Existing models of the hydrogeology in the Blue Ridge Province in the eastern United States generally assume a simplified two-layered system consisting of shallow unconsolidated and relatively homogeneous and porous regolith with a water-table aquifer that slowly supplies water downward to the underlying variably fractured crystalline bedrock. In these models, interconnected fractures in the crystalline bedrock act as conduits for predominantly downward vertical and limited horizontal flow. Fracture density is depthà limited and correlated with proximity to topographic lineaments. Current models consider the porous regolith as the primary water storage reservoir for the entire aquifer system. In this research, detailed hydrogeologic studies in the Blue Ridge Province in Floyd County, Virginia reveal a substantially different framework for groundwater flow. Recent acquisition of two-dimensional surface resistivity profiles collected using a variety of array techniques combined with borehole geophysical logs revealed new insights into this geologically complex province. Dipole-dipole arrays were particularly important in gathering high resolution resistivity profiles that document horizontal and vertical resistivity variation reflecting changes in subsurface geology and anomalous low resistivity areas in crystalline bedrock associated with fault zones. The shallow regolith contains unsaturated areas and also localized sand and clay prone facies with water table and confined aquifer conditions residing locally. Hydraulic heads between the shallow aquifer and the deeper fractured bedrock aquifer can vary by 20 m vertically. Within the crystalline bedrock are anomalous lower resistivity intervals associated with ancient fault shear zones. Brecciated rock adjacent to the shear zones, and the shear zones themselves, can be hydraulically conductive and serve as pathways for groundwater movement. Aquifer testing of the regolith-bedrock fracture system occurred over a 6-day period and produced rapid and relatively uniform drawdowns in surrounding wells completed in the fractured bedrock aquifers. The shallow aquifers experienced minimal drawdowns from the aquifer test indicating low vertical hydraulic conductivity and limited communication between the shallow and deeper bedrock aquifers. Water chemistry and chlorofluorocarbon (CFC) age dating analyses indicated significant differences between water samples from the shallow and deep aquifers. A new conceptual model for Blue Ridge aquifers is proposed based on these research findings. / Ph. D.

Multiple saline tracers were used to explore the role of geologic structure on groundwater recharge at the Fractured Rock Research Site in Floyd County, Virginia. Tracer migration was monitored through soil, saprolite, and fractured crystalline bedrock for a period of 3 months with chemical, physical, and geophysical techniques. Potassium chloride (KCl) and potassium bromide (KBr) tracers were applied at specific locations on the ground surface to directly test flow pathways in a shallow saprolite and deep fractured rock aquifer. Previous work at the Fractured Rock Research Site have identified an ancient thrust fault complex that is present in the otherwise competent metamorphic bedrock; fracturing along this fault plane has resulted in a highly transmissive aquifer that receives recharge along the vertically oriented portion of the fault zone. A shallow aquifer has been located above the thrust fault aquifer in a heterogeneous saprolite layer that rapidly transmits precipitation to a downgradient spring. Tracer monitoring was accomplished with differential electrical resistivity, chemical sampling, and physical monitoring of water levels and spring discharge. Tracer concentrations were monitored quantitatively with ion chromatography and qualitatively with differential resistivity surveys. KCl, applied at a concentration of 10,000 mg/L, traveled 160 meters downgradient through the thrust fault aquifer to a spring outlet in 24 days. KBr, applied at a concentration of 5,000 mg/L, traveled 90m downgradient through the saprolite aquifer in 19 days. KCl and KBr were present at the sampled springheads for 30 days and 33 days, respectively. Tracer breakthrough curves indicate diffuse flow through the saprolite aquifer and fracture flow through the crystalline thrust fault aquifer. Heterogeneities in the saprolite aquifer had a large effect on tracer transport, with breakthrough peaks varying several days over vertical distances of several meters. Monitoring saline tracer migration through soil, saprolite, and fractured rock provided data on groundwater recharge that would not have been available using other traditional hydrologic methods. Travel times and flowpaths observed during this study support preferential groundwater recharge controlled by geologic structure. Geologic structure, which is not currently considered an important factor in current models of Blue Ridge hydrogeology, should be evaluated on a local or regional scale for any water resources investigation, wellhead protection plan, or groundwater remediation project. / Master of Science

Les capteurs distribués à fibre optique (aussi nommés DAS) sont une nouvelle technologie d'acquisition sismique qui utilise des câbles traditionnels à fibre optique pour fournir une mesure de la déformation le long du câble. Ce système d'acquisition est largement utilisé dans les profils sismiques verticaux (PSV). Le couplage est un facteur clé qui a une grande influence sur la qualité des données. Alors que, pour les acquisitions PSV, les géophones sont attachés à la paroi du puits, le câble de fibre optique est soit cimenté derrière le tubage, soit attaché avec des pinces rigides au tubage ou simplement descendu dans le puits. Cette dernière stratégie de déploiement donne généralement le plus petit rapport signal sur bruit, mais est considérée comme la plus rentable en particulier pour les installations dans des puits existants. Cette thèse porte sur la problématique du couplage du DAS quand le câble est simplement descendu dans le puits. Nous développons des modèles numériques pour analyser les données réelles. L'interprétation de ces résultats nous permet de conclure qu'un contact immédiat du câble avec la paroi du puits avec une force de contact calculée est nécessaire pour fournir des bonnes conditions de couplage. Sur la base de ces résultats, nous proposons des solutions pour optimiser davantage les acquisitions avec le système DAS. Nous modifions numériquement la force de contact et les propriétés élastiques du câble DAS et démontrons comment ces modifications peuvent améliorer mais aussi détériorer la qualité des données. Enfin, nous proposons un algorithme de détection du couplage qui permet d'assurer l'acquisition de données réelles avec un rapport signal / bruit élevé. / Distributed Acoustic Sensing (DAS) is a new technology of seismic acquisition that relies on traditional fibre-optic cables to provide inline strain measurement. This acquisition system is largely used in vertical seismic profiling (VSP) surveys. Coupling is a key factor influencing data quality. While geophones and accelerometers are clamped to the borehole wall during VSP surveys, the fibre cable is either clamped and then cemented behind the casing, or attached with rigid clamps to the tubing, or loosely lowered into the borehole. The latter deployment strategy, also called wireline deployment, usually acquires the lowest level of signal but is regarded as the most cost-effective in particular for existing well installations. This PhD thesis addresses the problematic of coupling of DAS using wireline deployment. We develop numerical models that are used to analyse real data. The interpretation of these results allows us concluding that an immediate contact of the cable with the borehole wall with a computed contact force is required to provide good coupling conditions. Based on those findings, we propose solutions to further optimise DAS acquisitions. We numerically modify the contact force and the elastic properties of the DAS cable and show how these modifications can improve but also deteriorate data quality. Finally, we propose a coupling detection algorithm that is applied to real datasets and allows ensuring the acquisition of data with a high signal-to-noise ratio.

