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  • 1
    ISSN: 1432-1157
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract A newly developed towed transient electromagnetic (TEM) system is capable of measuring the electrical conductivity of the uppermost 5–10 m of sediments on the sea floor. The profiles of conductivity may be interpreted to give the porosity and likely texture of the bottom sediments. Recent tests of the system have demonstrated that data may be collected continuously in a surveying mode. Results from Knight Inlet, British Columbia, are in good agreement with sea floor samples from the area. Applications for the system include the rapid identification of sediment types for dredging operations, geotechnical surveys, or reconnaissance mapping of Quaternary geology.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 37 (1989), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The magnetometric resistivity (MMR) method uses a sensitive magnetometer to measure the low-level, low-frequency magnetic fields associated with the galvanic current flow between a pair of electrodes. While the MMR anomalies of simple structures such as dikes and vertical contacts have been determined analytically, there is a lack of systematic information on the expected responses from simple three-dimensional bodies. We determine the characteristic anomalies associated with square, plate-like conductors, which are excellent models of many base metal mineral deposits.The anomalies of plates of finite size are determined numerically using an integral equation method. A plate is subdivided into many sections and the current flow within each section is solved by equating the electrical field within each section to the tangential electrical field just outside it. When the plate size is small in relation to either the depth or the transmitter spacing, the shape and amplitude of the anomaly produced is closely approximated by a current dipole model of the same length and depth. At the other extreme, a large plate is represented by a half-plane. The dipole and half-plane models are used to bracket the behaviour of plates of finite size.The form of a plate anomaly is principally dependent on the shape, depth and orientation of the plate. A large, dipping plate near the surface produces a skewed anomaly highly indicative of its dip, but the amount of skew rapidly diminishes with increased depth or decreased size. Changes in plate conductivity affect the amplitude of the anomaly, but have little effect on anomaly shape. A current channelling parameter, determined from the conductivity contrast, can thus be used to scale the amplitude of an anomaly whose basic shape has been determined from geometrical considerations.The separation into geometrical and electrical factors greatly simplifies both the interpretation and modelling of MMR anomalies, particularly in situations with multiple plates. An empirical formula, using this separation, predicts the anomaly of two or more parallel plates with different conductances. In addition, the relation between the resolution of two vertical, parallel plates of equal conductance and their separation is determined.The ability of the integral equation method to model plate-like structures is demonstrated with the interpretation of an MMR anomaly in a survey conducted at Cork Tree Well in Western Australia. The buried conductor, a mineralized graphitic zone, is modelled with a vertical, bent plate. The depth to the top of the plate, and the plate conductance, is adjusted to fit the anomaly amplitude as closely as possible. From the modelling it would appear that this zone is not solely responsible for the observed anomaly.
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 32 (1984), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The cross-hole variant of the magnetometric resistivity (MMR) method requires two bore holes in the vicinity of a conductive target. In the first, two fixed current electrodes are located, one above the other. They are linked to a low frequency current source by cables, the whole system forming a vertical current bipole. In the second, a sensitive coil measures the axial magnetic field as a function of depth. For a uniform earth, if both holes are vertical, the measured component vanishes by symmetry. However, the presence of a local conductor channels the current and causes an anomalous magnetic component which is interpreted to indicate the position, shape and relative conductance of the target.Mineral deposits are often lamellar in form. The conductive disc is the simplest bounded lamella for which MMR responses may be computed. It is excited by a single current source on its axis. The second source and the surface of the earth are assumed to be far away, a valid assumption for down-hole measurements.The numerical method introduces a new integral equation describing the interaction of current dipoles located in the plane of the disc. The equation is solved analytically for a disc of infinite radius, a layer, and the result is compared with a corresponding known boundary value solution.The computed radial current in the disc and the magnetic field generated by it are described in terms of a current channelling number. The magnitude of the computed field is of the order of one nanoTesla for a typical mining problem.
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  • 4
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    In:  [Talk] In: MARELEC 2011, 20.-23.06.2011, San Diego, USA .
    Publication Date: 2017-07-26
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 5
    Publication Date: 2018-07-10
    Description: A typical marine controlled-source electromagnetic system consists of an electric dipole transmitter and one or more electric dipole receivers. The objective of a survey is to determine the seafloor resistivity by recording the electromagnetic transients, which diffuse through the earth from the transmitter to the receivers. Accurate knowledge of system geometry is crucial for proper interpretation; errors in the position and orientation of the transmitter and/or the receivers propagate into errors in the predicted seafloor resistivity. We show theoretically that for certain multireceiver set-ups and crustal electrical profiles that the geometry and the seafloor resistivity may be determined independently. A specific example is an experiment proposed in association with NEPTUNE Canada. Here, we have already deployed an electric dipole transmitter with a known orientation in a known location. A cabled streamer of receivers may be towed by a survey vessel in the vicinity of the transmitter on a known heading. For this configuration, an eigenparameter analysis of two seafloor models consisting of (1) a halfspace and (2) a resistive layer buried within a halfspace shows that the resistivity structure of the seafloor can be independently resolved from the cable location. Further studies of these two models also indicate that the position of the streamer must be roughly known in advance on the order of a hundred metres to be used as a suitable starting model in a non-linear inversion. The crucial information is contained in the parts of the pulse which travel through the seawater and which act as a calibration path. Such information is absent for a static DC method.
    Type: Article , PeerReviewed
    Format: text
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  • 6
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    Unknown
    AGU
    In:  Geophysical Research Letters, 25 (19). pp. 3647-3650.
