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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Calculation of a Synthetic Gather using the Aki-Richards Approximation to the Zoeppritz Equations

Ganssle, Graham 15 December 2012 (has links)
A synthetic seismic gather showing amplitude versus offset can be analyzed by the interpretive geophysicist to predict rock properties useful in oil exploration. Reflection coefficients derived from measured well log data are convolved with a Ricker wavelet to create a synthetic seismic trace. The Zoeppritz equations describe the propagation of an acoustic wave across an interface between two viscous media of different acoustic impedances with respect to increasing offset angle. The Aki-Richards linear approximation is used to create a synthetic seismic gather with offset angles up to fifty degrees. This gather is compared to a synthetic gather created using commercially available software.
2

Adsorption of Radionuclides on Clay Minerals

Warinner, J. Ernest 01 January 1962 (has links)
No description available.
3

Vertical Axis Rotation in the Silurian Hills: A Cenozoic Overprint on the Mesozoic U.S. Cordilleran Magmatic Arc

Comstock, John E. 01 December 1997 (has links)
New, detailed field work in the Silurian hills, southeast of Death Valley, reveals a pattern of a complex Cenozoic brittle fault overprint of rocks containing evidence of at least three episodes of Mesozoic thermal-ductile deformation. The Cenozoic brittle fault overprint of rocks containing evidence of at least three episodes of Mesozoic thermal-ductile formation. The Cenozoic faulting consists of five distinct sets of structures ordered by cross cutting relationships. The oldest, BF1, are fragments of reverse faults trending NW and dipping 45-60 SW. BF2 consist of NW trending, en echelon, sinistral strike-slip faults. BF3 is a conjugate set of N-NE trending high-angle normal faults with 100's m offset. BF4 is a complex array of conjugate strike-slip and normal faults. NNW trending dextral and NNE trending sinistral faults offset and are offset by N and NW trending conjugate sets of high-angle normal faults. The younges set, BF5, are low-angle normal faults with no consistent basal surface or transport direction. These are interpreted to be right lateral, strike-slip extension (dextral transtension, BF4 & BF5) preceded by right lateral, vertical axis rotation, and strike-slip shortening (dextral transrotation and transpression, BF2 & BF3), all related to the interaction of the Garlock fault and northwest trending dextral shear. 30° of clockwise vertical axis rotation is inferred to have affected the Silurian Hills since the Middle Miocene as the result of BF2 & BF3 deformation. The Mesozoic deformations are; 1, a greenschist facies, low-strain but tightly folded N vergent, crystalline basement through Pahrump Group section; 2, a greenschist facies, high-strain, 100's m thick, mylonite zone with stretching lineations trending NE-SW; and 3, an upper amphibolite facies (high T, low P), high-strain E-W trending, S plunging Pahrump Group section, syntectonically intruded by multiple igneous phases U-Pb dated from 177 Ma to 97 Ma. The two Pahrump Group sections are interpreted to form as the result of NW directed backthrusting in the hinterland of the Sevier orogeny and were juxtaposed by NNW normal, extensional transport along the mylonite zone after crustal thickening. K-Ar biotite cooling ages suggest the mylonite zone developed during the Late Cretaceous to Eocene time. Finally, the protolith of the tectonostratigraphic Riggs Formation to be two formations of the Pahrump Group, the Crystal Spring and Beck Spring Formations.
4

Modeling of Critically-Stratified Gravity Flows: Application to the Eel River Continental Shelf, Northern California

