1,721,015 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
Get PDFGeophysical 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
Seafloor massive sulphide exploration using deep-towed controlled source electromagnetics: Navigational uncertainties
Get PDFDeep-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
Spatial and temporal evolution of rifting and continental breakup in the Eastern Black Sea Basin revealed by long-offset seismic reflection data
Get PDFThe 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
Corrigendum to "Switching of a paleo-ice stream in northwest Svalbard" [Quat. Sci. Rev. 30 (2011), 1710-1725]
No full textIce streams are the fast-flowing zones of ice sheets that can discharge a large flux of ice. The glaciated western Svalbard margin consists of several cross-shelf troughs which are the former ice stream drainage pathways during the Pliocene–Pleistocene glaciations. From an integrated analysis of high-resolution multibeam swath-bathymetric data and several high-resolution two-dimensional reflection seismic profiles across the western and northwestern Svalbard margin we infer the ice stream flow directions and the deposition centres of glacial debris that the ice streams deposited on the outer margin. Our results show that the northwestern margin of Svalbard experienced a switching of a major ice stream. Based on correlation with the regional seismic stratigraphy as well as the results from ODP 911 on Yermak Plateau and ODP 986 farther south on the western margin of Spitsbergen, off Van Mijenfjord, we find that first a northwestward flowing ice stream developed during initial northern hemispheric cooling (starting not, vert, similar2.8–2.6 Ma). A switch in ice stream flow direction to the present-day Kongsfjorden cross-shelf trough took place during a glaciation at not, vert, similar1.5 Ma or probably later during an intensive major glaciation phase known as the ‘Mid-Pleistocene Revolution’ starting at not, vert, similar1.0 Ma. The seismic and bathymetric data suggest that the switch was abrupt rather than gradual and we attribute it to the reaching of a tipping point when growth of the Svalbard ice sheet had reached a critical thickness and the ice sheet could overcome a topographic barrier
A 3-D seismic tomographic study of spreading structures and smooth seafloor generated by detachment faulting – the ultra-slow spreading Southwest Indian Ridge at 64˚30’E
No full textAt ultra-slow spreading ridges, with full spreading rates less than ∼20 mm/yr, spreading is accommodated both by highly spatially and temporally segmented magmatism, and tectonic extension along large-scale detachment faults that exhume ultramafic material to the seafloor. In the most magma-poor regions, detachment faulting alternates in polarity over time, producing a “flip-flopping” effect of subsequent detachment dips. The resulting seafloor in these regions displays a morphology termed “smooth seafloor” comprising elongate, broad ridges with peridotite/serpentinite lithologies. We conducted tomographic travel-time inversion of a 3-D wide-angle seismic data set acquired over a region of smooth seafloor around 64°30′E along the Southwest Indian Ridge (SISMOSMOOTH; Cruise MD199), to produce a seismic velocity volume through the crustal section and into the uppermost mantle. We observe patterns of velocity anomalies that correspond with variations in the bathymetry arising from the mode of spreading and are interpreted as changes in the degree of alteration with depth resulting from spatial and temporal variations in fluid-rock interaction, controlled by faulting and tectonic damage processes. The detachment faults do not show simple planar structures at depth but instead mirror the shapes of the bathymetric ridges that they exhume. Magmatic input is overall highly limited, but there is one region on the lower part of an exhumed detachment footwall where a thickness of volcanic material is observed that suggests a component of syn-tectonic volcanism, which could contribute to detachment abandonment
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