The Detection Method Of Multi-resistivity Data Fusion Three-Dimensional Tomography And Engineering Application

Electrical resistivity tomography(ERT)is a branch of the electrical prospecting method.As it is sensitive to geoelectric targets with different resistivity,it has been widely used in the detection of hydrogeology,geotechnical engineering,mineral resources,environmental engineering,and archaeology.The detection resolution of surface ERT decreases with the depth increases.This method may not be sufficient to accurately locate the interface of the anomalies,and it cannot delineate the individual anomalies distributed side by side vertically.Owing to the limitation of acquisition methods,the 2D inversion profiles of the surface ERT have an inverted trapezoidal shape.The detection resolution of cross-hole ERT decreases with the increase of borehole spacing.If the cross-hole ERT method is used for identifying geological anomalies close to a borehole,the corresponding shape obtained based on the inversion imaging results tends to be distorted considerably.Since this method adopts the principle of "perspective to penetration",it is difficult to determine the anomalies distributed side by side horizontally.As the limited detection range of R shape survey line layout of borehole-surface ERT,the imaging ability of the borehole-surface ERT in the peripheral low-sensitivity area is poor.All the drawbacks cause great interferences when applying to interpret the anomalies distribution in an engineering project.At present,the conventional ERT arrangements are dominated by 2D profiles.If there is a large number of 2D profiles carried out through one survey,it is necessary to combine the specific mileage to achieve a better interpretation of the distribution of anomalies.However,comprehensive geophysical methods are generally tested by comparing the detection results derived from conventional methods.The data derived from different methods are not strongly correlated and the spatial intuitiveness of the 2D inversion results is poor.Aiming at the above problems,this paper conducted a multi-resistivity data fusion 3D imaging based on PCA method.The main research contents are as follows:(1)First of all,according to the spatial distribution of different anomalies in real engineering projects,we established some geoelectric models for numerical simulation;summarized the imaging features of different ERT methods in detecting water-filled karst caves and boulders.Besides,we conducted data fusion of different ERT inversion results based on PCA.It is found that the ERT method is sensitive to the geoelectric targets of physical differences through numerical simulation.Each method has its shortcomings.Inversion data fusion can significantly improve the characterization ability of the single method for anomalies and make the location of adverse geological bodies more accurate.(2)We chose an open space to carry out the outdoors model test.By using the iron bucket filled with water to simulate the karst cave,it is verified the feasibility of the data fusion method applied to practical engineering.The model test shows that the results are consistent with the conclusion obtained from numerical simulation,which verifies the distribution of the anomalies could be more accurately distinguished through data fusion.This method has a good ability to identify multiple geoelectric targets distributed side by side in a shallow layer.(3)In order to solve the poor visibility and data correlation of anomalies in 2D detection profile,we established the 3D coordinate conversion formula of two sets of 2D data inside the tunnel and the surface.The inversion data of the 2D detection profile at different positions under the same engineering environment are put into the same coordinate system to realize the 3D imaging,which improves the data correlation and makes the detection results more intuitive.Secondly,the imaging effect of 3D imaging method is analyzed,and the advantages and limitations of this method are discussed in details.(4)Three kinds of ERT methods were carried out through the projects of water inrush in Xiangxi Daba tunnel and boulder detection in the Dalian subway.By comparing the image features of the single method and inversion results after data fusion,the inversion data fusion makes up for the insufficiency of the method of comparative analysis of different geophysical data results based on experience and makes the comprehensive description of the same target by different geophysical data more accurate.Inversion data fusion improves poor imaging of the cross-hole ERT near the borehole and reduces the distribution of pseudo-anomalies near the electrode.In this study,the vertical resolution of the surface ERT,and the horizontal resolution of the cross-hole ERT were both improved,while the imaging ability of the borehole-surface ERT in the peripheral low-sensitivity area was enhanced.In addition,the 3D imaging of the 2D detection data after fusion is realized by the 3D coordinate conversion formula,which interprets the corresponding detection results more intuitively and comprehensively.The application of this method in the Xiangxi tunnel project and Dalian subway boulder detection proves that this method can overcome the limitations of a single ERT method;delineate the distribution range of anomalies more accurately;provide key technical guidance for tunnel safety construction.Finally,the author summarizes the main research results by comparing the domestic and foreign references and puts forward some prospects.