    Publication Date: 2018-02-13
    Description: Vertical gradient electromagnetic sounding (VGS) on the Endeavour segment of Juan de Fuca mid‐ocean ridge reveals the presence of a 2D ridge‐parallel, conductivity anomaly. If the anomaly is caused mainly by melt in a conventional upper mantle upwelling zone alone, then the conductivity of the zone is about 0.6 S/m. The corresponding Archie's law melt fraction exceeds 0.10. A significantly lower melt fraction requires a sheet‐like, well interconnected melt. Upwelling zone conductivity can be reduced by a third if the anomaly is broadened and a crustal conductor is added to the model.
    Type: Article , PeerReviewed
    Format: text
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  • 7
    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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  • 8
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The eastern part of Middle Valley, on the northern Juan de Fuca Ridge, is characterized by locally high heat flow, evidence of hydrothermal activity, and near-surface mineralization. A magnetometric offshore electrical sounding (MOSES) survey was conducted in this area to determine the electrical structure of the sediment and upper crust. Two perpendicular lines of data were obtained across a ‘mound’, a 500 m diameter uplifted hill where sulphide mineralization and hydrothermal activity have been observed. The best-fitting model for the east-west regional line consists of a sediment layer of resistivity 0.70 ± 0.06 Ωm and thickness 500 ± 100 m overlying basaltic basement of resistivity 2.3 ± 0.6 Ωm. Basement porosities are estimated to be of the order of 10-14 per cent, using measured vent fluid temperatures of 275-300d̀C, consistent with previous estimates of the porosity in fractured oceanic crust. Results for the north-south line along the mound indicate the presence of a 1.35 ± 0.15 Ωm, 100 ± 20 m thick resistive cap overlying deeper sediments of much lower resistivity, of the order of 0.1 Ωm. The resistivity structure across a localized high heat flow anomaly is similar but the resistive cap is absent. The phase lags calculated from the resistivity model deviate significantly from the measured lags and provide the clearest evidence for transverse isotropy, the interbedding of very conductive with less conductive layers. The conductive thin layers are two orders of magnitude less resistive than the resistive layers because of massive sulphide mineralization, a result now confirmed by drilling. Additional data were gathered at a site centred about the southern end of a vent field 3–4 km northwest of the hydrothermal mound; the resistivities can be fit using either the regional model or a 1.1 Ωm halfspace. There are some anomalous values, which appear to be spatially correlated with basement topographic highs.
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We describe the conception, design, construction and testing of a towed electromagnetic system capable of mapping the near-surface electrical conductivity of the sea-floor. The transmitter and receiver coils are arranged coaxially, and dragged along the sea-floor. The transmitter coil is 2 m in length and 1 m in diameter, and contains 100 turns of wire. It is energized from the surface by a constant voltage, whose polarity is reversed every 5 ms. The resulting transient magnetic field is detected in the receiver coil. Received signals are amplified and sent back to the surface for processing and analysis.Following a transition in the transmitter current, two distinct transients are observed at the receiver. These events correspond to electromagnetic energy which has diffused through the sea-water and less conductive sea-floor, respectively. The onset, amplitude and decay of the first transient are primarily a function of the conductivity structure of the sea-floor.A successful survey with the system was carried out in shallow coastal waters east of Vancouver Island. The survey yielded 20 conductivity measurements along three lines. The data are stacked 512-fold, and the shape and amplitude of the resulting noise-reduced signal are compared with theoretical signals using a generalized linear inversion process. The shape, amplitude, and delay time of the received signal are indicative of the conductivity of the bottom sediments. The resulting model is a layer of mud of conductivity 1.2 S m-1 and variable thickness overlying rock or sediment with a conductivity of about 0.1 S m-1. The model is consistent with seismic log profiles obtained during the survey, and with conductivity values expected for surficial, marine sediments.
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  • 10
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A natural-source electromagnetic sounding of the earth made near a surface conductivity anomaly will resolve different features of the underlying conductivity structure than a sounding in a more uniform region. the surface-conductivity anomaly deflects horizontal electric currents induced by an external source into a vertical plane converting transverse-electric (TE) mode currents into the transverse (TM) mode. the resulting current distribution involves both vertical current flow and spatial variations with shorter wavelengths than the external field, providing increased resolution of resistive layers and of the conductivity structure at shallow depths. We exmine the sensitivity of the converted-mode response for the vertical-gradient sounding (VGS) method in order to plan electromagnetic soundings in a narrow ocean strait such as the Strait of Georgia between Vancouver Island and the Canadian mainland.An integral-equation method is used to model the current system induced by a mode converter, consisting of a known conductivity structure, such as a body of ocean water. It is shown that the depth of penetration of the secondary current distribution produced by the mode converter depends on both the horizontal scale of the feature and the distance from its edge. Within this depth range the current system is strongly perturbed by the existence of either conductive or resistive layers. the sensitivity of the VGS response (the ratio of the horizontal magnetic field at the base and surface of the mode converter) is examined using forward modelling of layered conductivity structures. the response is found to be dependent on both the TE and TM current systems. For a narrow ocean strait such as the Strait of Georgia, a measurement of the converted-mode VGS response along a line of sites on the floor of the strait, will provide resolution of conductive and resistive layers in the upper 10 km. the appropriate frequency range over which the VGS response should be measured in the strait is 10−2 Hz to 10 Hz.In our investigation of mode conversion we examine both the frequency- and time-domain response. Snap shots showing the current system evolving in the earth after a step or impulse illustrate the interaction of the EM signals with resistive and conductive layers. We show that the time-domain response can be used in a ‘geometrical sounding’analogous to seismic refraction to determine the conductivity structure. Finally we examine the limitations on the accuracy of the frequency and time-domain VGS response imposed by natural signal levels and instrument sensitivity.
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