Characterizing pore networks for use in modified capillary bundle models
This study investigates the use of three-dimensional (3D) soil pore characteristics (volume, surface area, tortuosity) for prediction of saturated hydraulic conductivity (Ks). With a CT toolset capable of directly quantifying microstructure within well defined soils, the author investigates semi-empirical variables used in established fluid transfer models. The most commonly relied upon semi-empirical variable in consulting has been the poorly defined porous media permeability. The use of an incorrect permeability, among other variables, can have a profound impact on calculated hydraulic conductivity and therefore estimated travel distance of pathogens or soluble toxins. Through this presentation, the method for quantifying an intact soil pore network, and use of the derived structural information in a modified capillary model will be demonstrated. The influence of highly connected pore structures will also be discussed and addressed through use of a Dinic operator. A positive correlation between predicted results and laboratory values was observed and encourages further refinement of this method.
X-ray computed microtomography analysis of the influence of different agricultural treatments on the topsoil porosity of a Grey Brown Luvisol from Ontario
One of the most important applications of X-ray computed tomography (CT) for the study of soil is the characterization of the shape and spatial distribution of pores. Analysis of 3D X-ray CT image data, related to different pore categories, can provide insight to soil structural changes, which have implications in water infiltration and soil aeration, resulting from agricultural practices. The aim of this study was to evaluate changes in the spatial characteristics of voids, due to tillage practices, in the Ap horizon of an Orthic Grey- Brown Luvisol (located at the Elora Research Station of the University of Guelph). Undisturbed oriented soil samples were collected from ten plots representing different tillage treatments: spring moldboard plow, spring moldboard plow, cultivate and pack, fall moldboard plow, cultivate and pack, spring tandem disc, no cultivator, fall offset disc, fall offset disc, cultivate and pack, fall chisel plow, cultivate and pack, zero zone till (soys twin rows), zero tillage (long term), and zero tillage (corn residue removed in row, soys twin rows). Since the utilization of standardized classes, in the quantification of similar features, proved to be necessary in order to obtain comparable results, categories of pores, separated according to their size, circularity and orientation were considered in the interpretation of data. Total volume of pores and volume percentage of each class were calculated, revealing substantial differences among the analyzed soil samples.
A Simulation Study on Segmentation Methods of the Soil Aggregate Microtomographic Images
Advances in X-ray microtomography open up a new way for examining the internal structures of soil aggregates in 3D space with a resolution of only several microns. However, processing of X-ray soil images in order to obtain reliable representations of pore geometries within aggregate pore remain to be established. Multiple approaches to the segmentation algorithms used to best separate gray-scale images into pores and solid material. Segmentation of soil volumes requires a combination of multiple interactive algorithms that identify specific properties of the studied features of each volume. Additionally, similar 3D objects with known pore geometries and connectivities are needed to provide specific information that identifies the most accurate segmentation of microtomographic images. The objective of this study was to compare the performance of segmentation methods on simulated soil aggregate images with various porosities as scenarios of the ground-truth standards. Simulations of the soil aggregate images were conducted on pore and solid spaces respectively. For the pore space, taking into consideration of partial volume and other pronounced artifacts, several layers of the pores at different scales were created and overlaid and random Gaussian noises were added. For the solid space, LU decomposition technique on a Gaussian random field with a specified mean and covariance structure was applied on a conditional data set of the known pore space. Several different kinds of segmentation methods, namely, entropy-based methods, indicator kriging methods and clustering methods, were examined and compared based on thresholding criterion such as non-uniformity measure and misclassification error. Majority filtering was applied to smooth the resulting images. We found that clustering methods uniformly outperformed two other methods, especially in the relatively low porosity cases. Moreover, the indicator kriging method performs better in high porosity cases, however, its performance in low porosity cases was not satisfactory due to the lack of generating a bimodal pattern for the histogram of the gray scale images.
