1,721,025 research outputs found
Developing low-cost frequency-domain electromagnetic and induced polarization geophysical instrumentation
No full textIncludes bibliographical references.2022 Spring.Climate change, expanding populations, and the rapid industrialization of low and middle income countries have created unprecedented challenges for local communities and their surrounding environments. Near-surface geophysical surveying can provide a wealth of data to help address these challenges in a cost-effective manner that is minimally invasive to local environments. Electrical and electromagnetic methods are two such families of techniques that are widely used in near-surface geophysics due to their wide applicability to near-surface problems, such as water exploration and environmental monitoring, the ease of data acquisition, and that electrical and electromagnetic geophysical data can greatly aid near-surface investigations with minimal processing. Recent development and the widespread availability of electrical components and cheap microcomputers have created new opportunities for developing purpose-built, low-cost, open-source geophysical instrumentation for near-surface investigation. This thesis reports the development of two such prototype instruments. First, a frequency-domain instrument that is capable of sensing conductive objects in near-surface environments and costing under US200 to construct. Both instruments have demonstrated the capability to be leveraged for near-surface humanitarian applications. Future development includes improving on the mechanical stability of the low-cost instrumentation and improving autologging capabilities for use in remote and time-lapse geophysical investigations
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Transient Electromagnetic Modelling and Imaging of Thin Resistive Structures: Applications for Gas Hydrate Assessment
No full textGas hydrates are a solid, ice-like mixture of water and low molecular weight hydrocarbons. They are found under the permafrost and to a far greater extent under the ocean, usually at water depths greater than 300m. Hydrates are a potential energy resource, a possible factor in climate change, and a geohazard. For these reasons, it is critical that gas hydrate deposits are quantitatively assessed so that their concentrations, locations and distributions may be established. Due to their ice-like nature, hydrates are electrically insulating. Consequently, a method which remotely detects changes in seafloor electrical conductivity, such as marine controlled source electromagnetics (CSEM), is a useful geophysical tool for marine gas hydrate exploration.
Hydrates are geometrically complex structures. Advanced electromagnetic modelling and imaging techniques are crucial for proper survey design and data interpretation. I develop a method to model thin resistive structures in conductive host media which may be useful in building approximate geological models of gas hydrate deposits using arrangements of multiple, bent sheets. I also investigate the possibility of interpreting diffusive electromagnetic data using seismic imaging techniques. To be processed in this way, such data must first be transformed into its non-diffusive, seismic-like counterpart. I examine such a transform from both an analytical and a numerical point of view, focusing on methods to overcome inherent numerical instabilities. This is the first step to applying seismic processing techniques to CSEM data to rapidly and efficiently image resistive gas hydrate structures.
The University of Toronto marine electromagnetics group has deployed a permanent marine CSEM array offshore Vancouver Island, in the framework of the NEPTUNE Canada cabled observatory, for the purposes of monitoring gas hydrate deposits. In this thesis I also propose and examine a new CSEM survey technique for gas hydrate which would make use of the stationary seafloor transmitter already on the seafloor, along with a cabled receiver array, towed from a ship. I furthermore develop a modelling algorithm to examine the electromagnetic effects of conductive borehole casings which have been proposed to be placed in the vicinity of this permanent marine CSEM array, and make preliminary recommendations about their locations.Ph
Imaging ahead of a tunnel boring machine with DC resistivity: a laboratory and numerical study