Scully, Malcolm E. 01 January 2001 (has links)
An analytical and numerical model are presented and applied to predict gravitydriven transport and deposition of fluid mud layers that form within the wave boundary layer on the continental shelf off the Eel River in northern California. Observations indicate that following floods of the Eel River down-slope transport of fluid mud trapped within the wave boundary layer is the dominant across-shelf transport mechanism. The models are based upon the assumption that following significant floods, an abundant supply of easily suspended fine sediment is delivered to the coastal ocean, allowing a negative feedback mechanism to maintain the near-bed Richardson number at its critical value. Thus, sediment-induced stratification effectively limits the amount of fine sediment that can be maintained in suspension, allowing the calculation of down-slope transport and deposition knowing only the appropriate near-bed velocity scale. Analytic predictions of mid-shelf mud transport and deposition are spatially and temporally consistent with field observations and provide strong evidence that gravitydriven processes control the emplacement and location of the Eel margin flood deposit. Analytic predictions of deposition suggest that the magnitude of wave energy is more important than the magnitude of the flood event in controlling the thickness of mid-shelf gravity-driven deposition following floods. Higher wave energy increases the capacity for critically-stratified gravity flows to transport sediment to the mid-shelf and results in greater deposition. The bathymetry of the Eel margin plays a critical role in gravitydriven transport and deposition. Analytic predictions indicate that gravity-driven deposition on the mid-shelf begins roughly 7-8 km north of the river mouth. Closer to the river mouth, the seaward increasing mid-shelf slope associated with the concave downward subaqueous delta causes gravity-driven flux divergence, preventing significant mid-shelf gravity-driven deposition and favoring sediment bypassing. Seaward decreases in shelf slope in the vicinity of the observed flood depo-center leads to greater flux convergence by gravity-driven flows, and hence greater deposition. The numerical model predicts gravity-driven deposition on the continental shelf for four consecutive flood seasons of the Eel River using realistic bathymetry, waves and river forcing. Results from the numerical model are consistent with observations of deposition on the mid-self and support the results of the analytical model that suggest wave intensity and bathymetry are the dominant factors controlling the location and magnitude of observed deposition. Despite significantly greater sediment input near the river mouth, little mid-shelf deposition is predicted in this region due to the increasing off-shelf slope. The numeric results suggest that gradients in the along-shelf components of bed-slope also favor gravity-driven deposition 10-30 km north of the river mouth. Including the influence of along-shelf currents had little impact on the location of midshelf deposition, providing further support for bathymetric control of flood sedimentation on the Eel margin. A significant fraction of sediment from the Eel River was predicted to leave the shelf as a gravity-driven flow during floods with large wave energy. However, in extremely large floods, gravity-driven processes were not capable of removing riverderived fine sediment from the inner-shelf.
5

Sediment dispersal and sequence development along a tectonically active margin: Late Quaternary evolution of the Ganges-Brahmaputra River delta

Goodbred, Steven Lee, Jr. 01 January 1999 (has links)
Situated in the Bengal Basin, the Ganges-Brahmaputra river delta comprises one of the largest fluviodeltaic systems in the world, comprising ∼ 100,000 km2 of floodplain and delta plain and a 40,000 km2 subaqueous delta on the shelf. Sediment load of the Ganges-Brahmaputra river is 109 t/yr, and seasonal flooding may inundate >70% of the delta during large events. Active tectonic processes have resulted in both uplift and subsidence in this structurally complex region. These general characteristics suggest that the Ganges-Brahmaputra delta represents a heretofore undescribed delta end-member, forming along a high-yield, high-energy, tectonically active margin. to investigate this view, stratigraphic, sedimentologic, and geochronologic data are used to evaluate processes, controls, and development of the system over different spatial and temporal scales in the Late Quaternary.;Results of a century-scale sediment accretion study using radioisotope geochronology indicate that ∼ 30% of fluvial sediment load is sequestered to the delta and not reaching the coastal ocean as previously assumed. A Holocene-scale sediment budget generated from radiocarbon-dated stratigraphy also reveals ∼ 30% of sediment discharge was sequestered to the delta during this time. Considered with offshore data, these sediment budgets indicate contemporaneous highstand strata formation across floodplain, shelf, and deep-sea depocenters. Radiocarbon-dated stratigraphy was used to reconstruct the Late Quaternary history of delta formation. Growth of the Ganges-Brahmaputra delta began ∼ 10,000--11,000 cal yr BP, notably 2000--3000 years prior to most of the world's deltas. During subsequent rapid sea-level rise, the immense sediment discharge was sufficient to maintain relative shoreline stability while most margin systems experienced major transgression. offset of radiocarbon dates from eustatic sea level indicate 2--4 mm/yr of subsidence in several areas of the delta, suggesting tectonic control on deltaic sediment trapping and sequence formation. Shallow vibracore stratigraphy from the delta reveals a cap of muddy sediments overlying largely sandy material, reflecting differences in preservation between floodplain and channel deposits. Over longer time frames, floodplain sediments are eroded through channel migration and avulsion, thus preferentially preserving channel sands. Overall, the role of tectonics in controlling deltaic processes and product in the Ganges-Brahmaputra river delta implies a fundamental distinction for deltas forming along active margins.
6

