1,721,035 research outputs found
Measuring the geotherm with gas hydrate bottom-simulating reflectors: a novel approach using three-dimensional seismic data from the eastern Black Sea
No full textGas hydrate-related bottom-simulating reflectors mark the phase boundary between hydrate and free gas in the subsurface, and therefore may be used to estimate geothermal gradients and hence heat flow. The depth and temperature of the phase boundary depend on the composition of the hydrate-forming gas and of the pore fluid. In the absence of direct sampling, these compositions remain unknown. We develop an alternative approach that is less sensitive to compositional uncertainties and can be applied when the bottom-simulating reflector is densely sampled in a region with significant seabed relief. We apply this approach to a three-dimensional seismic dataset from the eastern Black Se
Localized rifting at Chagos Bank in the India-Capricorn plate boundary zone
No full textChagos Bank is a region of thick crust in the central Indian Ocean where a concentration of seismicity indicates that extension is ongoing. We have modeled bathymetry and gravity data to show that the crust-mantle boundary shallows near a seabed trough, consistent with a total extension of ~15 km over a zone ;50 km broad. This estimate is consistent with 5–10 m.y. of deformation at the present rate of relative India-Capricorn motion; the ~15 km of extension is toward the lower limit of the total extension expected from magnetic anomaly reconstructions, but would be accumulated in 1–3 m.y. in a regime having the present-day seismicity. We attribute the localization of seismicity beneath Chagos Bank to a weak rheology caused by its thick crust
Geophysical characterisation of the ocean–continent transition at magma-poor rifted margins
Full text linkGeophysical characterisation of the ocean-continent transition (OCT) at magma-poor riftedmargins has focused primarily on the determination of P wave velocities using wide-angleseismic techniques. Such experiments have shown that the OCT is heterogeneous, but thattypically velocities increase gradually with depth from ~5.0 km/s at top basement to ~8.0km/s at ~5 km deeper, without a large and abrupt Moho transition. The velocity variationwith depth is similar to that of old fracture zone crust, and appears to differ from that ofoceanic crust formed at ultra-slow spreading rates, though sampling of the latter is limited.Typically, the OCT is characterised by weakly lineated, low amplitude magnetic anomalies;the interpretation of these anomalies remains controversial. The oceanward limit of the OCTremains poorly defined on many margins
Assessment of gas hydrate saturations in the Makran accretionary prism using the offset dependence of seismic amplitudes
No full textWe estimate the saturations of gas hydrate and free gas based on measurements of seismic-reflection amplitude variation with offset (AVO) for a bottom-simulating reflector coupled with rock-physics modeling. When we apply the approach to data from a seismic line in the Makran accretionary prism in the Arabian Sea, the results reveal lateral variations of gas-hydrate and free-gas saturations of 4–29% and 1–7.5%, respectively, depending on the rock-physics model used to relate seismic velocity to saturation. Our approach is simple and easy to implement
A case study: travel time inversion for P-wave velocity using OBS data of South China Sea
No full textIt is very important for converting the seismic data from the time domain to the depth domain. Here we discuss the approaches of inverse modeling of travel times for determination of the P-wave velocity (Vp). The migration section of the single channel seismic data is used to define the model horizons and help to control their geometry. Wide angle hydrophone data of OBS are used to determine P-wave travel times. The picked travel times from various shots are inverted for P-wave interval velocities using RayInvr, which calculated theoretical travel times via ray tracing. Damped least squares optimization is performed to fine tune the fits between observed and calculated travel times. In the end, the Vp curve is achieved and the results are compared with that derived from the conventional hyperbolic curve velocity analysis method, the shape of the two curves are similar, and the velocity increases in the layer where gas hydrates are present
Lateral coexistence of ductile and brittle deformation shapes magma-poor distal margins: An example from the West Iberia-Newfoundland margins
Full text linkHighlights
• An unprecedented detailed tectono-thermal history of a magma-poor margin is revealed.
• Deformation mechanisms laterally vary across active faults during extreme extension.
• Mantle hydration occurs through brittle deformation in the footwalls of active faults.
• Detachments form through ductile shearing in the hangingwalls of active faults.
• Detachment formation is a byproduct but not a root cause of margin asymmetry.
