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  • Books
  • Articles  (11)
  • Express Letters, Geodynamics and Tectonics  (6)
  • Express Letters, Marine Geosciences and Applied Geophysics  (5)
  • Oxford University Press  (11)
  • Geological Society of London
  • MDPI Publishing
  • Geosciences  (11)
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  • Books
  • Articles  (11)
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  • Oxford University Press  (11)
  • Geological Society of London
  • MDPI Publishing
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  • Geosciences  (11)
  • 1
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    Ground topography effects on near-surface elastic full waveform inversion (2016)
    Oxford University Press
    Details
    Publication Date: 2016-08-10
    Description: The effects of neglecting ground surface topography variations in elastic full waveform inversion are investigated using two classes of synthetic example. The models include various high-contrast velocity and density anomalies, as they are often observed in near-surface applications. The first type of example shows that failing to account for even small amplitude fluctuations in topography introduces velocity artefacts in the near-surface part of the tomogram as well as degrades significantly the spatial resolution of features at greater depths. The disturbances are particularly severe when the topographic fluctuations have wavelengths comparable to the minimum seismic wavelength. The second type of synthetic example considers long wavelength topography variations of various amplitudes. It is found that neglecting topography with an amplitude fluctuation greater than half the minimum seismic wavelength leads to appreciable inversion image artefacts. Therefore, the incorporation of surface topography, even if it appears minor, is essential for successful elastic full waveform inversion of land seismic data.
    Keywords: Express Letters, 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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  • 2
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    Target-oriented Marchenko imaging of a North Sea field (2016)
    Oxford University Press
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    Publication Date: 2016-02-12
    Description: Seismic imaging provides much of our information about the Earth's crustal structure. The principal source of imaging errors derives from simplicistically modeled predictions of the complex, scattered wavefields that interact with each subsurface point to be imaged. A new method of wavefield extrapolation based on inverse scattering theory produces accurate estimates of these subsurface scattered wavefields, while still using relatively little information about the Earth's properties. We use it for the first time to create real target-oriented seismic images of a North Sea field. We synthesize underside illumination from surface reflection data, and use it to reveal subsurface features that are not present in an image from conventional migration of surface data. To reconstruct underside reflections, we rely on the so-called downgoing focusing function, whose coda consists entirely of transmission-born multiple scattering. As such, we provide the first field data example of reconstructing underside reflections with contributions from transmitted multiples, without the need to first locate or image any reflectors in order to reconstruct multiple scattering effects.
    Keywords: Express Letters, 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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  • 3
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    Frequency scaling of seismic attenuation in rocks saturated with two fluid phases (2016)
    Oxford University Press
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    Publication Date: 2016-11-14
    Description: Seismic wave attenuation is frequency dependent in rocks saturated by two fluid phases and the corresponding scaling behaviour is controlled primarily by the spatial fluid distribution. We experimentally investigate the frequency scaling of seismic attenuation in Berea sandstone saturated with two fluid phases: a liquid phase, water, and a gas phase, air, carbon dioxide or nitrogen. By changing from a heterogeneous distribution of mesoscopic gas patches to a homogeneous distribution of pore scale gas bubbles, we observe a significant steepening of the high-frequency asymptote of the attenuation. A transition from one dominant attenuation mechanism to another, from mesoscopic wave-induced fluid flow to wave-induced gas exsolution dissolution (WIGED), may explain this change in scaling. We observe that the high-frequency asymptote, for a homogenous pore scale gas bubble distribution, scales in accord with WIGED.
    Keywords: Express Letters, 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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  • 4
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    Capture of the Canary mantle plume material by the Gibraltar arc mantle wedge during slab rollback (2015)
    Oxford University Press
    Details
    Publication Date: 2015-04-16
    Description: Recent evidence suggests that a portion of the Canary plume travelled northeastwards below the lithosphere of the Atlas Mountains in North Africa towards the Alboran domain and was captured ~10 Ma ago by the Gibraltar subduction system in the Western Mediterranean. The capture would have been associated with the mantle return flow induced by the westward-retreating slab that would have dragged and trapped a portion of the plume material in the mantle wedge of the Gibraltar subduction zone. Such material eventually contaminated the subduction related volcanism in the Alboran region. In this work, we use scaled analogue models of slab–plume interaction to investigate the plausibility of the plume capture. An upper-mantle-scaled model combines a narrow (400 km) edge-fixed subduction plate with a laterally offset compositional plume. The subduction dominated by slab rollback and toroidal mantle flow is seen to increasingly impact on the plume dynamics as the area of influence of the toroidal flow cells at the surface is up to 500 x 1350 km 2 . While the plume head initially spreads axisymmetrically, it starts being distorted parallel to the plate in the direction of the trench as the slab trench approaches the plume edge at a separation distance of about 500 km, before getting dragged towards mantle wedge. When applied to the Canary plume–Gibraltar subduction system, our model supports the observationally based conceptual model that mantle plume material may have been dragged towards the mantle wedge by slab rollback-induced toroidal mantle flow. Using a scaling argument for the spreading of a gravity current within a channel, we also show that more than 1500 km of plume propagation in the sublithospheric Atlas corridor is dynamically plausible.
