Research On Cross-well Electromagnetic Forward Modeling And Imaging Methods

Cross-well electromagnetic detection is a low frequency remote detection method developed on the basis of the single well method.It uses a magnetic dipole source to transmit electromagnetic wave signals at a certain frequency in one well,and the signal is collected at the receiving well as far as a few hundred meters away.Cross-well electromagnetic imaging is based on the amplitude and phase of the signal received,which can be used to obtain the resistivity distribution between the wells through an inversion process.In this paper,the numerical simulation based on the integral equation method is used to simulate the formation model to obtain the electromagnetic response.Then we analyze the effects of resistivity variation and resistivity contrast on the electromagnetic response,to explore the key factors that affect the imaging quality,and provide effective theoretical guidance for the subsequent formation imaging.In this paper,we derive the Green's function of the magnetic field in a uniform formation,and the magnetic field from the magnetic dipole,which lays a theoretical foundation for the numerical simulation of cross-well electromagnetic response and imaging.The Born iteration method is used for formation imaging to obtain the formation resistivity distribution between the wells.To obtain the formation resistivity image,we first compute the electromagnetic response of the given true formation model according to the theoretical formula,and then use it as the input data for the inversion.During the inversion process,the theoretical magnetic field response is calculated according to the given initial formation resistivity.Then continuously modify the formation resistivity until the error between theoretical magnetic field response and the inversion data is within the pre-specified value to finally obtain the inverted formation resistivity image.In this paper,we verify the forward and inversion algorithm through three formation models: a model with an abnormal block,a stratified model and an irregular model.From the inversion results of the model with an abnormal block,the approximate location of the abnormal block distribution can be identified,including the depth of the abnormal block and the lateral position between the wells.At the same time,the target resistivity and background resistivity can be recovered relatively well.Then the imaging effect of the layered formation model is verified,and the layer number,layer boundary and layer thickness of the layered formation model can be determined;finally,in order to verify whether the algorithm is suitable for the imaging of more complex formation models,we investigate three types of irregular models: a "concave" model,a "convex" model and a "cross" model.The imaging can also determine the general shape,the location and the resistivity value of the formation.Through forward numerical simulation and the imaging of three types of models,it theoretically verifies the feasibility of the inversion algorithm.Finally,the imaging on the measured data verifies that the algorithm also applies to real situation.This is of great practical significance in the application of the exploration of oil and mineral resources,and it also has value for the further research on cross-well electromagnetic imaging.