Measurement and Analysis of Physical and Hydraulic Properties of Unsaturated Peat Using 3D Micro- CT Scanning
The hydraulic conductivity of unsaturated peat with a wetting phase is controlled by the peat structure which affects the air-filled porosity, pore size distribution and shape. This study investigates how the size and geometry of pores affects the flow of water through peat soil. To examine the effects of pore size and geometric characteristics of the air-filled pores on unsaturated hydraulic conductivity, detailed air-filled pore geometry data were obtained with high resolution (45 μm) 3D X-ray computed tomography and digital image processing. High-resolution 3D X-ray CT images offer a unique opportunity to observe the inner pore structure and distribution on a scale useful for analysis. Pores structure and configuration were found to be irregular, with fractal surface behaviour that controls the rate of water flow through peat soils. The fractal dimension obtained from the box counting method suggests pores with higher pore roughness (smaller values of the fractal dimension) have a higher tortuosity and lower connectivity. Estimates of unsaturated hydraulic conductivities made for the purpose of test the sensitivity of pore-shape and geometry parameters by directly measure the value of the shape coefficients using high resolution, 3D imagery on hydraulic properties of peats and evaluate how the structure of the peat affects its parameterization. We also studied the ability to quantify these factors for different soil moistures to define how the factors controlling the shape coefficient vary with changes in soil pressure. We quantify the pore structures of peat soil that affects the hydraulic conductivity in the unsaturated condition, and demonstrate the validity of our estimation of peat unsaturated hydraulic conductivity by making a comparison with standard permeameter-based estimates. The purpose of this study is to advance the understanding of the mechanisms and factors that control water flow through unsaturated peat soils, using 3D size and geometric distribution of the air-filled pores derived with image analysis. This understanding is important in characterizing peat properties and its heterogeneity for monitoring the progress of complex flow processes at the field scale in peatlands.
The Influence of Vegetation Canopy Structure on Active Layer Thaw Within the Sub-Arctic Discontinuous Permafrost Zone
Much of the sub-arctic discontinuous permafrost zone is dominated by a range in peatland ecosystems, each with their own characteristic soil frost dynamics. Soil thaw within the discontinuous permafrost zones of the Canadian sub-arctic is driven by the surface energy balance. The following study examines the influence of canopy structure on frost table (FT) depth and rates of thaw by: 1. relating measurements of FT depth to canopy structure using airborne scanning light detection and ranging (lidar) and hemispherical photographs taken below vegetated canopies; and 2. quantifying the spatial influences of canopy structural characteristics on the radiation balance (direct and diffuse incident radiation) within raised peat plateaus, connected bogs, fens, and isolated bogs. The results of this study indicate that peat plateaus, being characterised by greater vegetation fractional cover, typically have shallower FT depths (r2 = 0.5, p = 0.03) than locations with lower biomass. Further, average ground surface elevation and canopy height are related to rates of FT thaw (r2 = 0.73, p < 0.01; and r2 = 0.22, p = 0.2, respectively). Within the larger basin, variability in the spatial extent of vegetation biomass has an important influence on cumulative direct and diffuse radiation incident on the ground surface, especially in areas where peat plateaus are adjacent to open fens, connected bogs, and isolated bogs. This indicates that rates of thaw at the edges of peat plateaus and areas surrounding isolated bogs will be exacerbated by increased incident radiation and less shadowing by the canopy, leading to the conversion of peat plateaus to fens or bogs. This hypothesis is tested by comparing the change in peat plateau area coverage in 2000 and 2008 using classified IKONOS imagery (2000) and airborne lidar (2008).
Evaluating the use of hyperspectral remote sensing for large-scale mineralogical determinations in dust-producing regions
The mineralogy of dust producing regions is critical to understanding the influence of dust on climate, the chemical evolution of the ocean, and the dust record in ocean sediments. In arid playa environments, where dust production is high, evaporites are typical components of surface sediments. Yet, soluble evaporites are not stored in geologic archives, such as marine sediments; as a result, we might severely underestimate dust fluxes in the past and, accordingly, misinterpret the effect of dust on climate. In this study, we evaluate hyperspectral remote sensing methods for estimating the soluble evaporite content of dust. Remote sensing is particularly valuable when investigating large-scale processes such as dust production, since it is possible to measure large areas efficiently. As part of our evaluation, we analyze surface samples and ground-based radiometric measurements collected from a large playa system (Black Rock Desert, NV) and compare these to hyperspectral satellite observations (Hyperion, EO-1 platform). X-ray diffraction analyses of surface sediments, coupled with bulk sediment geochemical measurements, are used to evaluate the hyperspectral data. Our goals are to assess the extent to which we can map evaporite distributions using hyperspectral data, and then combine this information with dust models to produce estimates of dust fluxes in arid environments. Since Hyperion data contain a significant amount of noise, we conduct a model study for comparison in which we generate synthetic surface reflectance data, reconstruct at-sensor radiance, and correct for atmospheric effects to recover surface reflectance. Since we are interested in dust sources globally, we apply our technique to Hyperion data from the Bodélé Depression in Chad, considered to be the most productive dust source on Earth.