Abstract
A long-standing problem in solid Earth science is to understand how low-angle normal faults form, their role in the development of tectonic asymmetry of conjugate margins, and how they relate to mantle hydration during continental breakup. The latter requires water to reach the mantle through active brittle faults, but low angle slip on faults is mechanically difficult. Here, we incorporate observations from high-resolution multichannel seismic data along the West Iberia-Newfoundland margins into a 2D forward thermo-mechanical model to understand the relationship between evolving rift asymmetry, detachment tectonics, and mantle hydration. We show that, during extreme extension, slip on active faults bifurcates at depth into brittle and ductile deformation branches, as a result of the cooling of the faults' footwall and heating of their hangingwall. The brittle deformation penetrates the Moho and leads to mantle hydration, while ductile deformation occurs in localized shear zones and leads to the formation of detachment-like structures in the distal margin sections. Such structures, as for example ‘S’ in the West Iberia-Newfoundland margins, are thus composed of several shear zones, active at low-angles, ∼25°-20°, and merging with the Moho at depth. The final sub-horizontal geometry of these structures is the result of subsequent back-rotation of these shear zones by new oceanward faults. Our results reproduce remarkably well the final sedimentary, fault, crustal architecture, and serpentinisation pattern observed at the West Iberia-Newfoundland margins. However, they challenge widely accepted ideas that such detachment-like structures formed by brittle processes, separate crust from mantle and caused conjugate margin asymmetry. Our model provides a quantitative framework to study hydrothermal systems related to serpentinization during extreme extension, their associated hydrogen, methane production, and the chemosynthetic life they sustain
Seafloor massive sulphide exploration using deep-towed controlled source electromagnetics: Navigational uncertainties
Full text linkDeep-towed geophysical surveys require precise knowledge of navigational parameters such as instrument position and orientation because navigational uncertainties reflect in the data and therefore in the inferred geophysical properties of the subseafloor. We address this issue for the case of electrical conductivity inferred from controlled source electromagnetic data. We show that the data error is laterally variable due to irregular motion during deep towing, but also due to lateral variations in conductivity, including those resulting from topography. To address this variability and quantify the data error prior to inversion, we propose a 2-D perturbation study. Our workflow enables stable and geologically reliable results for multicomponent and multifrequency inversions. An error estimation workflow is presented, which comprises the assessment of navigational uncertainties, perturbation of navigational parameters, and forward modelling of electric field amplitudes for a homogeneous and then a heterogeneous subseafloor conductivity model. Some navigational uncertainties are estimated from variations of direct measurements. Other navigational parameters required for inversion are derived from the measured quantities and their error is calculated by means of error propagation. Some navigational parameters show direct correlation with the measured electric fields. For example, the antenna dip correlates with the vertical electric field and the depth correlates with the horizontal electric field. For the perturbation study each standard deviation is added to the navigational parameters. Forward models are run for each perturbation. Amplitude deviations are summed in quadrature with the stacking error for a total, laterally varying, data error. The error estimation is repeated for a heterogeneous subseafloor model due to the large conductivity range (several orders of magnitude), which affects the forward model. The approach enables us to utilize data from several components (multiple electric fields, frequencies and receivers) in the inversion to constrain the final model and reduce ambiguity. The final model is geologically reasonable, in this case enabling the identification of conductive metal sulphide deposits on the seafloor.</p
Marine dipole–dipole controlled source electromagnetic and coincident-loop transient electromagnetic experiments to detect seafloor massive sulphides: effects of three-dimensional bathymetry
