1,721,024 research outputs found
Ocean Island Densities and Models of Lithospheric Flexure
No full textEstimates of the effective elastic thickness (Te) of the oceanic lithosphere based on gravity and bathymetric data from island loads are commonly significantly lower than those based on the wavelength of plate bending at subduction zones. The anomalously low values for ocean islands have been attributed to the finite yield strength of the lithosphere, to erosion of the mechanical boundary layer by mantle plumes, to pre-existing thermal stresses, and to overprinting of old volcanic loads by younger ones. A fifth possible contribution to the discrepancy is an incorrect assumption about the density of volcanic loads. We suggest that load densities have been systematically overestimated in studies of lithospheric flexure, potentially resulting in systematic underestimation of effective elastic thicknesses and overestimation of the effects of hotspot volcanism. We illustrate the effect of underestimating load density with synthetic examples and an example from the Marquesas Islands. This effect, combined with the other effects listed above, in many cases may obviate the need to invoke hotspot reheating to explain low apparent elastic thicknesses
(Table 2) Physical properties of IODP rock samples measured under various confining pressures
No full textIn supplement to: Falcon-Suarez, Ismael Himar; Bayrakci, Gaye; Minshull, Tim A; North, Laurence J; Best, Angus; Rouméjon, Stéphane; Expedition 357 Scientists (2017): Elastic and electrical properties and permeability of serpentinites from Atlantis Massif, Mid-Atlantic Ridge. Geophysical Journal International, 211, 686-699, https://doi.org/10.1093/gji/ggx34
(Table 1) Description of IODP rock samples from the Atlantis Massif, near the Mid-Atlantic Ridge
No full textIn supplement to: Falcon-Suarez, Ismael Himar; Bayrakci, Gaye; Minshull, Tim A; North, Laurence J; Best, Angus; Rouméjon, Stéphane; Expedition 357 Scientists (2017): Elastic and electrical properties and permeability of serpentinites from Atlantis Massif, Mid-Atlantic Ridge. Geophysical Journal International, 211, 686-699, https://doi.org/10.1093/gji/ggx34
Comparison of two- and three-dimensional full waveform inversion imaging using wide-angle seismic data from the Deep Galicia Margin
Full text linkFull waveform inversion (FWI) is a data-fitting technique capable of generating high-resolution velocity models with a resolution down to half the seismic wavelength. FWI is applied typically to densely sampled seismic data. In this study, we applied FWI to 3D wide-angle seismic data acquired using sparsely spaced ocean bottom seismometers (OBSs) from the Deep Galicia Margin west of Iberia. Our dataset samples the S-reflector, a low-angle detachment present in this area. Here we highlight differences between 2D, 2.5D and 3D-FWI performances using a real sparsely spaced dataset. We performed 3D FWI in the time domain and compared the results with 2D and 2.5D FWI results from a profile through the 3D model. When overlaid on multichannel seismic images, the 3D FWI results constrain better the complex faulting within the pre- and syn-rift sediments and crystalline crust compared to the 2D result. Furthermore, we estimate variable serpentinisation of the upper mantle below the S-reflector along the profile using 3D FWI, reaching a maximum of 45 per cent. Differences in the data residuals of the 2D, 2.5D and 3D inversions suggest that 2D inversion can be prone to overfitting when using a sparse dataset. To validate our results, we performed tests to recover the anomalies introduced by the inversions in the final models using synthetic datasets. Based on our comparison of the velocity models, we conclude that the use of 3D data can partially mitigate the problem of receiver sparsity in FWI
CSEM Survey of a Methane Vent Site, Offshore West Svalbard
No full textIn July 2012, the scientific cruise JCR269B on RRS James Clarke Ross conducted two Controlled Source ElectroMagnetic (CSEM) surveys over the West Svalbard margin to complement previously collected seismic data. The objective was to evaluate hydrate and free gas saturations in the submarine sediments in this area
Integrated geophysical characterization of crustal domains in the eastern Black Sea
Full text linkRifting may lead ultimately to continental breakup, but the identification and characterization of the resulting crustal distribution remains challenging. Also, spatial and temporal changes in breakup magmatism may affect the geophysical character of the newly formed oceanic crust, resulting in contrasting interpretations of crustal composition and distribution. In the Eastern Black Sea Basin (EBSB), the evolution from rifting to breakup has been long debated, with several interpretations for the distribution of stretched continental and oceanic crust. We interpret basement morphological variations from long-offset seismic reflection profiles, highlighting a northwest-to-southeast transition from faulted and tilted continental blocks to a rough and then smoother basement. We model magnetic anomalies to further constrain the various basement domains, and infer the presence of a weakly magnetized, stretched continental crust in the northwest, and a 0.4-3.8 A/m layer coinciding with the smooth basement in the central and southeastern area. We conclude that the EBSB oceanic crust extends farther to the northwest than was suggested previously from an abrupt change in crustal thickness and lower-crustal velocity. The apparent discrepancy between these different types of geophysical evidence may result from changes in magma supply during breakup, affecting the thickness and velocity structure of the resulting oceanic crust.</p
