Date of Award
Master of Science (MS)
Changes in barometric pressure propagate into the subsurface where they can affect water level measurements in wells and cause vertical strain. Previous strain sensors were developed to measure vertical strain, but they were limited to measurements at a single depth, making it difficult to evaluate strains from a migrating pressure wave. Technology known as Coherence-length-gated Microwave Photonics Interferometry (CMPI) uses optical fiber sensors to measure strain at multiple locations. It has the potential to detect strain caused by variations in barometric pressure at multiple depths in the subsurface. The goal of this study is to evaluate the feasibility of using this technology to record the changes in vertical strain in the vadose zone from small fluctuations in air pressure. Propagation of these air pressure fluctuations is affected by hydrogeological properties such as water content and permeability. A second goal of this study is to evaluate how these properties affect the air pressure distribution, strain, and gas-phase diffusivity with a long-term goal of using strain to monitor the vadose zone. The focus of this study is on a suite of laboratory experiments that used a sand-filled column with an open head space. The CMPI fiber was packaged and embedded along the axis of the column to measure the strain at multiple locations while air pressure transducers were installed through the wall of the column. Small periodic fluctuations in air pressure, similar to barometric pressure fluctuations, were created in the head space of the column using an audio driven speaker driven at 4 Hz using a sinusoidal signal created by a function generator. These fluctuations propagated along the column where they were measured by the transducers and recorded as functions of time. Initial tests were conducted using dry sand, but then additional tests were conducted after injecting water into the column, which changed the hydrologic and mechanical properties. Another experiment was conducted after creating a thin barrier to air flow, which was designed to simulate a thin layer of saturated ground during rainfall. A final experiment was conducted by changing the grain size of the material to create a heterogeneity in the upper half of the column. The suite of six experiments was conducted to highlight effects of variations in water content and heterogeneities on air pressure and vertical strain. Numerical simulations were also created to evaluate the laboratory data and to provide a baseline analysis for the results. The numerical simulations used the governing equations to linear poroelasticity and two-phase flow with boundary conditions representing the experiments. The conceptual model for strain caused by barometric pressure recognizes that a fluctuating barometric pressure causes fluctuations in the air pore pressure that decrease in amplitude and lag in time with increasing depth. The pressure distribution results in two different loads that cause strain: 1.) a mechanical load as the barometric pressure acts on the ground surface; and 2.) pressure loading in the pore space. The results show that the total vertical strain is a contribution of both of these loads, and this is the strain that was observed in the laboratory. The lab experiments show that both air pressure and strain follow the sinusoidal inputs that propagate with depth. The amplitude of the air pressure and strain decrease as a function of depth in all the conditions that were evaluated. For example, in the dry sand, the amplitude of the air pressure decreases from 25 Pa at the head space to 5 Pa at a depth of 70 cm, whereas the amplitude of the strain decreases from 0.15 to 0.06 µε over the same depth interval. In many cases the amplitudes decrease is an approximately negative exponential functions of depth. When water was injected into the sand and the saturation increased, the amplitude of the air pressure increased in the partially saturated sand but decreased sharply to zero where the soil was saturated. The strain decreased with depth, but the transition from partially to fully saturated conditions had little effect on the strain profile, even though it had a major effect on the air pressure. This apparently is because strain is caused by variations in both water and air pressure. The fluctuating air pressure caused the water pressure to fluctuate, which caused strain throughout the column. Phase delay of the air pressure and the strain both increase as linear to bi-linear functions of depth. The phase delays indicate that pressure propagates at a velocity of 7 to 8 m/s in dry or partially saturated sand, but it drops by more than an order of magnitude in the vicinity of the capillary fringe. The phase delays indicate the strain propagates faster than the pressure. The data indicate that the strain propagates from 1.2 to more than 4x faster than the pressure. Switching from sand to silt in the column caused the velocity of the pressure to drop by roughly half for both pressures, but curiously, it had little effect on the velocity of the strain. The air pressure data from the laboratory experiments was used to estimate the gas-phase diffusivity. Gas-phase diffusivity values for dry sand ranged between approximately 1.4 – 1.7 m2/s whereas it was between 0.5 – 1.5 for the sand that had been wetted and then drained. This slight decrease is particularly likely from the increased water content that was present after the sand had been filled and then drained. A method of estimating the pressure diffusivity using strain was developed, and it indicates the diffusivity was 2.5 m2/s for dry sand and 0.2 m2/s for drained sand. These values are slightly larger to slightly smaller than values estimated from pressure data, but the values are similar enough to be encouraging. Simulations of strain caused by barometric pressure changes were conducted using methods of poroelasticity for partially saturated material coupled to analyses of pressures in two-phase flow. Available parameters (e.g., permeability and elastic modulus) were adjusted slightly for calibration. The results show that the simulations are remarkably similar to the observed pressures and strain in the experiments. This similarity provides a preliminary validation of both the approach used for the simulations, and the methods used to measure strain. One implication of this result is that it may be feasible to interpret strain data by inverting strain data by inverting poroelastic analyses, Simple formulas for estimating permeability and elastic modulus from strain data were developed for the project and give reasonable results that could be used as input for poroelastic inversions. This work indicates that strain caused by barometric pressure can be measured and analyzed. This is important because it provides insights on a process that was poorly understood. These insights will improve the correction of strain measurements for effects of barometric pressure, and they could lead to improve methods for monitoring hydrologic processes in the vadose zone.
Costantino, Olivia Lynn, "Evaluating the Feasibility of Measuring and Analyzing Strain Caused by Changes in Barometric Pressure in the Vadose Zone" (2021). All Theses. 3642.