Context.
The interior of a neutron star is usually assumed to be made of cold catalyzed matter. However, the outer layers are unlikely to remain in full thermodynamic equilibrium during the formation ...of the star and its subsequent cooling, especially after crystallization occurs.
Aims.
We study the cooling and the equilibrium composition of the outer layers of a non-accreting neutron star down to crystallization. Here the impurity parameter, generally taken as a free parameter in cooling simulations, is calculated self-consistently using a microscopic nuclear model for which a unified equation of state has recently been determined.
Methods.
We follow the evolution of the nuclear distributions of the multi-component Coulomb liquid plasma fully self-consistently, adapting a general formalism originally developed for the description of supernova cores. We calculate the impurity parameter at the crystallization temperature as determined in the one-component plasma approximation.
Results.
Our analysis shows that the sharp changes in composition obtained in the one-component plasma approximation are smoothed out when a full nuclear distribution is allowed. The Coulomb coupling parameter at melting is found to be reasonably close to the canonical value of 175, except for specific values of the pressure for which supercooling occurs in the one-component plasma approximation. Our multi-component treatment leads to non-monotonic variations of the impurity parameter with pressure. Its values can change by several orders of magnitude reaching about 50, suggesting that the crust may be composed of an alternation of pure (highly conductive) and impure (highly resistive) layers. The results presented here complement the recent unified equation of state obtained within the same nuclear model.
Conclusions.
Our self-consistent approach to hot dense multi-component plasma shows that the presence of impurities in the outer crust of a neutron star is non-negligible and may have a sizeable impact on transport properties. In turn, this may have important implications not only for the cooling of neutron stars, but also for their magneto-rotational evolution.
We propose a new approach for imaging the subsurface using a stochastic wavefield of interface waves present in the ambient seismic field. Unlike seismic interferometry, our technique does not rely ...on cross correlations to obtain the Green's function between two seismic receivers. Rather, it relies on the local measurements of phase velocity obtained directly from the ratio between second‐order temporal and spatial derivatives of the wavefield. We process 10 min of ambient seismic noise recording made using a large and dense array installed over Ekofisk. We image a subsidence‐induced geomechanical imprint on the Scholte wave phase velocities in the near surface. This resulting phase velocity pattern is verified by comparison to results from a seismic‐noise cross‐correlation tomography.
Key Points
Presented a new method for seismic noise imaging suitable for short recordings
Applied to a data set, we find a subsidence‐induced pattern in velocity
Potential for improved imaging and monitoring of seismic noise on all scales
SUMMARY
Surface waves are a particular type of seismic wave that propagate around the surface of the Earth, but which oscillate over depth ranges beneath the surface that depend on their frequency of ...oscillation. This causes them to travel with a speed that depends on their frequency, a property called dispersion. Estimating surface wave dispersion is of interest for many geophysical applications using both active and passive seismic sources, not least because the speed–frequency relationship can be used to infer the subsurface velocity structure at depth beneath the surface. We present an inversion scheme that exploits spatial and temporal relationships in the scalar Helmholtz (wave) equation to estimate dispersion relations of the elastic surface wave data in both active and passive surveys, while also reconstructing the wavefield continuously in space (i.e. between the receivers at which the wavefield was recorded). We verify the retrieved dispersive phase velocity by comparing the results to dispersion analysis in the frequency-slowness domain, and to the local calculation of dispersion using modal analysis. Synthetic elastic examples demonstrate the method under a variety of recording scenarios. The results show that despite the scalar approximation made to represent these intrinsically elastic waves, the proposed method reconstructs both the wavefield and the phase dispersion structure even in the case of strong aliasing and irregular sampling.
We show that a reliable and statistically significant group velocity time‐lapse difference between 2004 and 2010 can be retrieved from ambient seismic noise in an offshore hydrocarbon exploitation ...setting. We performed a direct comparison of Scholte wave group velocity images obtained using regularized tomography. We characterize the expected variation in group velocity images from the 2004 or 2010 recordings that result from fluctuations in the cross correlations by looking at cross correlations of portions of the recordings. We prove that the time‐lapse difference is statistically significant. The time‐lapse group velocity image from ambient noise data shows strong similarities with a time‐lapse phase velocity map obtained from controlled source data. The most striking features are a northern and a southern group velocity increase due to compaction and subsidence as a result of reservoir production.
Key PointsWe present a new approach for time‐lapse seismic monitoringWe apply the new method to a large data set in a hydrocarbon exploration settingWe detect velocity changes in the subsoil caused by reservoir compaction
Seismic interferometry is applied to continuous seismic recordings spanning 5 days and over 2200 stations at the Valhall Life‐of‐Field Seismic array in the Norwegian North Sea. We retrieve both ...fundamental‐mode and first‐overtone Scholte waves by cross correlation. Ambient seismic noise tomography using the vertical component of this dense array produces group velocity maps of fundamental‐mode Scholte waves with high repeatability from only 24 h of recording. This repeatability makes daily reservoir‐scale near‐surface continuous monitoring of the subsurface feasible. Such monitoring may detect production‐related changes over a long time scale (months to years) and may be useful for early detection of short time scale hazards (days to weeks) such as migrating gases and fluids. We validate our velocity maps by comparing them with maps obtained independently from controlled‐source data.
Key Points
Noise correlation retrieves fundamental and first wave modes in OBC array.
Group velocity maps show subsurface features up to 250m below the sea floor.
Daily highly repeatable group-velocity maps shows noise-monitoring feasibility.
Mystifying mass in the right ventricle de Ridder, S P J; Polak, P E; ter Woorst, J ...
European heart journal cardiovascular imaging,
03/2020, Letnik:
21, Številka:
3
Journal Article
The authors present a novel Bayesian approach to adaptively select frequency samples to obtain a rational macromodel of device responses over a broad frequency range while performing as few ...electromagnetic simulations as possible. The method leverages a Bayesian approach to vector fitting to construct a data-driven uncertainty measure. The presented technique is demonstrated by application to a double semi-circular patch antenna and is shown to accurately and efficiently construct a rational macromodel over the frequency range of interest.
We formulated an anisotropic eikonal tomography approach for phase velocities based on a two‐dimensional elliptical‐anisotropic wave equation. We can fit the parameters of the ellipse directly from ...measured first‐order traveltime surface gradients and constrain these parameters to vary smoothly over space. The method is applied to Scholte waves in virtual seismic sources from stations in the Life of Field Seismic Ocean Bottom Cable array installed over the Ekofisk field. The fast directions of the azimuthally anisotropic Scholte wave velocities form a large circular pattern over the Ekofisk field. This pattern dominates the Scholte wave phase velocities at Ekofisk between 0.7 and 1.1 Hz. It results from the overburden stress state and from seafloor subsidence induced by decades of hydrocarbon extraction.
Key Points
Presented a method for eikonal tomography for elliptical‐anisotropic dispersion
We apply the method to a data set and find a large circular pattern over Ekofisk
This pattern is consistent with a production‐altered overburden stress state
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