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  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Analytical chemistry 36 (1964), S. 142-144 
    ISSN: 1520-6882
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Analytical chemistry 32 (1960), S. 1132-1135 
    ISSN: 1520-6882
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Marine controlled-source electromagnetic experiments are designed to measure the electrical conductivity of the sea-floor. The apparatus consists of a transmitter, typically an electric current dipole, and a series of remote receivers. Variations in the current through the dipole cause correlated variations in the electric and magnetic fields at the receivers. The signals contain information about the electrical conductivity of the crustal rocks. Electrical conductivity is related to such critical physical parameters as porosity, temperature, composition, fluid content and texture.
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A 2-D electromagnetic inverse problem relevant for geophysical exploration is considered. an exact theory is described which proves Fréchet differentiability of the data and the existence of optimal, or best-fitting, solutions. the earth is represented by a 2-D electrical conductivity structure with a known, horizontal strike direction and the data are surface measurements of electric field due to telluric earth currents flowing along-strike. First, the TE-mode equations governing the forward problem are cast in variational form. It is then shown that there exists at least one admissible conductivity that minimizes the discrepancy between the observations and variational solutions to the forward problem. Admissible conductivities are drawn from any compact subset of the class of bounded, positive functions. Standard techniques from elliptic partial differential equations, non-linear functional analysis and optimization play key roles in establishing the result. In particular, the Lax-Milgram lemma assures that a unique solution to the forward problem exists. an implicit function theorem approach is used to establish the regularity of the mapping from the conductivity to the data. the existence of optimal models follows from the well-known fact that a continuous functional attains its minimum when varied over a compact set. Previously, Fréchet differentiability and existence theories for geo-electromagnetic inverse problems have been limited to induction in 1-D media. However, characterization of the optimal models in two dimensions, analogous to Parker's D+ conductivity models in one dimension, is impeded by the lack of closed-form solutions to the forward problem.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: New oceanic crust is being formed along active segments of the global mid-ocean ridge (MOR) system. The presence of an axial magma chamber and associated zones of partial melt and hydrothermal activity located up to several kilometres beneath the seafloor is central to almost all recently proposed theories of crustal formation. Seismic images of the top few kilometres beneath the fast spreading East Pacific Rise (EPR) have already been obtained. Reflection profiles place strong constraints on the geometry of the axial magma chamber but refraction data provide only coarse estimates of the subsurface temperature, distribution of partial melt and porosity, parameters required to distinguish between the proposed petrological models. Electrical conductivity is directly related to all these critical parameters and therefore electromagnetic experiments should be designed to help characterize the ridge environment.We determine the magnetic field B(t) on the seafloor caused by a sudden change in current in a 2-D electric dipole aligned perpendicular to the strike of the ridge. The finite element technique is used to solve the governing differential equation numerically in the Laplace s-domain. The transformation to the time domain is by the Gaver-Stehfest method. We show that the extraction of two basic parameters from the response curve Ḃ(t) can provide sufficient information to identify the more important features of the petrology. The parameters are the response amplitude Ḃmax, which is the maximum derivative of δtB(t), and the diffusion time γT, the time at which this maximum occurs. The behaviour of γ as a function of distance from the source is analogous to that of first arrival time in refraction seismology. The value of γ is a weighted integral of the conductivity along the most resistive path between the source and the receiver.A highly conductive, partially molten magma chamber beneath the ridge axis slows the rate of diffusion of electromagnetic fields across the ridge, increasing γ but also reducing Ḃmax at sites on the side of the ridge opposite the transmitter. A melt lens ponding as a thin layer on top of the chamber increases Ḃmax at the ridge crest and increases γ at sites on the far side. Hydrothermal fluid circulation in the uppermost 2 km of the crust reduces Ḃmax everywhere across the ridge but increases γ only at sites within 0–3 km of the ridge crest. Electromagnetic energy in this case can reach the more distant points via paths which by-pass the fluids.Inferences made from the results of 2-D modelling indicate that a practical experiment would require a 104 A m horizontal electric dipole (HED) transmitter located 5 km off-axis and receivers with a sensitivity of at least 1 pT s−1 over a time window up to 10 s.
    Type of Medium: Electronic Resource
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  • 6
    Publication Date: 2016-12-25
    Description: In recent years, marine controlled source electromagnetics (CSEM) has found increasing use in hydrocarbon exploration due to its ability to detect thin resistive zones beneath the seafloor. It is the purpose of this paper to evaluate the physics of CSEM for an ocean whose electrical thickness is comparable to or much thinner than that of the overburden using the in-line configuration through examination of the elliptically polarized seafloor electric field, the time-averaged energy flow depicted by the real part of the complex Poynting vector, energy dissipation through Joule heating and the Fréchet derivatives of the seafloor field with respect to the subseafloor conductivity that is assumed to be isotropic. The deep water (ocean layer electrically much thicker than the overburden) seafloor EM response for a model containing a resistive reservoir layer has a greater amplitude and reduced phase as a function of offset compared to that for a half-space, or a stronger and faster response. For an ocean whose electrical thickness is comparable to or much smaller than that of the overburden, the electric field displays a greater amplitude and reduced phase at small offsets, shifting to a stronger amplitude and increased phase at intermediate offsets and a weaker amplitude and enhanced phase at long offsets, or a stronger and faster response that first changes to stronger and slower, and then transitions to weaker and slower. These transitions can be understood by visualizing the energy flow throughout the structure caused by the competing influences of the dipole source and guided energy flow in the reservoir layer, and the air interaction caused by coupling of the entire subseafloor resistivity structure with the sea surface. A stronger and faster response occurs when guided energy flow is dominant, while a weaker and slower response occurs when the air interaction is dominant. However, at intermediate offsets for some models, the air interaction can partially or fully reverse the direction of energy flux in the reservoir layer toward rather than away from the source, resulting in a stronger and slower response. The Fréchet derivatives are dominated by preferential sensitivity to the reservoir layer conductivity for all water depths except at high frequencies, but also display a shift with offset from the galvanic to the inductive mode in the underburden and overburden due to the interplay of guided energy flow and the air interaction. This means that the sensitivity to the horizontal conductivity is almost as strong as to the vertical component in the shallow parts of the subsurface, and in fact is stronger than the vertical sensitivity deeper down. However, the sensitivity to horizontal conductivity is still weak compared to the vertical component within thin resistive regions. The horizontal sensitivity is gradually decreased when the water becomes deep. These observations in part explain the success of shallow towed CSEM using only measurements of the in-line component of the electric field.
