1,720,967 research outputs found
Continental lithospheric temperatures: A review
Get PDFThermal structure of the lithosphere exerts a primary control on its strength and density and thereby its dynamic evolution as the outer thermal and mechanic boundary layer of the convecting mantle. This contribution focuses on continental lithosphere. We review constraints on thermal conductivity and heat production, geophysical and geochemical/petrological constraints on thermal structure of the continental lithosphere, as well as steady-state and non-steady state 1D thermal models and their applicability. Commonly used geotherm families that assume that crustal heat production contributes an approximately constant fraction of 25–40% to surface heat flow reproduce the global spread of temperatures and thermal thicknesses of the lithosphere below continents. However, we find that global variations in seismic thickness of continental lithosphere and seismically estimated variations in Moho temperature below the US are more compatible with models where upper crustal heat production is 2–3 times higher than lower crustal heat production (consistent with rock estimates) and the contribution of effective crustal heat production to thermal structure (i.e. estimated by describing thermal structure with steady-state geotherms) varies systematically from 40 to 60% in tectonically stable low surface heat flow regions to 20% or lower in higher heat flow tectonically active regions. The low effective heat production in tectonically active regions is likely partly the expression of a non-steady thermal state and advective heat transport
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
Geoscience Data Analysis Course: Course materials
No full textThis repository contains data used for a Geoscience Data Analysis short course run by the University of Adelaide
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
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
Understanding Marine Magnetotellurics
Get PDFThe theory of plate tectonics proposes that the Earth’s lithosphere is separated into rigid plates which are capable of motion through interactions with the underlying asthenosphere. Following its adoption in the 1960’s, it has become the prominent theory used to understand geodynamic processes within the Earth. The mechanisms of tectonism are of particular interest due to their implications regarding the formation of economic resources. Despite numerous studies attempting to characterise these mechanisms, the lithosphere-asthenosphere rheological contrast (LARC) remains an enigmatic component of plate tectonic theory. The magnetotelluric (MT) method is of particular interest when investigating the upper mantle as it is primarily sensitive to electrical conductivity. From electrical conductivity, conclusions regarding the temperature, pressure, physical and chemical state, porosity, and permeability of rocks can be inferred. This thesis examines laboratory conductivity measurements with ocean-bottom MT data collected from the Pacific Ocean. From these data, I create an upper mantle reference model for electrical conductivity and propose a hybrid MT impedance which improves the bandwidth and confidence intervals of ocean-bottom MT data. Between 50 km and 100 km depth, MT data predicts variable electrical conductivity structures. My reference model is able to encapsulate this variability with a function which varies according to hydration, partial melt, and the age of the overlying oceanic lithospheric. For 200 km to 400 km, the hydrated end-member of this reference model predicts the convergence of conductivity structures with increasing depth observable in published conductivity structures. As such, this reference model constrains the presence of hydration and partial melt within the LARC for a range of lithospheric ages and is a representative model of the Earth’s oceanic lithosphere and asthenosphere. Following an analysis of ocean-bottom EM field observations, I observe the attenuation of magnetic field variations by the conductive ocean water. This attenuation results in MT impedances which are difficult to interpret using available modelling algorithms. In contrast, the calculation of a hybrid MT impedance using ocean-bottom electric and continental magnetic fields is observed to improve the signal-to-noise ratio. This improved signal-to-noise ratio extends the usable bandwidth of ocean-bottom MT data from just over one decade to approximately four decades. It is important to note that this impedance represents the normalisation of ocean-bottom electric fields using continental magnetic fields. As a consequence, alterations must be made to modelling algorithms before attempting to reproduce hybrid impedances. Finally, a case study is conducted to assess the validity of my reference model and hybrid impedance. To do so, structurally simple forward models of both standard and hybrid impedances are calculated. The conductivity structure of this model was constrained by a 3-Dimensional inversion of continental MT data, controlled source EM (CSEM) data, and my reference model. From this model, hybrid impedances are observed to reproduces four decades of data measured by numerous receivers. From this evidence, I conclude that my reference model constrains the LARC and that it realistically represents the upper mantle. Additionally, I conclude that my hybrid impedance is a useful alternative to traditional MT impedance when conducting oceanbottom MT studies.Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 202
Constrainting Subglacial Heat Flux in Antarctica from Thermal Conductivity and Subglacial Lakes
Get PDFDeveloping accurate models for the dynamics of ice sheets requires detailed knowledge of
the temperature field within. An important constraint on internal ice sheet temperature
is provided by geothermal heat flux, the heat flow from the solid Earth to the base of the
ice sheet (Fowler 2006). This flow of heat is not uniform, varying as a result of differences
in thermal properties (i.e., thermal conductivity and heat production) and variations in
heat transfer across the lithosphere asthenosphere boundary. Since temperature can affect
a range of ice properties, from strain rate to hardness and melting rate (Paterson 1994),
it is important that we have a detailed understanding of the heat flux both below and
within the Antarctic Ice Sheet so that I can accurately map internal temperature. In
this thesis, I examine the heat flux in Antarctic environments as well as the properties
and factors that distort it. I also take an indirect approach to test geothermal heat flux
models by using melting associated with subglacial lakes as a constraint.
