Study On The Three-Component Apparent Resistivity Calculation Of Grounded Source Transient Electromagnetic Method

In complex terrain areas,loop source transient electromagnetic method exploration faces many problems,such as wire frame laying difficulties,low work efficiency,and the observation data is seriously affected by the terrain,the detection reliability is reduced.The grounded source transient electromagnetic method adopts the electrode power supply mode,and the layout of the emission source is flexible and convenient.The area measurement can be carried out after one-time source distribution,which greatly improves the work efficiency.At present,the processing and interpretation methods of grounded source transient electromagnetic data are still dominated by vertical components.This single component interpretation has less information and low detection accuracy.It is necessary to develop three-component processing and interpretation technology.As the main parameter to characterize the electrical characteristics of abnormal bodies,the accuracy of apparent resistivity calculation is very important for the reliability of transient electromagnetic data interpretation.However,the current three-component apparent resistivity calculation of grounded source transient electromagnetic method is still not perfect,and the traditional early or late calculation methods are difficult to meet the detection requirements.Therefore,it is necessary to develop a three-component all-time apparent resistivity calculation method.Based on this,this thesis studies the calculation method of three-component apparent resistivity of grounded source transient electromagnetic method by combining theoretical analysis,numerical simulation and measured processing.The main research contents and results are as follows:(1)Based on the vector finite element method,the response characteristics of the three components of the grounded source transient electromagnetic method of the typical geophysical model are studied.The simulation results show that the four components of Hx,Hy,Hz and Ex have the characteristics of obvious response signal,easy identification of abnormal characteristics and sensitive response to threedimensional inclined abnormal body.When the inclination angle of the abnormal body changes,the response curves of these four components will have different degrees of depression,center point offset and other morphological changes;when the tendency of the abnormal body changes,the response curves of these four components will show left-right symmetry or central symmetry.(2)On the basis of the translation algorithm,the correction coefficient is introduced,and the correction coefficient t of ?Hx/?t,?Hy/?t and ?Hz/?t,the correction coefficient r/(2+(?))of ?Ex/?t in the all time apparent resistivity calculation are obtained respectively,and the accurate calculation of the multicomponent all time apparent resistivity of the grounded source transient electromagnetic method is realized.(3)The typical geoelectric model is established,and the accuracy of the all time apparent resistivity calculation of each component is analyzed by forward modeling and theoretical calculation.The calculation results show that the apparent resistivity curves obtained by ?Hx/?t,?Hy/?t,?Hz/?t and ?Ex/?t are consistent with the electrical changes of the layered formation,and the calculation accuracy is high.It is also suitable for the apparent resistivity calculation of the non-layered formation model.(4)The related factors affecting the apparent resistivity calculation are quantitatively analyzed.The results show that the resolution of each component is higher at short offset.The shielding effect is weak when the surface low resistivity layer is less than 0.1 times the thickness of the second layer.When the buried depth of the intermediate low resistivity thin layer remains unchanged,and the thickness ratio of the intermediate low resistivity thin layer to the first layer varies from 0.02 to 0.1,the minimum thickness of the thin layer that can be distinguished by each component is between 10-20 m.(5)The three-component apparent resistivity calculation method has been applied in the grounded source transient electromagnetic method detection of Ziyang fault,and good results have been achieved.