College of Science Theses and Dissertations
Permanent URI for this collectionhttps://hdl.handle.net/1969.6/1175
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Browsing College of Science Theses and Dissertations by Author "Ahmed, Mohamed"
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Item An Evaluation of the Feasibility of the Time-Lapse Electrical Resistivity Tomography Method in Quantifying Submarine Groundwater Discharge in Fine Sediment and Highly Saline, Shallow Embayments(2019-12) Stearns, Joseph; Murgulet, Dorina; Prothro, Lindsay; Ahmed, Mohamed; Williams, DeidreThere is uncertainty surrounding the application of the time-lapse electrical resistivity tomography (ERT) method in quantifying submarine groundwater discharge (SGD). The technique has been proven effective in areas with significant differences in salinity between surface water and discharging groundwater. However, there are inherent limitations associated with the method when studying embayments with fine bottom sediments, highly saline porewaters, and shallow surface waters. To evaluate this approach, a 68-hour time- lapse ERT study was conducted at University Beach (UB) in Corpus Christi, Texas, constrained by concomitant measurements of naturally occurring isotope tracers and groundwater characterizations along the ERT profile. Surface water continuous 222Rn measurements were conducted to calculate SGD rates. Subsurface fluid conductivity measurements along the profile were constrained using inversion models via Archie’s Law (AL) and the Waxman-Smits equation (WSE). The average ERT-derived SGD rate for the study period was 26±1cm∙d-1. Average tracer-derived rates using the deep well endmember were 69±26, 49±1, 80±2, and 21±1 cm∙d-1 corresponding to 222Rn, 223Ra, 224Ra, and 226Ra, respectively. Over the course of the study 222Rn SGD estimates increased, while ERT and radium isotope estimates tended to decrease. There were no significant differences between the model accuracy of AL (R2=0.5, p<0.01) and the WSE (R2=0.5, p<0.01), thus there is no evidence that negatively charged clay particles in the subsurface matrix had any latent influence on the ERT measurements. Temperature, however, was found to be the predominant source of error in the ERT. The influence of temperature is often evident in the uppermost 1-2 m of images as temperature inflections drive the conductive recirculation of seawater. When temperature corrections were well constrained, the inversion models were significantly more successful at deriving modeled subsurface fluid conductivities closer to those observed, reducing error by up to 20%. Consequently, precise constraints on temperature are necessary to perform an effective experiment. ERT ultimately produced conservative SGD estimates relative to the radiogenic tracers. This is related to a lack of contrast between ambient and groundwater subsurface fluid conductivities. Thus, in the investigated environment, ERT may be used for qualitative and exploratory purposes, in the absence of other validating in-situ measurements.Item Improving assessments of water resources characterization and relationships to climate variabilities(2022-07-07) Gyawali, Bimal; Murgulet, Dorina; Ahmed, Mohamed; Liu, Chuntao; Tissot, PhilippeWith the increasing vulnerability of water resources due to climate change and the associated more frequent and intensive extreme events, there is a growing need to monitor the spatial and temporal variability in water resources. Coastal aquifers are a major source of freshwater for almost 40% of the world’s population living in coastal regions. Changes in groundwater level impact groundwater discharge to surface waters, which in turn affects the amount of streamflow resulting in variability in freshwater inflow to estuaries. However, lack of continuous spatial and temporal coverage of groundwater elevation data hinders direct measurements of groundwater storage (GWS). The first objective of this dissertation is to quantify the spatial and temporal variation in coastal GWS using uninterrupted monthly Gravity Recovery and Climate Experiment (GRACE) derived terrestrial water storage (TWSGRACE) utilizing the Texas Gulf Coast Aquifer as a benchmark. To accomplish this, the data gap in TWSGRACE was filled with reconstructed values from multi-linear regression (MLR) and artificial neural network (ANN) models. The reconstructed TWSGRACE was integrated with land surface model products and ground-based measurements to examine the long- and short-term trends in GWS in response to changing climate conditions. This study found a significant decline (0.35 ± 0.078 km3·yr?1, p-value: < 0.01) in GWS during the study period 2002-2019 and was able to capture extreme climate events (i.e., drought and flood). The second objective provides an innovative approach to deriving a continuous TWSGRACE record for improved evaluation of GWS at a global scale. The approach integrates three machine learning techniques (deep-learning neural networks [DNN], generalized linear model [GLM], and gradient boosting machine [GBM]) and eight climatic and hydrological input variables to fill GRACE record gaps and reconstruct the TWSGRACE data record at both global grid and basin scales. This study’s models’ performances were found to be superior to those from 61% of previous similar studies and comparable to 21%. The reconstructed TWSGRACE data captured the occurrences of extreme hydro-climatic events over the investigated basins and grid cells. In addition, the third objective evaluates the impact of individual and coupled ocean-atmospheric phenomena (El Niño Southern Oscillation [ENSO], Pacific Decadal Oscillation [PDO], and Atlantic multidecadal oscillation [AMO]) on hydrological variables (i.e., precipitation, streamflow, and freshwater inflow) across