Heat and mass transfer through disturbed soil: Multiscale experimental and modeling investigation.

摘要

Landmines are one of the most prolific, human-made environmental hazards impacting the world. Although there are numerous technologies used to detect buried landmines, none enable a perfect find rate, in part, due to the heterogeneous nature of the environment in which they are buried. Variations in environmental conditions such as soil moisture and climate (e.g., temperature, diurnal fluctuations) impact detection performance. Improved understanding of the environmental conditions associated with minefield emplacement is needed to enable improvement in the algorithms used by detection technologies (e.g., infrared, ground penetrating radar), thus increasing their performance and probability of detection rates. However, there is a lack of understanding of the effect of the mine placement on the heat and mass transfer dynamics in the vicinity of the mine. More specifically, very little is known about how soil disturbance, a process that changes the soil thermal and hydraulic properties of the soil surrounding the mine, due to the placement and burial of the mine effects the soil moisture and temperature conditions in the vicinity of the mine. This is important because understanding these impacts enables increased ability to compare progressively complex models to measured aspects of interest specific to landmine emplacement conditions. The purpose of this research is to better understand the effect of soil disturbance (i.e., loosening the soil) and mixing (i.e., combining different soil types) on heat and mass transfer behavior in the vicinity of buried landmines. The aim is that this knowledge can help future research efforts to improve algorithms associated with various detection technologies. This research integrates a field experiment and numerous laboratory experiments with analytical modeling. In the first task, the thermal conductivity of mixed sands are evaluated at the small scale, providing critical knowledge of the unique behavior. Results indicate that for the test sands studied, knowledge of soil density enables identification of both saturated and dry thermal conductivity which enhances modeling of the thermal conductivity-saturation relationships. Experimental data were used to test thermal conductivity-saturation models. The analytical models varied in their ability to capture the thermal behavior, demonstrating the need for a physically based thermal conductivity-saturation model. The second task compares several approaches used to determine evaporation with several laboratory evaporation and evapotranspiration experiments in an effort to determine an appropriate method that can be applied to studies of landmine detection, specifically, disturbed soil conditions. Results demonstrate that the methods vary in their ability to capture atmospheric versus diffusion dominated evaporative stages for the test soils and boundary conditions studied. Although no one method is applicable for all boundary and initial conditions, the sensible heat balance and heat pulse method enabled the highest level of agreement between measured and modeled evaporation from bare soil experiments. Additionally, the ability of this method to isolate evaporation under evapotranspiration conditions has the potential to isolate transpiration which is significant for many agricultural applications as well as modeling efforts. The third task investigates the impact of soil disturbance and mixing on heat and mass transfer behavior under varying climate conditions at the laboratory scale. Using the methods established in Task 2, I could quantitatively understand the evaporation rates from soils under different conditions (e.g. disturbed or loose conditions compared to undisturbed or tight conditions) using both in-situ and remotely sensed temperature and soil moisture data. Results demonstrate that the disturbance and mixing cause a significant increase in evaporation compared to undisturbed soil conditions. Under disturbed conditions without mixing, the increase evaporation occurred in part to due capillary pumping from the loose soil into the tight soil. Additionally, higher evaporation rates were observed from the upstream tight region compared to the downstream tight region. Finally, the fourth task is a field scale proof of concept demonstration. The purpose of this task is to obtain a data set that includes aspects of tasks 1-3, thus testing our understanding of soil disturbance at the field scale. Experimental results demonstrate distinct behaviors in soil moisture and temperature distributions above and around buried objects that change with climate forcings (i.e., temperature and rain events).