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  • 1
    Electronic Resource
    Electronic Resource
    Resistivity structure of the underconsolidated sediments of the Cascadia Basin (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 118 (1994), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The Cascadia Basin is located on the abyssal plain of the Juan de Fuca Plate, off the west coast of Vancouver Island. The sediments of the Cascadia Basin are very thick, over 2 km, and are underconsolidated because of a high rate of sedimentation. Seismic results suggest that the sediment acoustic velocities are low, and increase slowly and monotonically with depth from the sea-floor. A vertical electric dipole (VED) survey was conducted to obtain the resistivity structure of the upper kilometre of sediments, to determine the physical properties of a thick sequence of abyssal basin sediments, and for comparison with seismic results. While models with multiple layers can be found which fit the data, the‘best-fitting’models share a common feature—decreasing resistivity with depth, which is suggestive of a gradient in the resistivity. The feature is consistent with the results of other geological and geophysical studies. In particular, the seismic results do not indicate any significant contrast in the physical properties, i.e. layering, in the upper 500 m. We extend the VED theory to the case where resistivity varies continuously with depth, and obtain a gradient model that fits the Cascadia Basin data marginally better than simple layered models. The model resistivity is combined with heat-flow measurements for the determination of physical properties versus depth; the physical property models, the porosity, for example, are not consistent with the physical properties derived from seismic survey results. The mismatch is diagnostic of the underconsolidation, and is primarily due to the effects of the high rate of sedimentation on the seismic velocity and on the heat flow, which in turn influence the physical properties.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1994.tb03996.x
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  • 2
    Electronic Resource
    Electronic Resource
    A study of the geoelectrical properties of peatlands and their influence on ground-penetrating radar surveying (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 42 (1994), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Geophysical surveys and chemical analyses on cores were carried out in three Ontario peatlands, from which we have gained a better understanding of the peat properties that control the geophysical responses. The electrical conductivity depends linearly on the concentration of total dissolved solids in the peat pore waters and the pore waters in turn bear the ionic signatures of the underlying mineral sediments. The ionic concentration, and thus the electrical conductivity, increase linearly from the surface to basement. The average bulk electrical conductivity of peatlands at Ellice Marsh, near Stratford, and at Wally Creek Area Forest Drainage Project, near Cochrane, are of the order of 25 mS/m. The Mer Bleue peatland, near Ottawa, has extremely high electrical conductivity, reaching levels of up to 380 mS/m near the base of the peat. The Mer Bleue peatland water has correspondingly high values of total dissolved solids, which originate from the underlying Champlain Sea glaciomarine clays. The dielectric permittivity in peats is largely controlled by the bulk water content. Ground penetrating radar can detect changes in water content greater than 3%, occurring within a depth interval less than 15 cm. The principal peatland interfaces detected are the near-surface aerobic to anaerobic transition and the peat to mineral basement contact. The potential for the successful detection of the basement contact using the radar can be predicted using the radar instrument specifications, estimates of the peatland depth, and either the bulk peat or the peat pore water electrical conductivities. Predicted depths of penetration of up to 10 m for Ellice Marsh and Wally Creek exceed the observed depths of 1 to 2 m. At Mer Bleue, on the other hand, we observe that, as predicted, a 100 MHz signal will penetrate to the base of a 2 m thick peat but a 200 MHz signal will not.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1994.tb00205.x
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