A study of the secondary turbidity maximum in the York River Estuary, Virginia

Lin, Jing 01 January 2001 (has links) (PDF)
A two-year period of monthly slack water survey results suggest that in addition to the classical estuary turbidity maximum (ETM), a second peak of bottom total suspended sediments (TSS) concentration, or a secondary turbidity maximum (STM), often exists in the mid-York Estuary. This STM, detected from majority of the slack water surveys, moves back and forth in the region of about 20--40 km from the York River mouth. Moreover, the distribution of potential energy anomaly indicates that the STM may be related to the stratification patterns of the water column. A mathematical analysis suggests that four processes may be important to the formation of the STM: convergence of bottom residual flow, tidal asymmetry, inhibition of turbulence diffusion by stratification, and local erosion. An intensive survey was conducted in the middle part of the York River. None of the four mechanisms was in favor of convergent sediment transport in the examined region. Accordingly, the STM did not show up in the slack water survey conducted two days before the intensive survey. The intra-tidal variations of the bottom TSS concentration was shown to be proportional to bottom shear stress in most stations, which indicates that bottom resuspension is an important source of TSS in this region. The survey results also suggest that lateral sediment transport is not negligible in the study area. A three-dimensional numerical model was applied to the York River system. The model reproduced the basic features in both the salinity and TSS fields. Sensitivity model studies confirmed the existence of the STM under low and mean flow conditions. Analysis to the model results shows that bottom resuspension is an important source of TSS in both the ETM and the STM. The location of the ETM is well associated with the null point of bottom residual flow under various flow conditions. Convergent bottom residual flow, as well as tidal asymmetry, were shown to be the most important mechanisms that contribute to the formation of the STM. Model results suggest that the association between the STM and the stratification pattern is due to geometric features.
7

New results from GPR at Legio: A Roman legionary base in the Jezreel Valley, Israel

Ernenwein, Eileen G., Adams, Matthew J., Tepper, Yotam 01 November 2020 (has links)
Legio is the base of the Roman II Triana and the VI Ferrata Legion, occupied from the early 2nd century to the early 4th century CE. It is the first of its kind to be excavated in the Eastern Roman Empire. Today the site sprawls beneath 30 hectares of pasture with slopes up to 15 degrees. Rapid, dense ground penetrating radar (GPR) survey with an antenna array would be ideal, but so far logistically impractical. The survey has proceeded since 2013 with a single 400 MHz antenna using parallel transects 0.5m apart for 5.85 ha to date. Like most Roman bases, Legio includes an extensive network of buildings and streets enclosed by rectangular fortifications. Unlike most Roman bases, however, it was constructed on a hillside with architectural components built by a combination of bedrock incision and above-ground construction. In addition, much of the site’s stonework has been robbed. These aspects demand topographic correction and interpretation using reflection profiles, depth slices, and 3D models. This paper presents data processing and results for the principia (central headquarters). Previous investigations were conducted at Legio and surrounding area by Tel Aviv University from 1998 to 2010. GPR and excavations since 2013 have been conducted as part of the Jezreel Valley Regional Project (JVRP)in association with the W.F. Albright Institute of Archaeological Research.
8

