A Study On EVRT Responses And Tomographic Analysis Of The Unsaturated Zone Around Cavity In Two-dimensional Space

In the field of hydrogeology and engineering geology, unsaturated zone variation around cavity is still in the stage of theory research. The monitoring of water migration around cavity is lack of techniques. The thesis is based on the physical model established in laboratory and high-density resistance data collected by ERT (High-density Electrical Resistivity Tomography). A study of water content in the spatial and temporal variation in draining processes around underground cavity was performed. In this thesis, a new method named EVRT (High-density Electrical Resistance Relative Variation Rate Tomography) based on ERT was introduced.By completing rounds of experiments, the results reveal the following conclusions.(1) Anomalous body experiments proved the effectiveness of EVRT method, based on which, rounds of NaCl solution experiments was constructed. The results revealed the EVRT method could reflect the change of solution concentration and the migration of NaCl solution. Meanwhile, salinity dela mination was found in the NaCl solution injection experiments by EVRT.(2) By setting different media in two different physical models (half-cylinder cavity model & cuboid cavity model), rounds of draining experiments were made which reflected that the spatial and temporal variation in EVRT around cavity could fit to the draining process in experiments and different media showed different water movement characteristics.(3) By researching the R-t (Resistance-time) curve of points at different positions around cavity, water content variations could be reflected by R-t curve and the water migration velocity could be calculated by the resistance inflection point from R-t curve. In addition, with the calculated velocity and Darcy’s Law, the water permeability coefficient in the media could be calculated, which were of great importance in quantitative research.(4) By studying the EVRT images around cavity at the initial time, the formation and development of unsaturated zone around cavity could be shown. The unsaturated zone formed fast at the very initial time and it didn’t form along cavity or expand around cavity but radiate from the draining holes on cavity. In addition, the extend scale of unsaturated zone were affected by the media characterization around cavity.(5) When ground water was in steady condition, the Seep2D software could simulate the water migration and water head distribution of the physical model, comparing with EVRT images, both methods could reflect the flow line and anomalous body characterization. But Seep2D could only simulate in the condition of steady water movement meanwhile EVRT could reflect water migration in unsteady and dynamic conditions.