Bayrakci, Gaye; Falcon Suarez, Ismael; Minshull, Tim; North, Laurence; Best, Angus. (Table 1) Description of IODP rock samples from the Atlantis Massif, near the Mid-Atlantic Ridge. PANGAEA 2017, 10.1594/PANGAEA.873533 [Output (Electronic)]
No full textIn supplement to: Falcon-Suarez, Ismael Himar; Bayrakci, Gaye; Minshull, Tim A; North, Laurence J; Best, Angus; Rouméjon, Stéphane; Expedition 357 Scientists (2017): Elastic and electrical properties and permeability of serpentinites from Atlantis Massif, Mid-Atlantic Ridge. Geophysical Journal International, 211, 686-699, https://doi.org/10.1093/gji/ggx34
Bayrakci, Gaye; Falcon Suarez, Ismael; Minshull, Tim; North, Laurence; Best, Angus. (Table 1) Description of IODP rock samples from the Atlantis Massif, near the Mid-Atlantic Ridge. PANGAEA 2017, 10.1594/PANGAEA.873533 [Output (Electronic)]
No full textIn supplement to: Falcon-Suarez, Ismael Himar; Bayrakci, Gaye; Minshull, Tim A; North, Laurence J; Best, Angus; Rouméjon, Stéphane; Expedition 357 Scientists (2017): Elastic and electrical properties and permeability of serpentinites from Atlantis Massif, Mid-Atlantic Ridge. Geophysical Journal International, 211, 686-699, https://doi.org/10.1093/gji/ggx34
The effect of heterogeneities in hydrate saturation on gas production from natural systems
Full text linkUnderstanding the rate and time evolution of gas release from natural gas hydrate systems is important when evaluating the potential of gas hydrate as a future energy source, or the impact of gas from hydrate on climate. The release of gas from hydrate is heavily influenced by a number of factors, many of which vary through the hydrate system. The fundamental heterogeneity of natural gas hydrate systems is often poorly represented in models. Here we simulate depressurisation-induced gas production from a single vertical well in 34 models with heterogeneous 2D distributions of hydrate that include layered, columnar or random configurations and comparable models with homogenous saturation distributions. We found that the temporal evolution of gas production rate follows a consistent trend for all models, but at any time the gas production rate across the models varied by up to ±35% in the first year of production, and by up to ±25% thereafter. The primary control on the gas production rate is the overall amount of hydrate in the system, but local variations in hydrate saturation cause significant fluctuations in the time evolution of production. These hydrate variations can cause changes in the gas flow path through the system and associated drops in gas production rate continuing for multiple years. Overall, our results suggest that small levels of heterogeneity in hydrate systems can cause variations in the gas production rate similar in scale to much larger variations in homogenous systems. Our work provides an error margin for previously modelled gas production rates, and a note of caution for potential commercial development of gas hydrate
Seismic characterisation of multiple BSRs in the Eastern Black Sea Basin
Full text linkLong offset seismic reflection data reveal the presence of four Bottom Simulating Reflectors (BSR0-3) within folded sediments of the Tuapse Trough, along the NE margin of the Eastern Black Sea Basin (EBSB). Multiple BSRs are observed in other sites worldwide, however, their origin and formation mechanisms are still debated. Here, we investigate the formation mechanisms of the EBSB multiple BSRs based on their seismic character and on their physical properties derived from reflected and refracted arrival seismic velocities. Seismic reflection data are downward continued to enhance refracted arrivals. A 2D travel-time velocity model of the sub-seabed, using combined travel-times from non-downward-continued reflected and downward-continued refracted signals, shows variations in the physical properties at the BSRs and nearby sediments. The P-wave velocity (VP) increase of 1.55–1.72 km/s between the seafloor and BSR0 (258 mbsf) reflects normal compaction trends in sediments, whereas the VP of 1.75–1.83 km/s between BSR0 and BSR1 (360 mbsf) is higher than that expected for sediments at that depth. Beneath BSR1, a VP decrease from 1.83 km/s to 1.61 km/s occurs within a 70-80 m-thick layer including BSR2 (395 mbsf) and extending to BSR3 (438 mbsf). Beneath BSR3, VP increases. Based on an analytical model linking seismic velocity to physical properties, these VP trends can be explained by a gas hydrate saturation from 0 to 2% between the seafloor and BSR0, reaching 4 ± 2% just above BSR1. A free gas saturation of up to 20–25% is estimated within the low-velocity zone between BSR1 and BSR3. BSR1 likely represents the present-day base of the gas hydrate stability zone (BGHSZ), which aligns with the theoretical BGHSZ assuming a geothermal gradient of 26–30 °C/km. Based on seismic polarities and results from travel-time analysis and rock physics modelling, we suggest that hydrate dissociation and recycling processes may explain the negative polarity of BSR2 and BSR3, which are still visible due to the presence of relict gas, and inferred higher gas hydrate saturations close to the present-day base of the stability zone at BSR1. Also, structural and stratigraphic controls seem to have favoured focused free gas flow and hydrate formation at the top of an anticlinal structure, thus likely controlling multiple BSR generation in the EBSB
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