.In the middle part of the loop source,the excitation field is mainly the vertical magnetic field and the intensity is smaller in the transverse direction.The magnetic field produced by 8 kinds of common size loops at different depths of the ground is calculated by taking the square loop used in actual production as an example.The excitation field generated by the large size loop source is not strong,but it is relatively stable with depth,and the excitation field is stronger.At a fixed depth of the target body,too large or too small size of the loop source can not stimulate the maximum intensity of a magnetic field.The results of this paper is very useful for us to select the size of loop source for the Downhole TEM.(2)The response characteristics of the typical concealed water bodies in the coal mine,and the influence of the three component response with the parameters changes is obtained.The three-component response of the typical concealed water bodies in the coal mine including the collapsed column,the small coal mine water-bearing area,the water-bearing fault and the roof sandstone rich water body,is simulated by the three-dimensional timedomain finite difference algorithm.The abnormalities of each model have obvious threecomponent magnetic anomaly response.The abnormal curve is based on "N" or "V" as the basic shape.The zero point and the extreme point is the position of the center of the abnormality on the line.The three-component anomaly curve form combination and the spatial orientation of the anomaly are the only matching relationships,while could determine the orientation of the anomalous body.The effects of the five factors have been studied by numerical simulation,including loop source size,low resistance cover,anomaly orientation,anomaly scale and distance.By increasing the size of the transmission loop source the response intensity of the total field and abnormal field could be improved,but the increase is gradually weakened,and abnormal field have a limit amplitude.Abnormal response amplitude is decreased with decreasing the thickness of the low-resistivity overburden.The change of the distance between the abnormal body and the line,only changes the amplitude of the abnormal curve,but does not change the shape of the curve.The farther away from the anomaly body,the weaker the abnormal signal.The distinguishable minimum signal strength can be determined by the instrument accuracy and the magnetotelluric noise as the basic threshold.The limit detection depth of the Downhole TEM is discussed from the relative relationship between the strength of the three components of the total field and the minimum signal threshold changed the depth of the survey line.(3)The floating coefficient spatial intersection algorithm suitabling for the horizontal line with different offset is developed based on the spatial directivity of the component vector of the horizontal current loop.By analyzing the three component vector distributions of the XY plane and the XZ plane of the magnetic field radiated by the horizontal current loop in the horizontal line,it is determined that the vector and the center of the current loop have clear orientation on the XY plane and XZ plane.The time domain finite difference algorithm is used to simulate the anomalous field of the anomalous body,and the distribution law of induced eddy current magnetic field with time is analyzed.By comparing the magnetic field generated by the horizontal current loop,it is considered that the external abnormal field can be replaced by the magnetic field radiated by the current loop located inside the abnormal body.By studying the intersection of the three components of the magnetic field on the horizontal line for the horizontal current loop center,the spatial convergence algorithm for the floating coefficient is developed,and the floating coefficient table based on different Y offset is established.When the intersection is made at the XZ plane,the Z component is adaptively adjusted according to the coefficient table,which makes the intersection of the XZ plane more accurate.The results of the four models in the different quadrants verifies the accuracy of the floating coefficient spatial intersection algorithm in the horizontal line.The calculation results show that the floating intersection algorithm is more suitable for the horizontal current loop.The increase of the inclination angle will lead to the decrease of the intersection accuracy.(4)Based on the three-component data of the anomalous field,the least squares algorithm based on current loop theory is used to invert the size,spatial attitude and center coordinates of the anomalous body.According to the data collected in the normal section of the horizontal survey line,the background field of the whole line is obtained by curve fitting using the polynomial fitting algorithm.The anomaly field is obtained by subtracting the background field from the total field data.By using the rotation formula of the three-dimensional cartesian coordinate system,the current loop forward is realized at any inclination angle.The eddy current field inside the anomaly is considered to be an equivalent current loop,which is given a variable such as the coordinates of the center point,the radius,and the tilt angle.The least squares inversion algorithm with constraints is used to invert the variables.The parameters of the current loop are calculated by the minimum fitting error,and the parameters such as the center coordinates,the spatial attitude and the size of the abnormal body are obtained.The inversion algorithm is validated by the data obtained in three ways,such as the plate body data calculated for the MAXWELL software,cube data computed time domain finite difference algorithm,field testing that the aluminum plate is placed on the ground as an abnormal body.The results show that the center coordinates,spatial attitude and size of the current loop obtained by inversion of the plate data are consistent with the model parameters.The center coordinates and spatial attitude of the current loop obtained by inversion of the cube data are in good agreement with the model parameters,but the dimensional results are unstable and there is a certain deviation from the model parameters.The center coordinates and spatial attitude of the current loop obtained by inversion of the field test data are consistent with the model parameters,but there are some phenomena that the dimensional results are unstable and there is some deviation.