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  • 1
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    Seismicity in the vicinity of the Tristan da Cunha hotspot: Particular plate tectonics and mantle plume presence (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Solid Earth, 122 (12). pp. 10427-10439.
    Details
    Publication Date: 2020-02-06
    Description: Earthquake locations along the southern Mid-Atlantic Ridge have large uncertainties due to the sparse distribution of permanent seismological stations in and around the South Atlantic Ocean. Most of the earthquakes are associated with plate tectonic processes related to the formation of new oceanic lithosphere, as they are located close to the ridge axis or in the immediate vicinity of transform faults. A local seismological network of ocean-bottom seismometers and land stations on and around the archipelago of Tristan da Cunha, allowed for the first time a local earthquake survey for one year. We relate intra-plate seismicity within the African oceanic plate segment north of the island partly to extensional stresses induced by a bordering large transform fault and to the existence of the Tristan mantle plume. The temporal propagation of earthquakes within the segment reflects the prevailing stress field. The strong extensional stresses in addition with the plume weaken the lithosphere and might hint at an incipient ridge jump. An apparently aseismic zone coincides with the proposed location of the Tristan conduit in the upper mantle southwest of the islands. The margins of this zone describe the transition between the ductile and the surrounding brittle regime. Moreover, we observe seismicity close to the islands of Tristan da Cunha and nearby seamounts, which we relate to ongoing tectono-magmatic activity.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1002/2017JB015017
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    How Offshore Groundwater Shapes the Seafloor (2018)
    AGU (American Geophysical Union) | Wiley
    In:  Eos: Earth & Space Science News, 99 .
    Details
    Publication Date: 2021-05-11
    Description: The MARCAN project, launched last January, is working to fill a gap in our knowledge of how freshwater flowing underground shapes and alters the continental margins.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1029/2018EO090691
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Hydrothermal circulation at the Cleft-Vance overlapping spreading center : results of a magnetometric resistivity survey (1998)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 103 (B6). pp. 12321-12338.
    Details
    Publication Date: 2018-04-12
    Description: We report on a magnetometric resistivity sounding carried out in the overlapping spreading center between the Cleft and Vance segments of the Juan de Fuca Ridge. The data collected reveal a strong three dimensionality in the crustal electrical resistivity structure on wavelengths of a few kilometers. Areas of reduced crustal electrical resistivities, with values approaching that of seawater, are seen beneath the neovolcanic zones of both active spreading centers. We interpret these reduced resistivities as evidence of active hydrothermal circulation within the uppermost 1 km of hot, young oceanic crust.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/98JB00599
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    The Electrical Properties of a 2D Conductive Zone underneath the Juan de Fuca Ridge (1998)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 25 (19). pp. 3647-3650.
    Details
    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
    DOI: 10.1029/98GL52807
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Verification of velocity-resistivity relationships derived from structural joint inversion with borehole data (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 40 (14). pp. 3596-3601.
    Details
    Publication Date: 2017-06-20
    Description: We present results of three-dimensional joint inversion of seismic, magnetotelluric, and gravity data over a marine salt dome. Such structures are difficult to image with a single method, and our results demonstrate how combining different techniques can yield improved results. More importantly, we examine the reliability of velocity-conductivity relationships derived from structure-coupled joint inversion approaches. Comparison with a seismic reflection section shows that our models match the upper limit of the salt. Furthermore, velocity and resistivity logs from a borehole drilled into the salt dome's flank match, within error, those recovered by the inversion. The good match suggests that the difference in length scale does not have a significant effect in this case. This provides a strong incentive to incorporate borehole data into the joint inversion in the future and substantiates approaches that use the relationships derived from joint inversion models for lithological classification.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/grl.50696
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Structure of the mantle beneath the Alboran basin from magnetotelluric soundings (2015)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 16 . pp. 4261-4274.
    Details
    Publication Date: 2018-02-27
    Description: We present results of marine MT acquisition in the Alboran sea that also incorporates previously acquired land MT from southern Spain into our analysis. The marine data show complex MT response functions with strong distortion due to seafloor topography and the coastline, but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3D inversion model has allowed us to image the structure in the underlying mantle. The resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath Gibraltar, which plunges nearly vertically beneath the Alboran. Our model contains three primary features of interest: a resistive body beneath the central Alboran, which extends to a depth of ~150 km. At this depth, the mantle resistivity decreases to values of ~100 Ohm-m, slightly higher than those seen in typical asthenosphere at the same depth. This transition suggests a change in slab properties with depth, perhaps reflecting a change in the nature of the seafloor subducted in the past. Two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle (or both). Of these, the one in the center of the Alboran basin, in the uppermost-mantle (20-30km depth) beneath Neogene volcanics and west of the termination of the Nekkor Fault, is consistent with geochemical models, which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2015GC006100
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Three-dimensional inversion of magnetotelluric impedance tensor data and full distortion matrix (2015)
    Wiley
    In:  Geophysical Journal International, 202 (1). pp. 464-481.