Heat can move both vertically and horizontally in order to find the path of least
thermal resistance to the surface. The path is dictated by the thermal conductivity of
the crustal material as heat will attempt to move through the most conductive material.
In a subglacial valley, or buried bedrock high, most heat will move through the more
conductive bedrock, resulting in heat being moved away from subglacial valleys and into
bedrock in regions of geological contacts whereby heat will move into the more conductive
of the two mediums. The result is the creation of localized regions where heat flux at the
base of the ice sheet can be 80 to 120% of the regional heat flux creating localized regions
of elevated/reduced temperature.
Having demonstrated the underlying bedrock thermal geology is critical to mapping the flow of heat through the Antarctic ice sheet, I collected the thermal conductivity on
49 Antarctic rock samples and combined them with a larger global database to develop
predictors for thermal conductivities in the inaccessible Antarctic lithosphere. From this
dataset, I determine oxide and mineral contributions to the effective thermal conductivity
for a range of igneous compositions. I exploit a correlation between high thermal
conductivities and low seismic velocities to produce an empirical model, which is applied
to a crustal tomography model to predict thermal conductivity of the Antarctic crust.
The largest lateral conductivity variations are found in a region with high conductivity
between 15 to 27 km, which also corresponds to an anomaly in proxy models for the
geothermal heat flux beneath the Antarctic Ice Sheet.
Several geothermal heat flux models for Antarctica have been made via proxy-based
estimates due to limited sampling across the continent. Proxy-based estimates have large
disagreements between each other (up to 50 mW m−2 in West Antarctica). To ascertain
accuracy, I test the proxy-based estimates using a basal heat flux constraint (BHFC)
assuming melting at the base of the ice sheet. In the presence of subglacial lakes, regions
where proxy-based estimates should exceed this constraint. I find that while results show
a subtle relation between lake and regions of elevated heat flux, a large number of lakes
are in regions of insufficient heat flux to generate melting. These results indicates that
current proxy models currently underestimate geothermal heat flux.
Since there is a relation between heat flux and lake locations, the proxy-based estimates
can be combined with other maps of Antarctic surface temperature, ice thickness, bedrock
elevation, crustal thickness, bedrock slope and ice velocity to predict lake melt source
regions. Three methods are tested, comparative property analysis, principal component
analysis and machine learning method using a Subspace KNN classifier. The comparative
analysis shows the properties of surface temperature, ice thickness and ice velocity to have
the greatest disparity between sub-glacial lakes and Antarctica but are unable to make a
clear prediction about the melt source for subglacial lakes. The PCA, while shown not
to be a good predictive map, is excellent at identifying regions of Antarctica as either
containing active lakes (with current water infill/outflow) or stable lakes (in which water levels remain constant). The Subspace KNN classifier meanwhile, is able to both identify
lake melt sources and type of lakes generated from those sources.
