Countermeasures for Symmetric Effect in Borehole Electrical Resistivity Tomography
|關鍵字:||井孔電探;鏡像效應;聯合反演;加權模型疊加;優化方法;borehole ERT;symmetric effect;combined inversion;weighted model stacking;optimization|
Electrical resistivity tomography (ERT) method, especially surface ERT method has been successfully applied in many fields. Researches about the borehole electrical resistivity tomography (BHERT) method were mostly concentrated on the applications, while ignored the theoretical problems in the BHERT. BHERT method mainly consists of borehole-to-surface and cross-hole method. Symmetric effect which is caused by the symmetrical sensitivity pattern is a special type of shadow effect in the BHERT method. The root cause of the symmetric effect was clarified and two countermeasures for symmetric effect were proposed in this research. Literatures about BHERT were reviewed in this research firstly. The symmetric effect exists when using the conventional configurations. And it was difficult to be removed even if the weighted inversion method was used. Combined data inversion method based on the in-panel/off-panel sensitivity calculation can reduce the symmetric effect, but the selection procedure was complex. Optimized arrays selected through special ranking functions were mainly focused on the enhancement of resolution and sensitivity, while some physical problems in the ERT method cannot be solved by using the optimized arrays. In the part of borehole-to-surface ERT, almost all possible borehole-to-surface electrode configurations were discussed. Through the sensitivity plot and the inversion results of each configuration, it was found that the concurrently exist of one current pole and one potential pole in the borehole would lead to the symmetrical sensitivity pattern in the inverted profile. And it was the root cause of the symmetric effect. An index was firstly defined in this study to evaluate the capability of each borehole-to-surface configuration to resist the symmetric effect. Based on this index and the characteristics of each configuration, optimal dataset was selected to conduct the combined data inversion, and then better results were obtained. Weighted model stacking technique which did not need the pre-knowledge of the subsurface was also proposed to solve the symmetric effect. As for the part of cross-hole ERT, discussions were also performed on all possible cross-hole configurations. And these configurations were compared with the optimized dataset proposed by previous researchers and the mostly used Bipole-Bipole array generated from the AGI software. The root cause of symmetric effect was found to be the same as in the borehole-to-surface measurements and the proposed countermeasures also worked in cross-hole method. Meanshile, inversion scheme of cross-hole ERT was examined and modified in this study. From the comparison of the general and the extended inversion model, the boundary condition in the inversion process was corrected. Above all, this research unveiled the root cause of symmetric effect and proposed effective countermeasures. Further investigations should consider the field noise in order to enhance the data quality.