The Comparative Interpretation Of Forward Modeling And Inversion For DC Resistivity Method And The Use Of It

The DC resistivity method is based on the resitivity difference of the crustal rocks and minerals.Through the observation research on the distribution of the electrial field created artificially,the electrical information of the underground was obtained.The distribution of the artificially created electric field was depended on the distribution of the underground media,such as the value of the resistivity,the abnormal body occurrence.The electrial information can be obtained through the observation and analysis of the underground electrical field.But the distribution of the underground electric field is also related with the topographic relief,the space of electrical poles,the boundary effect of the abnormal electrial interface,the combination of abnormal bodies. The exploration of the DC resistivity method is to obtain the resistivity distribution of the underground media.The above influencing factors of the effect of the DC resistivity method can change the resistivity data in different degree.The resistivity data obtained from the DC resistivity method is the comprehensive reflection of the multiple factors.If the analysis of the influencing factors is not profound and comprehensive,the characteristics of the underground media are difficult to distinguish,and,the wrong information and conclusions was got sometimes.In this paper, the finite-element method is used for numerical forward modeling, and,the physical model is used for physical forward modeling.The zody algorithm and the smooth constrained least-squares method are used for one-dimension and two-dimension inversion of the measured resistivity data.In this paper,multiple factors was studied,such as DC current density profile,the near-source data distrubution characteristics,the influence of topographic relie on the electrical field and resistivitycurve,the method to delineate the boundary on two-dimension slope profile,the effect of boundary,the resistivity characteristics of the layered media and space-focal abnormalities on two-dimension geoelectric section.And the influence of the distance between the power-supply poles,topographic relief,the occurrence of abnormalous body,and the relationship between abnormal bodies was discussed in order to distingush the effect of them on resistivity prospecting.The further discussion is done about the lateral resolution ability and the use of "boundary coefficient" to delineate the boundaries of multiple abnormal bodies.Through the forward modeling and inversion and the statistical relationship of different models,a lot of results was obtained.Through the study of the influence of topography on resistivity prospecting, The topography has changed the boundary conditions of the DC electric field. The distribution characteristics of the DC electric field was Changed. In the negative terrain conditions, the current line gathered in the negative terrain,and, the current density-vector increases in negative-terrain area, which performed as higher apparent-resistivity anomaly. In the positive terrain conditions, the current line transpireed in the positive terrain, the current density-vector decreases in negative-terrain area, which performed as lowered apparent-resistivity anomaly. These two types of abnormalities are not the reflection of underground abnormal bodies.They are caused by the change of topography which changesd the boundary conditiom of electric field.The relationship between depth and the power-supply polar distance was discussed in this paper.Through the analysis of the current density-vector profile,depth-conversion parameters were gathered. And the conversion parameters decreases gradually with the increase of power supply polar distance.In pratice, the maxium and minum power supply polar distance should be suitably selected according to the relationship between the exploration depth and powre-supply polar distance.The DC resistivity value is gathered under linear distribution conditions of the current density vector. The near-source electrical field spread in spherical divergence. Through the improvement of the coefficient between potential difference and apparent resistivity,the range of efficient resistivity data can be incresed to four to five spave between the power-supply poles.The change of occurrence of abnormal body can change the apparent resistivity curve in different degree. There is a close relationship between the apparent resistivity curve and the occurrence of abnormal body. Through the analysis of two-dimensional resistivity slope profile, more characteristics can be be delineated about the boundary of the anomalous body and Internal structure of the undergroud media. Through the analysis of the resistivity curves of different forwarding modeling, the horizontal resolution capability of the DC resistivity prospecting was studied. As the depth increases, the resolution capability of the DC resistivity method becomes worsen Nnormally,the resistivity anomalies in deep areas is greater than the true distribution of the anomalous body.Due to the influence of boundary effects, the influence of resistivity interface on the electrical field, the current line and the current density vector is located in the electrical interface and its vicinity within a certain range. On the apparent resistivity and inversion geoelectric section, the boundary effect performed as the transition area of the resistivity.On Geoelectric section, the resistivity value of the boundary is depended on the resistivity values of the bodies which constitute the interface. Through the geoelectric section response characteristics and the statistical relationship of the different resistivity layered electrical interface models, the background space local anomalous body models. The boundary coefficient was introduced and used. Through a large number of physical models and practical applications,it is accurace and practical to use the boundary coefficient to delineate the electrical interface.In this paper, the smooth constrained least squares method was used in order to do the inversion of the measured resistivity data with terrian.Then, the statistical relationship of the boundary coefficient was used in order to delineate the electrical interface. Better practical results is achieved, and the accuracy of the exploration is improved greatly.Through the analysis of the influencing factors on DC resistivity method and the application of a large number of numerical models and physical models, the results of this paper was used in selection of hydropower station site, pipe and cable detection, metal mineral exploration and geological hazard assessment.These effects test and verify the practicality and accuracy of the research in this paper.The results of this article is used to guide the design of practical work, selection for exploration parameters, identifying the original data, the design of data processing, the principle of exploration results interpretion.And, the results is confirmed comparing with the geology and drilling data.The research in this paper have been confirmed to be meaningful for theoretical study and practical application.