An improved Time-Lapse resistivity tomography to monitor and estimate the impact on the groundwater system induced by tunnel excavation

Time-Lapse Electrical Resistivity Tomography (TL-ERT) is an effective geophysical method for monitoring groundwater migration for its high sensitivity to the conductivity changes of the rock mass. However, existing Time-Lapse resistivity inversion still has two problems to be appropriately improved: false anomalies in initial model propagating into subsequent inversions and the limitation due to equal sensitivity to resistivity variations along different directions. In this paper, Time-Lapse resistivity inversion is improved with two additional approaches. First, prior temporal gradient constraint is applied into Time-Lapse inversion equation to enhance the sensitivity to the vertical or horizontal variation of the model parameters evolved over time during iteration. Second, the observation datasets are normalized with the same ratio before inversion to filter the false anomalies of initial model and to highlight the significant information on model changes over time. The effectiveness of the improvement is verified by a numerical inversion test and a physical model test. Furthermore, this paper describes the successful application of the improved Time-Lapse resistivity inversion method to monitor the groundwater migration caused by water inrush in tunneling located at Cen-xi tunnel, Guangxi Province of China. Surface ERT arrangement in the site has appropriately indicated the existence of the subsurface runoff pathway of underground water, and then Cross-hole ERT were implanted to monitor the groundwater migration. The results acquired by the proposed Time-Lapse inversion method could effectively provide guidance to hazard management and groundwater environmental recovery in the site.