Full text linkSeafloor massive sulphides (SMSs) are regarded as a potential future resource to satisfy the growing global demand of metals including copper, zinc and gold. Aside from mining and retrieving profitable amounts of massive sulphides from the seafloor, the present challenge is to detect and delineate significant SMS accumulations, which are generally located near mid-ocean ridges and along submarine volcanic arc and backarc spreading centres. Currently, several geophysical technologies are being developed to detect and quantify SMS occurrences that often exhibit measurable contrasts in their physical parameters compared to the surrounding host rock. Here, we use a short, fixed-offset controlled source electromagnetic (CSEM) system and a coincident-loop transient electromagnetic (TEM) system, which in theory allow the detection of SMS in the shallow seafloor due to a significant electrical conductivity contrast to their surroundings. In 2016, CSEM and TEM experiments were carried out at several locations near the Trans- Atlantic Geotraverse hydrothermal field to investigate shallow occurrences of massive sulphides below the seafloor. Measurements were conducted in an area that contains distinct SMS sites located several kilometres off-axis from the Mid-Atlantic ridge, some of which are still connected to hydrothermal activity and others where hydrothermal activity has ceased. Based on the quality of the acquired data, both experiments were operationally successful. However, the data analysis indicates bias caused by three-dimensional (3D) effects of the rough bathymetry in the study area and, thus, data interpretation remains challenging. Therefore, we study the influence of 3D bathymetry for marine CSEM and TEM experiments, focusing on shallow 3D conductors located beneath mound-like structures.We analyse synthetic inversion models for attributes associated with 3D distortions of CSEM and TEM data that are not sufficiently accounted for in conventional 1D (TEM) and 2D (CSEM) interpretation schemes. Before an adequate quantification of SMS in the region is feasible, these 3D effects need to be studied to avoid over/underestimation of SMS using the acquired EM data. The sensitivity of CSEM and TEM to bathymetry is investigated by means of 3D forward modelling, followed by 1D (TEM) and 2D (CSEM) inversion of the synthetic data using realistic error conditions. Subsequently, inversion models of the synthetic 3D data are analysed and compared to models derived from the measured data to illustrate that 3D distortions are evident in the recorded data sets
Spatial and temporal evolution of rifting and continental breakup in the Eastern Black Sea Basin revealed by long-offset seismic reflection data
Full text linkThe age and distribution of the syn-rift and early post-rift infill records the spatial and temporal distribution of extension and breakup processes in a rift basin. The Eastern Black Sea Basin (EBSB) is thought to have formed by back-arc extension during Cretaceous to Early Cenozoic time. However, a lack of direct constraints on its deep stratigraphy leaves uncertainties over the time, duration and location for rifting and breakup processes in the basin. Here we use the enhanced imaging provided by 2D long-offset seismic reflection profiles to analyze the deep structural and stratigraphic elements of the EBSB. Based on these elements, we infer the presence of two distinct Late Cretaceous syn-rift units, recording initial extension (rift stage 1) over the continental highs (Shatsky Ridge and the Mid Black Sea High), followed by strain localization along the major basin-bounding faults and rift migration towards the basin axis (rift stage 2). Overlying these units, Palaeocene(?)-Eocene and Oligocene units show a syn-kinematic character in the NW, with evidence for ongoing extension until Oligocene time. Towards the SE, these sequences are instead post-kinematic, directly overlaying a basement emplaced during breakup. We interpret the Palaeocene(?)-Oligocene units to record the time spanning from the initiation of breakup (Late Cretaceous-Palaeocene, in the SE) to the end of extension (Oligocene, in the NW). The first ubiquitously post-rift infill is the Lower Miocene Maykop Formation. Our results highlight the along-strike temporal variability of extension and breakup processes in the EBSB
Tracking and monitoring fin whales offshore northwest Spain using passive acoustic methods
No full textFin whales produce regular vocalizations with a dominant frequency of ~20 Hz, similar to that of large airgun sources used in marine seismic surveys. Thus these vocalizations may be recorded on ocean-bottom seismometers (OBSs) or hydrophones (OBHs) deployed during such surveys. We used data recorded by an array of 72 OBSs/Hs with 4–6.5 km spacing, deployed offshore northwest Spain during June–August 2013, to study fin whale movements in this area. Whale vocalizations were identified automatically using signal-processing techniques and localized using time-delay estimates between several instruments. Airgun shooting took place during the deployment period, but we found no evidence for a correlation between vocalization detection rate and the presence or absence of shooting. Our analysis focused on six fin whale tracks identified during this period. Uncertainties in depth led to poor confidence intervals, preventing effective analysis of diving behavior for the identified tracks. In the horizontal plane, the localizations had a higher degree of confidence. Use of a Kalman filter resulted in smoother tracks. Mean swim velocities range from 2 to 15 km/hr, and the animals traveled distances of 1.5–15 km in the periods analyzed
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