    Keywords: Marine Geosciences and Applied Geophysics
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 7
    Publication Date: 2016-12-11
    Description: Electromagnetic responses reflect the interaction between applied electromagnetic fields and heterogeneous geoelectrical structures. Quantifying the relationship between multiscale electrical properties and the observed electromagnetic response is therefore important for meaningful geologic interpretation. We present here examples of near-surface electromagnetic responses whose spatial fluctuations appear on all length scales, are repeatable and fractally distributed, supporting the notion of a ‘rough geology’ exhibiting multiscale hierarchical structure. Bounded by end member cases from homogenized isotropic and anisotropic media, we present numerical modelling results of the electromagnetic responses of textured and spatially correlated, stochastic geologic media, demonstrating that the electromagnetic response is a power law distribution, rather than a smooth response polluted with random, incoherent noise as commonly assumed. Our modelling results show that these electromagnetic responses due to spatially correlated geologic textures are examples of fractional Brownian motion. Furthermore, our results suggest that the fractal behaviour of the electromagnetic responses is correlated with degree of the spatial correlation, the contrasts in ground conductivity, and the preferred orientation of small-scale heterogeneity. In addition, the EM responses acquired across a fault zone comprising different lithological units and varying wavelengths of geologic heterogeneity also support our inferences from numerical modelling.
    Keywords: Marine Geosciences and Applied Geophysics
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 8
    Publication Date: 2011-12-31
    Description: Swarms of the salp Thalia democratica periodically occur off southeast Australia following the austral spring bloom of phytoplankton. In October 2008 a filament of upwelled water was advected south by the adjacent East Australian Current and formed a 30 km diameter cold-core eddy (CCE). The three-dimensional structure of a subsurface swarm of T. democratica within the eddy was examined using both oblique and vertical hauls and an optical plankton counter (OPC) deployed on a towed body. The CCE displayed distinct uplift of the nutricline and elevated fluorescence. Net samples show the zooplankton community was dominated by T. democratica, comprising 73%–88% of zooplankton abundance. The size distribution of T. democratica measured from net samples was 0.5–5 mm and was used to interpret the OPC transects, which showed the swarm formed a 15 km diameter disc located 20–40 m deep in the center of the eddy. The maximum salp abundance was in the pycnocline and coincided with the subsurface fluorescence maximum. The mean abundance of T. democratica size particles within the disc was 5003 individuals m−3 (ind. m−3), contrasted with only 604 ind. m−3 at the outer edge of the eddy. The vertically concentrated and horizontally constrained disc-shaped salp swarm occurred at the interface of salp-bearing inner shelf water and nutrient-rich upwelled water in a CCE. The physical processes that formed the CCE on the inshore edge of the western boundary current led to the largest density of salps recorded.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
    Publication Date: 2009-02-01
    Print ISSN: 1468-8115
    Electronic ISSN: 1468-8123
    Topics: Geosciences
    Published by Wiley-Blackwell
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  • 10
    Publication Date: 2012-08-28
    Description: The Tasman Sea is unique - characterised by a strong seasonal western boundary current that breaks down into a complicated field of mesoscale eddies almost immediately after separating from the coast. Through a 16-year analysis of Tasman Sea eddies, we identify a region along the southeast Australian coast which we name ‘Eddy Avenue’ where eddies have higher sea level anomalies, faster rotation and greater sea surface temperature and chlorophyll a anomalies. The density of cyclonic and anticyclonic eddies within Eddy Avenue is 23% and 16% higher respectively than the broader Tasman Sea. We find that Eddy Avenue cyclonic and anticyclonic eddies have more strongly differentiated biological properties than those of the broader Tasman Sea, as a result of larger anticyclonic eddies formed from Coral Sea water depressing chl. a concentrations, and for coastal cyclonic eddies due to the entrainment of nutrient-rich shelf waters. Cyclonic eddies within Eddy Avenue have almost double the chlorophyll a (0.35 mg m−3) of anticyclonic eddies (0.18 mg m−3). The average chlorophyll a concentration for cyclonic eddies is 16% higher in Eddy Avenue and 28% lower for anticyclonic eddies when compared to the Tasman Sea. With a strengthening East Australian Current, the propagation of these eddies will have significant implications for heat transport and the entrainment and connectivity of plankton and larval fish populations.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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