A physical model experiment on the hydrogeologic applications of GPR.

摘要

Ground Penetrating Radar (GPR) has been used to image the shallow subsurface of the earth in hydrogeologic applications with environmental concerns. The focus of this study is to: (1) determine the sensitivity of GPR to changes in hydrogeologic conditions, (2) determine the response of GPR to the presence of light non-aqueous phase liquids (LNAPLs), and (3) determine if there is a GPR response to vapor phase LNAPLs. These tests were conducted by simulating certain hydrogeologic conditions with a physical model consisting of fluid injected into a tank filled with sand and gravel. The first model experiment was conducted to investigate the ability of GPR to monitor small changes in the water table level, and the sensitivity of GPR to changes in moisture content in the vadose zone by simulating a rising and falling water table. After the first experiment was conducted with a fluctuating water table model, the second model experiment was conducted by introducing gasoline into the bottom of the model tank to simulate a subsurface discharge from a leaking pipe or tank. Then, a rising and falling water table with gasoline was simulated to investigate the influence on the GPR response of different phases of organic hydrocarbons (gasoline) distributed by a fluctuating water table with gasoline. GPR data were obtained at different stages of changing liquid levels, and measurements were repeated over an extended period of time. A 1-D GPR forward modeling was performed to illuminate the relationship between the dielectric properties and hydrogeologic properties such as porosity and fluid saturations of each medium in the tank model.; The results from the model experiment proved the sensitivity of GPR for monitoring changes in the moisture content of porous media in the vadose zone as a function of time and the effectiveness of GPR for monitoring minor fluctuations of the water table. The results also demonstrate a potential of GPR for detecting possible vapor phase effects of volatile hydrocarbons in the vadose zone as a function of time, and for detecting the effects of residual phase of hydrocarbons in the water saturated system. In addition, the results of this study provide a potential tool of GPR to estimate hydrogeologic parameters such as fluid saturations in the porous media, and provide the basis for a strategy that has the potential to successfully detect and delineate LNAPL contamination at field sites where zones of residual LNAPL are present in the subsurface.