This work improves our ability to accurately map the geothermal heat flux at the base
of the Antarctic ice sheet by giving proxy modellers by showing the importance of bedrock
thermal conductivity as well as mapping it over a large section of Antarctica. We also
show future avenues of research that can improve upon or use these geothermal heat flux
models including mapping the melt sources of subglacial lakes.Thesis (Ph.D.) -- University of Adelaide, School of Geology and Earth Science, 202
Electrokinetic Methods and Applications in Australian Aquifer Settings: High-Dimension Electrical Tomography Imaging and Neural Network Filtration Techniques
Get PDFBeing the driest continent in the world, there is a significant reliance on groundwater resources within many communities and industries throughout Australia. Particularly in regional areas with low rainfall and surface runoff resources, the underlying groundwater availability plays a pivotal role in population capacity and economic prosperity. Whilst the importance of groundwater resources is indisputable, many aspects of its real world homeostatic processes, in both macro and micro scales, remain difficult to decipher and explain. Within Australia’s fractured rock aquifer systems, attributed with storage of the largest volume of groundwater resources nationally, there is still only fragmented understandings of several of their principal components and capacities. This is inclusive even of key aquifer characteristics, such as total volume estimations, regeneration sources, and their flow or transportation methods. Improved modeling capabilities and techniques based on prominent and robust hydrogeological principals are continually emerging from advancing technologies, new data sources and forward thinking. However, within the field data retrieval facet of hydrological research a seemingly slower evolution is taking place. A vast quantity of aquifer information is still derived directly from intrusive observation wells. Although the plethora of information these wells can yield in modelling is invaluable, there are some profound limitations that must still be addressed. Wells are costly to establish due to drilling expenses, can only provide single point information, and can also be disruptive to the homeostasis of the system. The self-potential method is an electro-kinetic geophysical method that has recently been re-identified as an immensely promising groundwater technique. It is a fast, passive, inexpensive surface technique which requires no drilling. Uniquely and most importantly however, it is the only geophysical method that is directly sensitive to not only the presence of groundwater, but also the physical flow of groundwater due to its generation of a measurable electrical signal. Previously regarded as a predominately qualitative geophysical tool, contributing factors including advancements in low-cost instrumentation and processing capabilities have meant self-potential surveys can now provide spatially significant quantitative data for a range of groundwater modelling inputs such as permeability. The method has been recurrently reviewed since its early conception in international geophysical literature through to modern times. However, only a small quantity of this peer reviewed research has been conducted within Australia. A lesser extent of published literature therefore deals in particularly with addressing the challenges of both our harsh climate, and surface and geological conditions. With our own unique geological and hydrogeological settings, current and future challenges regarding securement of groundwater resources, and increasingly common practice of industrial geotechnical processes such as fracking, all research and findings are vital contributions to furthering our understanding of potential groundwater applications for self-potential methods on home soil. This research thesis provides analyses of multiple electro-kinetic field research projects. New self-potential datasets have been collected in the Adelaide Hills targeting stimulated fractured-rock aquifers up to 40m below surface - a considerably deep target for the method, particularly within highly conductive Australian geological conditions. Previously collected geophysical datasets from the Adelaide Hills have been reprocessed from two to four-dimensions utulising newly constructed algorithms, then reanalysed with supporting geophysical datasets. And finally, a long term (46 day) self-potential monitoring program was conducted at a commercial-use porous media aquifer to investigate novel techniques in both autonomous groundwater flow presence investigation, and environmental noise filtering methodologies for a given self-potential dataset. This research endeavors to draw further conclusion on the self-potential methods prospective as a value-adding and commercial viability modern geophysical technique in Australian groundwater research. Additionally, employing use of artificial neural networks (machine learning) for the self-potential autonomous detection and environmental noise filtration methods, we highlight the current gap in geophysical literature regarding the combination of these techniques. A light is drawn to the combined techniques immensely promising future of potential applications and contributions within the wider electrical geophysics data automation and filtration space. Much akin to our continual pursuit for mineralisation deposits, Australia is searching deeper than ever before for crucial groundwater supplies as shallower sedimentary aquifers are becoming fully utilised or depleted. As we move forward towards this new era of deepening natural resources, we must further develop both old and new tools which can enhance clarity of understanding within these challenging hydrogeological systems.Thesis (MPhil) -- University of Adelaide, School of Physical Sciences, 201
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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