    Keywords: Express Letters, Geodynamics and Tectonics
    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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  • 5
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    On guided versus deflected fields in controlled-source electromagnetics (2015)
    Oxford University Press
    Details
    Publication Date: 2015-04-17
    Description: The detection of electrically resistive targets in applied geophysics is of interest to the hydrocarbon, mining and geotechnical industries. Elongated thin resistive bodies have been extensively studied in the context of offshore hydrocarbon exploration. Such targets guide electromagnetic fields in a process which superficially resembles seismic refraction. On the other hand, compact resistive bodies deflect current in a process which has more similarities to diffraction and scattering. The response of a real geological structure is a non-trivial combination of these elements—guiding along the target and deflection around its edges. In this note the electromagnetic responses of two end-member models are compared: a resistive layer, which guides the electromagnetic signal, and a resistive cylinder, which deflects the fields. Results show that the response of a finite resistive target tends to saturate at a much lower resistivity than a resistive layer, under identical survey configurations. Furthermore, while the guided electromagnetic fields generated by a buried resistive layer contain both anomalous horizontal and vertical components, the process of electromagnetic deflection from a buried resistive cylinder creates mainly anomalous vertical fields. Finally, the transmitter orientation with respect to the position of a finite body is an important survey parameter: when the distance to the target is much less than the host skin depth, a transmitter pointing towards the resistive cylinder will produce a stronger signal than a transmitter oriented azimuthally with respect to the cylinder surface. The opposite situation is observed when the distance to the target is greater than the host skin depth.
    Keywords: Express Letters, 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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  • 6
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    Geodetic model of the 2015 April 25 Mw 7.8 Gorkha Nepal Earthquake and Mw 7.3 aftershock estimated from InSAR and GPS data (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-18
    Description: We map the complete surface deformation of 2015 M w 7.8 Gorkha Nepal earthquake and its M w 7.3 aftershock with two parallel ALOS2 descending ScanSAR paths’ and two ascending Stripmap paths’ images. The coseismic fault-slip model from a combined inversion of InSAR and GPS data reveals that this event is a reverse fault motion, with a slight right-lateral strike-slip component. The maximum thrust-slip and right-lateral strike-slip values are 5.7 and 1.2 m, respectively, located at a depth of 7–15 km, southeast to the epicentre. The total seismic moment 7.55 x 10 20 Nm, corresponding to a moment magnitude M w 7.89, is similar to the seismological estimates. Fault slips of both the main shock and the largest aftershock are absent from the upper thrust shallower than 7 km, indicating that there is a locking lower edge of Himalayan Main Frontal Thrust and future seismic disaster is not unexpected in this area. We also find that the energy released in this earthquake is much less than the accumulated moment deficit over the past seven centuries estimated in previous studies, so the region surrounding Kathmandu is still under the threaten of seismic hazards.
    Keywords: Express Letters, Geodynamics and Tectonics
    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
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    Incomplete separability of Antarctic plate rotation from glacial isostatic adjustment deformation within geodetic observations (2015)
    Oxford University Press
    Details
    Publication Date: 2015-11-25
    Description: Geodetic measurements of Antarctic solid Earth deformation include signals from plate rotation and glacial isostatic adjustment (GIA). Through simulation, we investigate the degree to which these signals are separable within horizontal GPS site velocities that commonly define plate rotation estimates and that promise new constraints on models of GIA. Using a suite of GIA model predictions that incorporate both 1-D and 3-D Earth rheologies, we show that, given the present location of GPS sites within East Antarctica, unmodelled or mismodelled GIA signal within GPS velocities produces biased estimates of plate rotation. When biased plate rotation is removed from the GPS velocities, errors as large as 0.8 mm yr –1 are introduced; a value commonly larger than the predicted GIA signal magnitude. In the absence of reliable forward models of plate rotation or GIA then Antarctic geodetic velocities cannot totally and unambiguously constrain either process, especially GIA.
