Numerical Simulation Of Cross-hole DC Resistivity CT Method In Karst Area

When using the ground resistivity method for exploration,because the detection accuracy and depth are limited,it is often impossible to accurately determine the specific depth and size of the karst.At this time,more accurate exploration methods are needed for detailed investigation.The cross-hole DC resistivity CT method is a new type of geophysical detection technology developed on the basis of the traditional resistivity method.It is based on the electrical difference between the target body and the surrounding rock.The electrode is placed in the hole for detection and acquisition.The potential or apparent resistivity information is obtained to achieve a method of identifying and accurately locating the target.Due to the particularity of the cross-hole observation mode,the detection imaging effect is not only related to the selection of the device,the setting of the hole,but also related to the karst's own properties.Under the comprehensive influence,it has brought great difficulties to the exploration and data processing interpretation.The purpose of this study is to combine the positive and negative numerical simulations to summarize the influencing factors of karst detection resolution and to invert the typical karst bad geological bodies to provide reference for theoretical research and engineering applications.Based on the research of ground DC resistivity method,the working principle,forward modeling theory and inversion theory of cross-hole DC resistivity CT method are studied.The device coefficients of the three devices are derived.The numerical values are developed by self-programming and professional software.simulation.Through the numerical simulation analysis,the abnormal response characteristics of each device are studied.The influencing factors of the detection resolution are studied.The imaging characteristics of the bad geological bodies in the karst area are studied,and the corresponding laws are summarized.In this paper,a single low-resistance and high-resistance model,a low-resistance model,and a horizontal layered model are established.Based on the finite element forward numerical simulation,the apparent resistivity response curve is drawn.When the two-pole device is detected,the low-resistance is expressed as High-resistivity resistivity response,high-resistance body performance as low-view resistivity response,and three-pole and four-pole devices can directly express electrical information of the measured area,response variability of different devices with apparent resistivity,for three devices preliminary understanding of the apparent resistivity response was established.From the perspective of inversion imaging,the influencing factors of the cross-hole DC resistivity CT method are studied.Through the control variable method,the sensitivity of each device is first analyzed,and the characteristics of each device are obtained.Combined with a variety of karst geologic models,the detection resolutions of the two-pole,three-pole and four-pole devices are analyzed to obtain a four-pole device.The comprehensive detection effect is the best.Then the effects of three observation parameters,such as hole spacing,hole depth and electrode distance,on the single anomalous body are analyzed in detail.It is concluded that the hole spacing and electrode spacing are the main influencing factors.And give suggestions: the electrode distance should be less than or equal to the target body diameter to effectively locate the target.Finally,the six karst self-property factors of the cover layer are analyzed,and the detection effects of single or multiple anomalous bodies are respectively analyzed.It is concluded that as the resistivity difference becomes larger,the resolution will increase,and the lateral resolution is sensitive to the change of the low-resistance difference;as the size of the abnormal body increases,the resolution will increase;the method can distinguish the vertical shape.Change;the distance between the anomalous body and the hole will affect the resolution.The closer the hole is,the stronger the abnormal response will be.The closer to the center of the two holes,the better the resolution will be;the depth will affect the resolution,and the resolution will be closer to the surface and bottom of the hole.difference.Recommendation: When setting up the hole,try to make the anomaly body at the center of the hole.When the karst is refined by the cross-hole resistivity CT method,the inversion imaging results are often unsatisfactory.Based on the previous analysis of the influence of device selection,parameter setting and karst's own properties on the detection effect,this paper analyzes the imaging characteristics of different karst target environments in a detailed way using common models close to the actual situation.Finally,the theoretical research is applied to the actual application.In the parameter setting,the selection of the hole spacing and the selection of the electrode distance are referenced,and the imaging feature law is summarized and applied to the inversion image interpretation.Through the example verification of karst detection,it is proved that the cross-hole DC resistivity CT method is suitable for detailed investigation of karst area,and the correctness of the forward and inversion algorithm is verified,and the necessity of numerical simulation imaging research is verified.It is shown that the research in this paper can not only provide reference for the theoretical study of the cross-hole DC resistivity CT method,but also provide the basis for the actual karst exploration,which has theoretical and practical application value.