<p>In 2009, the Regional Municipality of Halton and McMaster University initiated a 2-year collaborative study (Georgetown Aquifer Characterization Study-GACS) of the groundwater resource potential of Quaternary sediments near Georgetown, Ontario. As part of that study, this thesis investigated the Quaternary infill stratigraphy of the Middle Sixteen Mile Creek (MSMC) and Cedarvale (CV) buried valley systems using newly acquired core and borehole geophysical data. Multi-parameter geophysical log suites (natural gamma, EM conductivity, resistivity, magnetic susceptibility, full-waveform sonic, caliper) were acquired in 16 new boreholes (16 m to 55 m depth), pre-existing monitoring wells and from archival data. Characteristic log responses (electrofacies) were identified and correlated with core to produce a detailed subsurface model of a 20-km² area to the southwest of Georgetown. Nine distinctive lithostratgraphic units were identified and their geometry mapped across the study area as structure contour and isochore thickness maps. The subsurface model shows that the CV valley truncates the Late Wisconsin MSMC stratigraphy along a channelized erosional unconformity and is a younger (post-glacial?) sediment-hosted valley system. Model results demonstrate the high level of stratigraphic heterogeneity and complexity that is inherent in bedrock valley systems and provides a geological framework for understanding groundwater resource availability.</p> <p>Principal component analysis (PCA) was applied to selected log suites to evaluate the potential for objective lithologic classification using log data. Gamma, resistivity and conductivity logs were most useful for lithologic typing, while p-wave velocity and resistivity logs were more diagnostic of compact diamict units. Cross plots of the first and second principal components of log parameters discriminated silts and clays/shales from sand/gravel and diamict lithofacies. The results show that PCA is a viable method for predicting subsurface lithology in un-cored boreholes and can assist in the identification of hydrostratigraphic units.</p> / Master of Science (MSc)

Undulating well trajectories are often drilled to improve length exposure to rock formations, target desirable hydrocarbon-saturated zones, and enhance resolution of borehole measurements. Despite these merits, undulating wells can introduce adverse conditions to the interpretation of borehole measurements which are seldom observed in vertical wells penetrating horizontal layers. Common examples are polarization horns observed across formation bed boundaries in borehole resistivity measurements acquired in highly-deviated wells. Consequently, conventional interpretation practices developed for vertical wells can yield inaccurate results in HA/HZ wells. A reliable approach to account for well trajectory and bed-boundary effects in the petrophysical interpretation of well logs is the application of forward and inverse modeling techniques because of their explicit use of measurement response functions. The main objective of this dissertation is to develop inversion-based petrophysical interpretation methods that quantitatively integrate logging-while-drilling (LWD) multi-sector nuclear (i.e., density, neutron porosity, photoelectric factor, natural gamma ray) and multi-array propagation resistivity measurements. Under the assumption of a multi-layer formation model, the inversion approach estimates formation properties specific to a given measurement domain by numerically reproducing the available measurements. Subsequently, compositional multi-mineral analysis of inverted layer-by-layer properties is implemented for volumetric estimation of rock and fluid constituents. The most important prerequisite for efficient petrophysical inversion is fast and accurate forward models that incorporate specific measurement response functions for numerical simulation of LWD measurements. In the nuclear measurement domain, first-order perturbation theory and flux sensitivity functions (FSFs) are reliable and accurate for rapid numerical simulation. Albeit efficient, these first-order approximations can be inaccurate when modeling neutron porosity logs, especially in the presence of borehole environmental effects (tool standoff or/and invasion) and across highly contrasting beds and complex formation geometries. Accordingly, a secondary thrust of this dissertation is the introduction of two new methods for improving the accuracy of rapid numerical simulation of LWD neutron porosity measurements. The two methods include: (1) a neutron-density petrophysical parameterization approach for describing formation macroscopic cross section, and (2) a one-group neutron diffusion flux-difference method for estimating perturbed spatial neutron porosity fluxes. Both methods are validated with full Monte Carlo (MC) calculations of spatial neutron detector FSFs and subsequent simulations of neutron porosity logs in the presence of LWD azimuthal standoff, invasion, and highly dipping beds. Analysis of field and synthetic verification examples with the combined resistivity-nuclear inversion method confirms that inversion-based estimation of hydrocarbon pore volume in HA/HZ wells is more accurate than conventional well-log analysis. Estimated hydrocarbon pore volume from conventional analysis can give rise to errors as high as 15% in undulating HA/HZ intervals. / text