Theses
Permanent URI for this collectionhttps://hdl.handle.net/1969.6/1140
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Browsing Theses 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 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 Quantifying land subsidence rates in the Coastal Bend of Texas using temporal gravimetry, campaign and permanent GNSS, and interferometric radar techniques(2022-12) Beattie, Amanda; Ahmed, Mohamed; Chu, Tianxing; Gebremichael, EsayasLand subsidence and local sea level rise are well-known, ongoing problems that are negatively impacting the Coastal Bend of Texas. Land subsidence in this area increases the vulnerability of coastal communities to effects of flood and hurricane events. In this study, four different approaches were used for quantifying land subsidence rates over the Coastal Bend of Texas on both local and regional scales. The first approach was a gravity survey, which employed a relative gravimeter to measure temporal gravity changes every two weeks for a period of two years (Oct. 2020 – Sept. 2022) on a local scale over six different areas around Corpus Christi, North Padre Island, Mustang Island, and Rockport. The second approach was to utilize campaign Global Navigation Satellite System (GNSS) elevation measurements, which were collected in a campaign congruent with the gravity survey. The third approach used the Interferometric Synthetic Aperture Radar (InSAR) to generate land deformation rates over the central Coastal Bend region on a reginal scale during from January 2017 to November 2021. The fourth approach used fifteen permanent GNSS stations to quantify land subsidence rates on a regional scale over a time period similar to that of the InSAR. The gravity- and campaign GNSS-derived deformation rates were compared with each other, while the InSAR and permanent GNSS rates were compared. Factors driving InSAR-derived subsidence rates were also investigated. While both local and regional integrations showed significant correlation, land subsidence results from the gravity and campaign GNSS surveys could be enhanced with an extended campaign period and more frequent data acquisition. Additionally, the gravity-derived land subsidence rates could be improved by selecting a more stable base station location, utilizing high precision relative gravimeter and/or an absolute gravimeter, and target a noise-free acquisition days and times. The InSAR-derived rates were consistent with the permanent GNSS-derived rates (R: 0.7). Results of this study showed that land subsidence rates exhibited spatial and temporal variations across the Coastal Bend of Texas. Four regions were identified to experience significant InSAR-derived land subsidence rates: inland towns (total: 17); coastal towns (total: 6); cities (total: 6); industrial plants (total: 8). Four coastal towns subsided at an average rate of -2 ± 3 mm/yr (range: -4 ± 4 to -0.4 ± 3 mm/yr), likely driven by sediment compaction and growth faulting. Three regions classified as city around the Corpus Christi urban area experienced an average subsidence rate of -2 ± 3 mm/yr (range: -4 ± 3 to -0.2 ± 3 mm/yr), with subsidence being attributed to increased groundwater extraction rates, sediment compaction, and growth faulting. Three inland towns experienced average subsidence rates of -4 ± 4 mm/yr (range: -8 ± 8 to -0.7 ± 3 mm/yr); two inland towns close in proximity to the coastline experienced subsidence that is likely attributed to a high density of growth faults, while the town located further inland could have experienced subsidence due to enhanced groundwater extraction activities. Seven industrial plants experienced subsidence at an average rate of -4 ± 6 mm/yr (range: -8 ± 6 mm/yr to -0.2 ± 5 mm/yr). The localized subsidence observed within these areas is thought to be driven by sediment compaction by overburden from storage tanks. Quantifying land subsidence is important in estimating local sea level rise, determining the local geoid, and improving understanding of their controlling factors. Land subsidence rates, generated from this study, could be useful for supporting coastal communities in mitigating the effects of natural forces and improving their resilience against them.