No full textIncludes bibliographical references.2019 Spring.Tunnel Boring Machines (TBMs) are powerful tools for tunneling and underground construction, which excavate material and install a segmental concrete tunnel liner for support. However, unknown ground conditions pose a significant risk to tunneling operations and any damage to the machine can be disastrous to a project. As such, there is a need for tools which look ahead of the TBM for potential hazards during tunneling, including water saturated zones, faults, boulders and metal pipes. Geophysical methods offer the capability to image ahead of tunneling in order to prevent damage to the machine or nearby infrastructure, thus improving tunneling operations. In particular, the DC resistivity method is useful because it is sensitive to a large range of conductivity variations in geological and man made materials. The research presented in my thesis consists of three parts: (1) a laboratory study of a scale model TBM and tunneling environment, (2) a series of forward models studying different survey designs, and (3) the inversion and imaging of synthetic data under different assumptions. I introduce several new survey designs that attach DC resistivity electrodes on a probe or probes, which are then pushed into the earth in front of the machine each time excavation stops. My laboratory data and forward modeling results show that this method reduces interference caused by the metallic TBM body, and increases the distance ahead of the machine at which a target may be detected: depending on the specific survey design, the TBM influence is minimal once the probe is pushed 20% to 55% of the TBM diameter ahead of the machine and targets can be detected up to 60% TBM diameter away. Finally, I invert synthetic data to produce ahead-of-tunneling images using different amounts of prior information (e.g. TBM and host rock resistivity) and perform a time-lapse inversion, which has not been done for DC resistivity on a TBM before. I conclude the inversions with two types of comparisons to the true model and show that including more prior information decreases model error, but does not necessarily improve how well a target can be imaged
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Transient Electromagnetic Modelling and Imaging of Thin Resistive Structures: Applications for Gas Hydrate Assessment
Get PDFGas hydrates are a solid, ice-like mixture of water and low molecular weight hydrocarbons. They are found under the permafrost and to a far greater extent under the ocean, usually at water depths greater than 300m. Hydrates are a potential energy resource, a possible factor in climate change, and a geohazard. For these reasons, it is critical that gas hydrate deposits are quantitatively assessed so that their concentrations, locations and distributions may be established. Due to their ice-like nature, hydrates are electrically insulating. Consequently, a method which remotely detects changes in seafloor electrical conductivity, such as marine controlled source electromagnetics (CSEM), is a useful geophysical tool for marine gas hydrate exploration.
Hydrates are geometrically complex structures. Advanced electromagnetic modelling and imaging techniques are crucial for proper survey design and data interpretation. I develop a method to model thin resistive structures in conductive host media which may be useful in building approximate geological models of gas hydrate deposits using arrangements of multiple, bent sheets. I also investigate the possibility of interpreting diffusive electromagnetic data using seismic imaging techniques. To be processed in this way, such data must first be transformed into its non-diffusive, seismic-like counterpart. I examine such a transform from both an analytical and a numerical point of view, focusing on methods to overcome inherent numerical instabilities. This is the first step to applying seismic processing techniques to CSEM data to rapidly and efficiently image resistive gas hydrate structures.
The University of Toronto marine electromagnetics group has deployed a permanent marine CSEM array offshore Vancouver Island, in the framework of the NEPTUNE Canada cabled observatory, for the purposes of monitoring gas hydrate deposits. In this thesis I also propose and examine a new CSEM survey technique for gas hydrate which would make use of the stationary seafloor transmitter already on the seafloor, along with a cabled receiver array, towed from a ship. I furthermore develop a modelling algorithm to examine the electromagnetic effects of conductive borehole casings which have been proposed to be placed in the vicinity of this permanent marine CSEM array, and make preliminary recommendations about their locations.Ph
Mechanical properties of the Niobrara
No full textIncludes bibliographical references.2016 Fall.The Niobrara Formation consists of alternating chalk and marl intervals. The chalk intervals comprise reservoir units while the marl intervals act as sources and seals. Mineralogy and kerogen content are vertically and laterally heterogeneous, and knowledge of mechanical properties improves calculation of minimum horizontal stress, a primary control of fracture growth. Permeability is enhanced by fracture networks, and the detection of these networks informs completion decisions. In this study laboratory ultrasonic and static measurements are used to derive relationships enabling the creation of an anisotropic stress model for a well in Wattenberg Field, CO, USA. The well log is also used to apply a fracture detection method that considers the fractured medium transversely isotropic with a horizontal axis of symmetry (HTI). Applying Hudson’s crack model to the laboratory anisotropy