    Details
    Publication Date: 2018-07-10
    Description: Galvanic distortion of magnetotelluric (MT) data due to small-scale surficial bodies or due to topography is one of the major factors that prevents accurate imaging of the subsurface. We present a 3-D algorithm for joint inversion of MT impedance tensor data and a frequency-independent full distortion matrix that circumvents this problem. We perform several tests of our algorithm on synthetic data affected by different amounts of distortion. These tests show that joint inversion leads to a better conductivity model compared to the inversion of the MT impedance tensor without any correction for distortion effects. For highly distorted data, inversion without any distortion correction results in strong artefacts and we cannot fit the data to the specified noise level. When the distortion is reduced, we can fit the data to an RMS of one, but still observe artefacts in the shallow part of the model. In contrast, in both cases our joint inversion can fit the data within the assumed noise level and the resulting models are comparable to the inversion of undistorted data. In addition, we show that the elements of the full distortion matrix can be well resolved by our algorithm. Finally, when inverting undistorted data, including the distortion matrix in the inversion only results in a minor loss of resolution. We therefore consider our new approach a promising tool for the general analysis of field MT data.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1093/gji/ggv144
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    A framework for 3-D joint inversion of MT, gravity and seismic refraction data (2011)
    Wiley
    In:  Geophysical Journal International, 184 (1). pp. 477-493.
    Details
    Publication Date: 2018-07-10
    Description: We present a 3-D joint inversion framework for seismic, magnetotelluric (MT) and scalar and tensorial gravity data. Using large-scale optimization methods, parallel forward solvers and a flexible implementation in terms of model parametrization allows us to investigate different coupling approaches for the various physical parameters involved in the joint inversion. Here we compare two different coupling approaches, direct parameter coupling where we calculate conductivities and densities from seismic slownesses and cross-gradient coupling, where each model cell has an independent value for each physical property and a structural similarity is enforced through a term in the objective function. For both types of approaches we see an improvement of the inversion results over single inversions when the inverted data sets are generated from compatible models. As expected the direct coupling approach results in a stronger interaction between the data sets and in this case better results compared to the cross-gradient coupling. In contrast, when the inverted MT data is generated from a model that violates the parameter relationship in some regions but conforms with the cross-gradient assumptions, we obtain good results with the cross-gradient approach, while the direct coupling approach results in spurious features. This makes the cross-gradient approach the first choice for regions were a direct relationship between the physical parameters is unclear.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1111/j.1365-246X.2010.04856.x
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Joint inversion in hydrocarbon exploration (2016)
    AGU (American Geophysical Union) | Wiley
    In:  In: Integrated Imaging of the Earth: theory and applications. , ed. by Moorkamp, M., Lelievre, P. G., Linde, N. and Khan, A. Geophysical Monograph Series, 218 . AGU (American Geophysical Union), Wiley, Washington, DC, pp. 167-190. ISBN 978-1-118-92905-6
    Details
    Publication Date: 2017-01-04
    Description: This chapter investigates the particular challenges for joint inversion in the context of hydrocarbon exploration. It reviews the current literature and introduces interesting approaches that have been developed in the context of hydrocarbon exploration and could potentially be used in other application areas as well. Joint inversion methods incorporating petro physical models of the reservoir can directly yield quantities of interest such as porosity and permeability, but have to be carefully tuned to the area under investigation. The chapter presents two detailed joint inversion case studies, one for subsalt imaging and one for sub basalt imaging. It also covers emulation of the forward calculation, a concept that can be applied generally in joint inversion, but has only been applied in hydrocarbon exploration. For a successful joint inversion, it is necessary to invert each dataset, construct different coupling approaches, and evaluate the impact on the final results.
    Type: Book chapter , NonPeerReviewed
    DOI: 10.1002/9781118929063.ch9
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    OceanRep: Book chapter
  • 10
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    Approximations for the 2-D coast effect on marine magnetotelluric data (2012)
    Wiley
    In:  Geophysical Journal International, 189 . pp. 357-368.
    Details
    Publication Date: 2019-09-23
    Description: Marine natural source electromagnetic data acquired on continental margins are often of considerable scientific and commercial interest. However, the large conductivity contrast between the ocean and coast causes this type of data to be severely distorted. For a 2-D coastal model, this distortion is most pronounced for the marine magnetotelluric and geomagnetic response function derived from induced currents flowing parallel to the coast. A maximal distortion occurs for a given period at a specific distance from the coast and causes severe anomalies in the magnitude and phase of the response functions. Based on a modelling study, we empirically relate the characteristic period and characteristic distance to physical parameters such as the ocean depth and the host resistivity. Based on a simple analytical approach, we test these approximations and show that maximum distortion occurs when destructive interference between the ocean and host response is at its highest. While the coast effect causes a large distortion in the marine responses we show through a resolution analysis that it does not mask subsurface conductivity anomalies but in fact increases the sensitivity to the seafloor.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1111/j.1365-246X.2012.05385.x
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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