Study On Response Characteristics Of DC Resistivity Method Based On Differential Excitation

In recent years,with the increasing demand for engineering exploration and environmental monitoring,geophysical exploration has played a key role.Among geophysical exploration methods,direct current(DC)resistivity method plays an important role in shallow exploration because of its advantages of complete theory,high construction efficiency and low exploration cost.Only by understanding the response characteristics and resolution of the exploration method to the underground medium,and on this basis,further improving the basic theory and improving the observation system,can we achieve high-resolution and high-precision electrical observation and inversion interpretation of the DC resistivity method,and then fully tap the potential of the application of the DC resistivity method in shallow exploration.Along this line of thought,this paper has carried out the research on the theory and response characteristics of DC resistivity method based on differential excitation,analyzed the distribution pattern of electric field in the underground media under differential excitation,and tried to reveal some laws of electrical characteristics to further improve the exploration theory of DC resistivity method.The specific contents are as follows:(1)The principle of differential excitation DC resistivity method and the response characteristics/resolution of homogeneous layered medium are studied.Firstly,the basic principle of DC resistance exploration and the mathematical principle and working mode of integral/differential excitation device are systematically expounded.Secondly,the expressions of electric field and potential of point source at any position in homogeneous layered medium are derived,and Hankel integral is calculated by using shape function integration method.The difference of response characteristics of traditional sounding array(including Pole-pole array,Pole-dipole array,Schlumberger array,Gradient array,Dipole-dipole array,and so on),Integrating array and Differential array to layered medium(different thicknesses/resistivity)is studied by using onedimensional(1D)forward modeling method.Thirdly,the basic theory of1 D inversion method for layered media is described in detail,and based on the 1D inversion method,the inversion effects of traditional sounding array,Integrating array and Differential array on high/low resistivity geoelectric model are compared and studied.Finally,the maximum resolving power of the Dipole-dipole array and the Differential array to the middle thin layer model(different thickness/resistivity)is studied.These test results show that the resolution of the Differential array to the thin underground layer is superior to other arrays.(2)The three-dimensional(3D)numerical simulation theory of mixed space-wavenumber domain of DC resistivity method is studied.Firstly,based on the idea of "mixed space-wavenumber mixed domain" method,which uses two-dimensional(2D)Fourier transform to transform 3D numerical simulation problems into 1D numerical simulation problems in wavenumber domain for solution,the computational theory of 3D numerical simulation of DC resistivity method applicable to anisotropic media is presented,including basic theory,numerical methods and key steps processing.Then,different numerical examples are used to verify and analyze various properties of the algorithm in isotropic and anisotropic media,including correctness,convergence,efficiency,adaptability,parallelism,and computation of undulating terrain.The test results show that the method has the characteristics of high precision,high efficiency,good adaptability and parallelism,and is suitable for the calculation of undulating terrain and the model with excessive number of nodes.(3)Based on the 3D numerical simulation method of mixed spacewavenumber domain DC resistance method,the abnormal response characteristics of electric field of differential excitation under the 3D model of isotropic medium and anisotropic medium are studied.Firstly,the normalized coefficient of electric field is defined,and the rationality of this parameter is verified by an example.Then,under the condition of isotropic medium,the difference of electric field and electric field normalization coefficient of different arrays(include Pole-dipole array,Dipole-dipole array and Differential array)under high resistance model and low resistance model is studied,and the cause of formation is analyzed.The difference of electric field and its normalized coefficient under different arrays is studied by using the model of no mineralization zone of magmatic intrusive rock body and the model of mineralization zone generated by magmatic intrusive rock body.The research shows that the differential array has rich launch modes,more options than other devices,and its exploration effect is better than other devices.Finally,under the condition of anisotropic medium,based on the model of magmatic intrusive rock mass with resistivity anisotropy in the metallogenic belt,the distribution form and cause of formation of electric field and electric field normalization coefficient of different devices are studied and compared with the results of resistivity isotropy in the metallogenic belt.Based on the prism and sphere model of resistivity anisotropy in half space,the influence of different Euler angles on the electric field and its normalization coefficient is studied by making the background medium isotropic and anisotropic respectively.The results show that the electrical characteristics of the medium have a great influence on the observation results,especially the background medium is anisotropic.Changing the Euler angle of the anomaly body can obtain different observation results.Therefore,the field observation and data inversion need to be combined with the actual geological data in order to better carry out the comprehensive interpretation of the actual geological data.(4)Based on the 3D numerical simulation method of mixed spacewavenumber domain DC resistance method,the abnormal response characteristics of electric field of differential excitation considering the 3D model of induced polarization(IP)effect are studied.Firstly,based on the model of mineralized zone generated by magmatic intrusion,the shape distribution of electric field and its normalized coefficient of Differential array under the condition of isotropic and anisotropic polarizability of the mineralized zone is studied.The results show that the effect of resistivity anisotropy on the observed results is much greater than that of polarization anisotropy.Then,taking well to well azimuthal exploration as an example,the difference of apparent resistivity anomaly response and induced polarization effect of different devices under high resistivity(low resistivity)and high polarizability is studied.The results show that the differential array has better response characteristics and exploration effect.Finally,the results of apparent resistivity anomaly response and IP effect of Pole-polar array and Differential array under different types of background media are studied.It is shown that the background media without IP effect has little influence on the results of apparent polarizability.Therefore,in actual production,the comparison of apparent resistivity and apparent polarizability parameters can be comprehensively considered for the exploration of polarized ore bodies under complex background media,so as to better analyze the spatial distribution characteristics of underground ore bodies and surrounding rocks.There are 116 figures,13 tables and 137 references.