data serves as a quality check for the method. Dry rock velocity and strain measurements were taken for the D chalk and marl facies of the Niobrara. Mineralogy, TOC, and vitrinite reflectance data were obtained for the laboratory samples and the studied reservoir facies. The samples measured in the laboratory exhibit lower velocity pressure sensitivity than is observed in the studied well, which is related to peak maturity of the lab samples. The dry rock velocity measurements are used to obtain the Thomsen parameters for both facies. The chalk samples tested are isotropic, and the marl samples are weakly anisotropic. Anisotropy decreases with increasing confining pressure, and the observed trend is commensurate with laboratory tests of peak-mature shales (Vanorio, 2008). Saturated velocities from Maldonado (2010) were used to observe the anisotropic response to pore fluid pressure. ε and γ both decrease with increasing pore and confining pressure. γ exhibits a greater response to increasing pore fluid pressure than to confining pressure for the measured facies. Empirical relationships between the laboratory-derived Thomsen parameters are applied to the dipole shear log, which is treated as a dry rock measurement. Static and dynamic elastic parameters are measured to further describe the dry rock mechanical properties. The parameters are compared with Voigt-Reuss-Hill effective medium theory and employed in uniaxial strain approximations of the minimum horizontal stress. The anisotropy present in the study well is considerably lower than that measured in the laboratory samples. The estimation of δ allows the application of the ANNIE approximation. The approximated constants are corrected to the static case using the laboratory-derived ratios, and the errors resultant from utilizing dynamic constants and assuming isotropy are discussed. Fracture density is estimated using a method developed by Conoco (Sil, 2013). The method considers the formation a homogenous, horizontal transverse isotropic (HTI) medium consisting of vertical, parallel sets of fractures. In Conoco’s method δN, the normal fracture weakness, is used to derive the fracture density, e. δT, the tangential fracture weakness, replaces δN in this method since δT is less prone to second-order effects. Fracture density is compared to the laboratory-derived anisotropy parameters using Hudson’s crack model
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Powerline contamination in time-domain electromagnetic data: experiment, theory, and basic building blocks
No full textIncludes bibliographical references.2017 Spring.Time-domain Electromagnetics (TEM) is a powerful tool to image the conductivity structure of the subsurface. These systems can be ground-based or can be mounted on an airborne platform, which allows for greater data density and resolution. Airborne electromagnetic (AEM) surveys are continually being pushed to regions with increasing culture; regions such as producing oil-fields with a need of ground-water contaminant monitoring, populated agricultural regions for water resource management, and/or mineral exploration around existing developments. Culture and cultural noise are proportional, and powerlines are a common and persistent source of cultural noise. Powerlines present a unique signature in TEM data. AEM data from Iowa, contracted by the USGS, exemplify these effects. The affected data are smoothly elevated and contamination can be seen as far as 400+ m away from a powerline. Standard data processing techniques simply cull the affected data from the dataset yielding a data loss in excess of 25%. While the problem is well documented in the literature, the basic building blocks of the physics behind the problem are not well understood. This thesis is an attempt to characterize and understand the powerline coupling problem, both theoretically and experimentally. A critical component of this work is the conceptual idea of a Skywire – a vertical loop of wire, simulating powerline grounding structures and their earth return. This concept was tested experimentally – by building a Skywire and performing TEM surveys around it, and studied theoretically – through modeling the coupling of a Skywire to an idealized TEM system. Both approaches show that powerline grounding structures are indeed the culprit, but also suggest that the mutual inductance between the earth and the Skywire plays a large role in the contamination. While the modeling does not fully capture the amplitudes and spatial extent of the contamination observed in AEM or ground-based data, many aspects of the problem were investigated. These aspects include Skywire resistance, Skywire self-inductance, Skywire–earth mutual-inductance, and differing geometries between airborne- and ground-based systems and Skywires. A methodology to repair affected TEM data and recover the earth structure has yet to be developed, however, I present a modeling algorithm and a way to quantify contamination levels in TEM data
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