Curvature Analysis of Aeromagnetic Data

Lee, Madeline 04 1900 (has links)
<p>Fundamentally the amplitude, sign, and frequency of a magnetic signal are inherently linked to curvature. This thesis employs curvature analysis as a semi-automated tool for source signal extraction from a magnetic field surface represented by a grid. The first step is to compute the full, profile, and plan curvatures from the magnetic grid. These values are used in two approaches to curvature analysis: statistical and lineament. The descriptive statistics mean, standard deviation, kurtosis, and skew are computed for quantitative analysis. Mean is used in conjunction with kurtosis and skew to assess frequency content of the signal, magnetization and source dip. Standard deviation characterizes low, moderate, and extreme curvatures. A rapid technique to statistical analysis is applied using a graphical approach with histograms and scatterplots. Histograms display frequency distribution and scatterplots display the relationship between different curvatures. Curvature in the maximum dip direction is used to systematically identify surficial lineaments characterized as continuous troughs or ridges. These lineaments may represent geological sources or remanent acquisition artefacts. Lineaments representing faults and dykes are used in conjunction with <em>a priori</em> knowledge to determine mineralization vectors since many ore deposits are structurally controlled. Quality control of the aeromagnetic grid levelling application may be assessed using spatial correlation of flight lines and magnetic lineaments. In this work curvature analysis is applied to simple synthetic models and two Canadian aeromagnetic data sets. Curvature analysis was applied to magnetic data from the Wopmay Orogen to identify bedrock contacts, fault configurations, and dyke swarms. The data was also used to show lineaments displayed as rose diagrams may be used as an alternative to standard Fourier power spectrums for assessment of levelling. Magnetic survey data from Southern Ontario was used to show a statistical approach to identify regional dip, dominant magnetization, and interference in anomalies.</p> / Doctor of Philosophy (PhD)
9

GEOPHYSICAL PROCESSING AND INTERPRETATION WITH GEOLOGIC CONTROLS: EXAMPLES FROM THE BATHURST MINING CAMP

Tschirhart, Peter 10 1900 (has links)
<p>With an ever-increasing consumption of natural resources new prospecting techniques are required to satisfy the demand. Geophysical methods are one tool commonly relied upon. New acquisition platforms or survey methodologies provide one way to expand the geophysical capabilities, but are expensive and slow to develop. New processing and interpretation techniques on the other hand provide a rapid means to reinterpret existing datasets with the goal of improving our geologic understanding of a project area. This thesis presents four new ways to extract additional geologic insights from a variety of geophysical datasets. All of the studies are based within the Bathurst Mining Camp, NB.</p> <p>A physical rock property database for the Bathurst Mining Camp is constructed and statistically analyzed in chapter two. Descriptive statistics include mean, standard deviation; first, second and third quartiles are calculated for density and magnetic susceptibility measurements and provided in tables for reference. Bivariate plots are then used to identify trends in the density-magnetic susceptibility relationship. We relate some of our findings to processes involved in the depositional and alteration history of the various lithologies. Comprehensive rock property databases provide valuable constraints for geophysical data processing and are essential for any subsequent geophysical modeling. This is demonstrated with two examples. A joint gravity-magnetic profile model is completed across the geologically complex Nine Mile Synform. The profile reveals deep structure in the Camp down to 5 km depth. A geologically constrained geophysical inversion model of the magnetic anomaly associated with the Armstrong B mineral deposit reveals this anomaly contains a strong magnetic remanence contribution. The influence of remanence is often ignored in magnetic interpretation and modeling, but vital to achieve a geologically correct solution. In this instance comparison of the calculated remanence direction with the expected Apparent Polar Wander Path defined direction suggests an age of mineralization that is compatible with geological evidence.</p> <p>A new approach to determine the optimum near surface residual magnetic signal is presented in chapter three. Additionally, a new way of locating remanently magnetized bodies is also introduced. This technique inverts frequency domain helicopter-borne electromagnetic data to yield apparent magnetic susceptibility. To locate those zones where the magnetic signal is dominated by remanence the inverted HFEM susceptibility is cross plot against the results of a traditional apparent susceptibility filter. The inverted HFEM susceptibility is independent of remanence while the apparent susceptibility assumes no remanence. Where remanence is present the TMI derived apparent susceptibility does not correlate with the HFEM. These differences are readily evident in a cross plot of the two susceptibilities. To determine a magnetic residual the inverted susceptibility is forward modeled as a series of vertical prisms with homogeneous susceptibility equal to the inverted susceptibility. This HFEM magnetic model is then used to reference the results of traditional wavelength separation methods. By design the HFEM information is restricted the near surface whereas all traditional regional / residual separation methods operate under wavelength assumptions. A case study using this methodology is presented on the western side of the Tetagouche Antiform.</p> <p>The use of a spatially variable density correction applied to ground gravity and gravity gradiometry in the BMC is examined in the fourth chapter. The influence of topography on gravity and gravity gradiometry measurements is profound and must be removed prior to interpretation. In geologic environments where there is a structural and/or stratigraphic control on the near surface mass distribution, using a single density value may introduce error into the reduced data. A regionally variable density correction is a means to compensate for this effect. Spectral information between the ground gravity and airborne gravity gradiometry is also compared in this chapter. Both systems are fundamentally recording the same geologic mass distribution albeit by different means. Where differences exist one system must be in error.</p> <p>The final chapter demonstrates a quantitative interpretative technique for geophysical data. Often interpretation of the geophysical data in a geological context is done qualitatively using total field and derivative maps. With this approach the resulting map product may reflect the interpreter’s bias. Source edge detection provides a quantitative means to map lateral physical property changes in potential and non-potential field data, but the field data must be transformed prior to SED computation. There are numerous transformation algorithms, all of which operate slightly differently. We demonstrate that by combining the output of several different SED computations through data stacking, the interpretable product of SED is improved. In two examples, a synthetic example and real world example from the Bathurst Mining Camp, a number of transformation algorithms are applied to gridded geophysical datasets and the resulting SED solutions combined. Edge stacking combines the benefits and nuances of each SED algorithm; coincident, or overlapping solutions are considered more indicative of a true edge, while isolated points are taken as being indicative of random noise or false solutions.</p> / Master of Science (MSc)
10

