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  • 1
    Electronic Resource
    Electronic Resource
    Shallow crustal structure beneath the Juan de Fuca Ridge from 2-D seismic refraction tomography (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 100 (1990), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The formation of oceanic lithosphere along ocean ridges, and the role that crustal magma chambers play in the accretionary process, continue to be fundamental issues in plate tectonics. To address these issues, a multireceiver airgun/ocean-bottom seismograph refraction line was shot across the Endeavour segment of the Juan de Fuca Ridge near 48° N, 129° W. 2-D traveltime tomography applied to the data results in a three-layer model for the upper crust. Layer 1 is 250–650 m thick, with v1= 2.5 km s−1 and ∇Vzv1= 0.5 s−1. Layer 2 is ∼800 m thick, v2= 4.8 km s−1 and ∇Vzv2= 1.0 s−1. Layer 1 and layer 2 probably represent the sequence of extrusives and the transition to layer 3 (v3= 5.8 km s−1, ∇Vzv3= 0.5 s−1) is associated with the extrusives-dike complex transition. An abrupt velocity transition between layer 1 and layer 2 may be a metamorphic front within the basalt pillows or it may be the depth to which surface fissures penetrate. A low-velocity anomaly (velocities decreased by 〈 0.45 km s−1) exists beneath the ridge in layer 2 and upper layer 3. It is interpreted as a zone of increased fracture porosity and/or permeability associated with axial hydrothermal circulation, and correlates reasonably well with a sub-axial reflector. No evidence is found for the existence of a crustal magma chamber in the depth range of 1.5–3.5 km sub-bottom. However, the velocity anomalies observed in layer 3 suggest that crustal temperatures in this layer are elevated by 150–250°C beneath and to the east of the ridge relative to temperatures west of the ridge.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1990.tb00690.x
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  • 2
    Electronic Resource
    Electronic Resource
    Two-Dimensional Seismic Refraction Tomography (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 97 (1989), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: An iterative tomographic inversion scheme is presented for determination of 2-D velocity structure from seismic refraction first-arrival traveltimes. the method is suited to refraction profiles where source/receiver spacings are denser than for conventional profiles. the inversion method is based on an iterative solution of the linearized problem, and allows for determination of continuous velocity variations as welt as geometry of subhorizontal interfaces. In each iteration, two-point ray tracing is performed using a shooting method to construct the linear system. the velocity field is defined using triangular cells within which the velocity gradient is constant, allowing analytic calculation of ray paths. Two different inversion techniques are considered, based on distinct linearized formulations of the forward problem. Inversion using a linearized traveltime-velocity Jacobian produced better results than a slowness formulation more akin to common series expansion-techniques. Resolution examples reveal horizontal smearing due to ray geometry, drop-off in resolution with depth, as well as the effect of source-receiver geometry and velocity structure on resolution. Inversion examples indicate that a global norm produces solutions closer to the true model than solutions calculated using a smallest perturbation approach, when a good starting model is available. Streak effects caused by inhomogeneous ray coverage and the removal of these effects are demonstrated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1989.tb00498.x
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