    Keywords: Express Letters, Geodynamics and Tectonics
    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
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    Dramatic effect of elasticity on thermal softening and strain localization during lithospheric shortening (2015)
    Oxford University Press
    Details
    Publication Date: 2015-12-13
    Description: We present two-dimensional numerical simulations for shortening a viscoelastoplastic lithosphere to quantify the impact of elasticity on strain localization due to thermal softening. The model conserves energy and mechanical work is converted into heat or stored as elastic strain energy. For a shear modulus G = 10 10 Pa, a prominent lithospheric shear zone forms and elastic energy release increases the localization intensity (strain rate amplification). For G = 5 x 10 10 Pa shear zones still form but deformation is less localized. For G = 10 12 Pa, the lithosphere behaves effectively viscoplastic and no shear zones form during homogeneous thickening. Maximal shearing-related increase of surface heat flux is 15–25 mW m –2 and of temperature at lower crustal depth is ~150 °C, whereby these peak values are transient (0.1–1 My). Elasticity and related energy release can significantly contribute to strain localization and plate-like behaviour of the lithosphere required for plate tectonics.
    Keywords: Express Letters, Geodynamics and Tectonics
    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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  • 9
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    Does wettability influence seismic wave propagation in liquid-saturated porous rocks? (2015)
    Oxford University Press
    Details
    Publication Date: 2015-11-07
    Description: In order to evaluate the effects of different fluids, we measured the P and S wave speeds through a series of synthetic epoxy-bonded carbonate composites with porosities from 21.9 to 31.5 per cent and saturated with air, with kerosene and with brine. These observed speeds were converted to the saturated bulk and shear moduli and compared to predictions made using Biot and Gassmann formulations. The observed bulk moduli agreed with those calculated for both kerosene and brine saturation as did the high frequency shear modulus under kerosene saturation. The observed shear modulus under water saturation, however, was significantly lower than the prediction. After excluding the currently known mechanisms of shear weakening, we suspect this disparity may be due to variations in the wetting of the epoxy that coats the pores surfaces. The kerosene completely wets this surface while the brine is only weakly hydrophilic with a wetting angle of 73.6°. At the molecular scale, this means that Stoke's no-slip boundary condition may not always apply as has been more recently demonstrated by many researchers in other disciplines such as microfluidic engineering but the implications for wave propagation in liquid-saturated rocks have not been considered.
    Keywords: Express Letters, 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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  • 10
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    Numerical simulation of the December 4, 2007 landslide-generated tsunami in Chehalis Lake, Canada (2015)
    Oxford University Press
    Details
    Publication Date: 2015-02-27
    Description: On December 4, 2007, a three million cubic metres landslide impacted Chehalis Lake, 80 km east of Vancouver, Canada. The failed mass rushed into the lake and parented a tsunami that ran up 38 m on the opposite shore, destroyed trees, roads and campsite facilities. Armed with field surveys and multihigh-tech observations from SONAR, LiDAR and orthophotographs, we apply the newly developed ‘Tsunami Squares’ method to simulate the Chehalis Lake landslide and its generated tsunami. The landslide simulation shows a progressive failure, flow speeds up to ~60 m s –1 , and a slide mass stoppage with uniform repose angle on the lakebed. Tsunami products suggest that landslide velocity and spatial scale influence the initial wave size, while wave energy decay and inundation heights are affected by a combination of distance to the landslide, bathymetry and shoreline orientation relative to the wave direction.
    Keywords: Express Letters, Geodynamics and Tectonics
    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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  • 11
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    An intraplate slow earthquake observed by a dense GPS network in Hokkaido, northernmost Japan (2014)
    Oxford University Press
    Details
    Publication Date: 2014-11-09
    Description: An intraplate slow earthquake was detected in northernmost Hokkaido, Japan, by a dense network of the global navigation satellite system. Transient abnormal acceleration of 〈12 mm was observed during the period 2012 July to 2013 January (~5.5 months) at several sites. The spatial displacement distribution suggests that a localized tectonic event caused localized deformation. Estimated fault parameter indicates very shallow-dip reverse faulting in the uppermost crust, with a total seismic moment of 1.75E + 17 N m ( M w 5.4). This fault geometry is probably consistent with detachment structure indicated by geological studies. A simultaneous earthquake swarm with the maximum magnitude M 4.1 suggests a possibility that the slow slip triggered the seismic activity for unknown reasons. This slow earthquake is slower than its moment would indicate, with a duration–magnitude scaling relationship unlike either regular earthquakes or subduction slow slip events. This result indicates that even if the area is under different physical property from subduction zones, slow earthquake can occur by some causes. Slow earthquakes exist in remote regions away from subduction zones and might play an important role in strain release and tectonic activity.
    Keywords: Express Letters, Geodynamics and Tectonics
    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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