Airborne Gravity Gradiometry as an Exploration Tool

Dohey, Tim 10 1900 (has links)
<p><strong>Airborne gravity gradiometry (AGG) is a relatively new technology to the mineral exploration industry which has been increasingly used over the past decade. AGG systems are capable of separating linear accelerations due to aircraft movement from the accelerations related that represent the gravity signal, resulting in a much higher resolution measurement than airborne gravity. The rapid and cost-effective deployment of an AGG survey gives it an advantage when compared to traditional ground gravity survey. With the momentum of existing AGG technology in the exploration industry and the multitude of next-generation AGG sensors currently in development the technique promises to be a valuable exploration tool for the foreseeable future. This thesis focuses on the capabilities of the AGG technology as an exploration tool, its niche within the exploration process, and how AGG compares to other gravity methods. An overview of the AGG method provides context for the aim of the study. A history of airborne gravimetry is presented, as well as a detailed technical description of AGG measurements and units. A summary of all existing airborne gravity and airborne gravity gradiometry technology is provided, along with the major research initiatives aimed at making more sensitive AGG sensors in the future. A discussion of the potential sources of error and uncertainty when working with AGG data highlights many of the technique’s obstacles that we will be closely examining within this study. The AGG case study which is examined includes an AGG dataset collected as part of a nickel exploration program to image prospective troctolite chambers in Northern Labrador, by Vale. The study focuses on the portion of the survey over the Voisey’s Bay main block that contains several economic nickel deposits, including the world-class Ovoid deposit. This area has been characterized both geologically and geophysically in the past, and contains multiple datasets, including ground gravity. Forward modeling is completed using Voisey’s Bay physical rock properties to calculate the response that could be expected over a nickel-bearing troctolite chamber. The methodology and considerations of AGG data acquisition are reviewed in the context of this survey and the dataset is then taken through a terrain correction involving the determination of the best possible background density choice. The limitations and potential pitfalls of the terrain correction are examined in relation to the digital elevation model being used. The problem of thick, variable overburden in portions of the survey is also examined. Several filtering techniques are completed on the data, including vertical integration and the removal of the regional signal. The AGG resolution is then quantitatively compared to the historical ground gravity data and an upward continued version of the ground gravity (representing the response of an airborne gravity survey) by using 2D power spectra and radially averaged power spectra plots. Although the ground gravity is found to contain better resolution in some areas due to its proximity to the ground, the more regular spatial sampling of the AGG survey provided resolution advantages in other areas. The much higher sensitivity of the AGG sensor resulted in a strong resolution advantage over the upward continued gravity. This comparison is extended to include the differences in interpretive products produced from each dataset, in the form of 3D gravity inversions. Inversions were completed on all three datasets and the results are compared. Although the resolution of an individual ground gravity measurement is greater than that of an AGG measurement, the uniformity of the AGG survey provides superior coverage and leads to a more detailed inversion model, particularly for features greater than ~200-400m, such as the prospective nickel bearing troctolite chambers. </strong></p> / Master of Science (MSc)

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