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  • English  (4)
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  • 1
    Publication Date: 2020-02-12
    Description: Passive continental margins offer the unique opportunity to study the processes involved in continental extension and break up as well as the role of hot-spot related magmatism. We conducted combined on- and offshore seismic experiments in Northern Namibia designed to characterize the Southern African passive margin at the interaction with the Walvis Ridge, to assess the interaction of the presumed plume with continental lithosphere and to determine the deep structure of the transition from the coastal fold belt to the stable craton, where the Walvis Ridge hits the African continent. The seismic project integrated three experiments, (A) an onshore, coast-parallel refraction seismic profile, (B) two onshore-offshore wide-angle seismic transects, and (C) a combined on- and offshore seismic experiment to image the sub-Moho velocity (Pn tomography) at the ocean-continent transition (Fig. 1). The knowledge of the lithospheric structure of the margin together with results from other geoscientific studies (e.g., conducted within the SPPSAMPLE, DFG Priority Program 1375, South Atlantic Margin Processes and Links with onshore Evolution) will help to address fundamental questions such as, how continental crust and plume head interact, what the extent and volumes of magmatic underplating is, and how and which inherited (continental) structures might have been involved and utilized in the break-up process.
    Language: English
    Type: info:eu-repo/semantics/report
    Format: application/pdf
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  • 2
    Publication Date: 2020-12-14
    Description: Gas hydrates are naturally-occurring solid compounds of gas and water within almost all sediment-rich continental margins. Due to the large amounts of methane stored in submarine gas hydrates, they might serve as future reservoirs for offshore marine gas production. Assessing the reservoir characteristics requires reliable estimates of both the gas and gas hydrate concentration, which can be best addressed using geophysical and geological investigations. Here, we demonstrate the power of joint interpretation of interdisciplinary geophysical techniques and geological laboratory experiments. Regional 2D multichannel seismic data provide the broad overview of a hydrate-bearing area. High-resolution 2D and 3D seismic reflection data provide detailed images of two working areas, the buried S1 channel-levee system at 1500 m water depth (well within the gas hydrate stability zone) and a slope failure location, located at 665 m water depth (top limit of the hydrate formation) next to the S2 channel. Detailed compressional and shear wave (Vs) velocity-depth models were derived from four component ocean-bottom seismic data, the latter from P- to S-conversion upon reflection. Due to their steep reflection angles, shear wave events result in less resolved Vs models. Nevertheless, in case of a change in elasticity of the sediment matrix due to gas hydrate cementation, shear wave events can be used as an indicator. As such, Vs can give insight into the nature of hydrate formation throughout the GHSZ. We present new developments in the application of common reflection surface, normal-incidence-point tomography and full waveform inversion techniques to enhance model resolution for the seismic data sets. 2D and 3D controlled-source electromagnetic measurements provide volume information of the resistivity-depth distribution models. Electrical resistivity of the sediment formation depends on its porosity and the resistivity of the pore fluid. Gas hydrate and free gas generally have much higher electrical resistivities than saline pore fluid, and can be assessed using empirical relationships if the porosity and pore fluid salinity are known. Calibration with logging data, laboratory experiments on hydrate- or ice-bearing sediments, and resulting velocity and resistivity values, guide the joint interpretation into more accurate saturation estimations. Beyond that, a joint inversion framework supporting forward calculation of specialized geophysical methods at distributed locations is under development. In this paper, we summarize these individual components of a multi-parameter study, and their joint application to investigate gas hydrate systems, their equilibrium conditions and preservation of bottom-simulating-reflectors. We analyze data from two working areas at different locations and depth levels along the slope of the Danube Fan, which are both characterized by multiple bottom simulating reflectors indicating the presence of gas hydrate. In the first working area we located two depth windows with indications for moderate 16%–24% gas hydrate formation, but no vertical gas migration. In the second working area we observed fluid migration pathways and active gas seepage, limiting gas hydrate formation to less than 10% at the BSR. Some discrepancies remain between seismic-based and electromagnetic-based models of gas and gas hydrate distribution and saturation estimates, indicating that further in-situ investigations are likely required to better understand the gas hydrate systems at our study areas and to calibrate the inversion processes, which will be required for a joint inversion framework as well.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 3
    Publication Date: 2023-08-09
    Description: Glacier and slope instabilities pose significant hazards in mountain areas, with a high potential impact on the population. Forecasting glacier and slope instabilities remains challenging as sensing technology focusing on the surface might fail to detect damage and changes in subsurface elastic properties leading to large-scale failures. Seismic methods, such as seismic interferometry, can help address this observational gap by quantifying changes in material integrity. Here, we discuss two case studies in which seismology elucidates the development of cryospheric hazards: a hanging glacier instability and permafrost degradation on an active rockslide. We first analyze seismic data from Switzerland's Eiger hanging glacier before a 15,000 m3 break-off event. Our approach, based on an analysis of multiple icequake waveforms, allows us to measure seismic source migration. Combined with an analytical model based on damage mechanics our results quantify crevasse extension between unstable and stable ice masses. We then move to the second study site, an active rock slope near "Spitze Stei" in the Kandersteg region, Switzerland. The time series of relative seismic velocity variations (dv/v) constrain the lateral and depth-dependent extent of changes in the rock's elastic properties caused by pore pressure increase and potentially by permafrost thawing. The presented case studies illustrate how seismology can give quantitative insights into material damage and allow separating effects of irreversible damage growth from reversible thermoelastic hydrologic variations. This knowledge is needed to better predict the development of large failures and thus improve warning systems.
    Language: English
    Type: info:eu-repo/semantics/conferenceObject
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  • 4
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    Publication Date: 2024-02-13
    Description: Within the Inter-Wind project we study wind turbine (WT) emissions with ground motion and acoustic measurements which are accompanied by the acquisition of meteorological parameters as well as psychological surveys of residents living in the vicinity of the wind farms. Measurements are conducted on the Swabian Alb in Southern Germany at wind farms Tegelberg and Lauterstein in multiple interdisciplinary campaigns. Here we focus on measurements with line and ring layouts which are directed at improving the prediction of ground-motion emissions of WTs. This dataset contains recorder log files. Seismic data is stored at GEOFON, network 4C (2020 - 2024, Ritter and Gaßner 2022).
    Language: English
    Type: info:eu-repo/semantics/workingPaper
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