several major estuaries located along the northwestern Gulf of Mexico (nGOM), which span a significant climatic gradient. Results show that the individual and coupled climate variability phenomena have a significant impact on all three hydrological variables and the magnitude of impact varies seasonally and spatially, with the strongest modulation on cold seasons and in the wet region (of Texas). The severity of droughts increases significantly when the La Niña phase of ENSO is coupled with the PDO/AMO cold/warm. This dissertation 1) offers a reliable approach to examine the long- and short-term trends in GWS, 2) provides a robust and effective approach to fill the data gaps in TWSGRACE that can serve as a reference tool to fill the data gaps in any hydrological system across the globe, and 3) improves the understanding of the relationship between estuarine hydrology and climate variability. The results from these studies serve as valuable tools for water management strategies and benefit water resources planning and disaster management in coastal areas.Item Investigating fault control on reservoir accumulation and spatial distribution using 3D seismic data and well logging data: A Case study from the Lower Oligocene Vicksburg Formation, Brooks County, Texas(2020-12) Turner, Ryan Lewis; Turner, Ryan Lewis; Ahmed, Mohamed; Mohamed, Ahmed; Coffin, Richard; Prothro, Lindsay; Bissell, Randy; Coffin, Richard; Prothro, Lindsay; Bissell, RandyIn southern Brooks County, Texas, the Lower Oligocene Vicksburg Formation (LOVF, Rupelian stage, approximately 33.9-27.82 million years ago), is being influenced by the Vicksburg Fault Zone (VFZ). The VFZ is characterized by listric-normal faults that have formed highly faulted rollover anticlines that are sought-after structural traps for hydrocarbon exploration. This research explored how secondary synthetic (dipping East), antithetic (dipping West), and coast-perpendicular faults are affecting the accumulation and spatial distribution of hydrocarbons within the La Rucias Field. Results indicate that synthetic, antithetic, and coast-perpendicular faults affecting the V-102, V-17, and V-19 horizons provide conduits for hydrocarbon migration. Antithetic faults and coast-perpendicular faults within the rollover anticline are terminating beneath the overlying shale seal layer between the V-16 and V-17, creating natural gas accumulation. While synthetic faults affect the overlying seal layer migrating gas out of the V-102, V-17, and V-19. Bidirectional faulting linking antithetic and perpendicular to the coast faults are acting as additional pathways for enhanced hydrocarbon accumulation. Spatial distribution of hydrocarbons within the La Rucias Field varies with the horizon being targeted. Productive V-102 reservoirs are located on the western flank of the rollover anticline, the V-17 and V-19 reservoirs are located on structural highs where antithetic faults are not affecting the overlying shale seal layer, and the most productive V-17 and V-19 reservoirs are being affected by bidirectional faulting terminating beneath the shale seal layer allowing accumulation and spatial distribution within the rollover anticline. Investigating the control of these fault systems enhances our understanding on subsurface fluid migrations and accumulations (oil, gas, groundwater, and contaminants) in the expanded Vicksburg productivity trends.Item Mapping optimal recharge and extraction locations for groundwater resources in Southern Sinai, Egypt: Modelling and geophysical constraints(2023-08) Elshalkany, Muhamed; Ahmed, Mohamed; Murgulet, Dorina; Sauck, WilliamGroundwater resources are the only long-term solution for the local Bedouin community who live in southern Sinai. However, the Bedouin community as a whole lacks a basic understanding of how these resources are developed, recharged, distributed, and how to use them sustainably. The present study addresses this issue by utilizing publicly available remote sensing data and techniques to model potential groundwater recharge and extraction locations. Furthermore, the study investigates the influence of structural elements, including faults and shear zones, on the spatial distribution of these locations. To calibrate and validate remote sensing-derived results, near-surface geophysical surveys such as Vertical Electrical Sounding (VES), Seismic Refraction (SR), and Ground Penetrating Radar (GPR) were employed. The findings of this study are as follows: (1) The study area comprises 15% high potential recharge regions, 37% moderate potential recharge regions, and 47% low potential recharge regions; (2) A total of 334 locations were identified at the intersections of two or more fault/shear zone systems, representing optimal sites for drilling sustainable groundwater wells; (3) Two trends of structural elements, namely NW-SE and NE-SW, were identified in southern Sinai. The spatial distribution of these structural elements, along with surface gradient, predominantly controls groundwater accumulation by providing preferred pathways for groundwater flow; and (4) Geophysical surveys indicated that areas where two or more faults and shear zones intersected exhibited thicker and shallower saturated zones (thickness 18-23.5 m; depth 5.5-12.5 m) compared to other areas (thickness 5.5-16 m; depth 7-13 m). The comprehensive findings of this study provide valuable insights into the potential recharge and extraction locations for sustainable groundwater use in southern Sinai. Moreover, the study highlights the significance of structural elements and their spatial distribution in controlling groundwater availability. The methodologies employed in this research can be utilized as a framework for similar studies in other regions with highly fractured basement terrains.