Inner core structure: Constraints from frequency dependent seismic anisotropy

2009 ◽  
Vol 341 (6) ◽  
pp. 439-445 ◽  
Author(s):  
Annie Souriau
Keyword(s):  
Seismic Anisotropy ◽  
Inner Core ◽  
Core Structure ◽  
Frequency Dependent

Observing the signature of the magnetic field's behaviour in the radial variation of inner core anisotropy

2020 ◽  
Author(s):  
Janneke de Jong ◽  
Lennart de Groot ◽  
Arwen Deuss
Keyword(s):  
Magnetic Field ◽  
Heat Flux ◽  
Seismic Anisotropy ◽  
Inner Core ◽  
Body Wave ◽  
Outer Core ◽  
Core Structure ◽  
Reversal Rate ◽  
Seismic Structure ◽  
The Magnetic Field

<p>The release of latent heat and lighter materials during inner core solidification is the driving force of the liquid iron flow in the outer core which generates the Earth's magnetic field. It is well known that the behaviour of the magnetic field varies over long time scales. Two clearly identifiable regimes are recognized, (i) superchrons and (ii) periods of hyperactivity (Biggin et al. 2012). Superchrons are characterized by an exceptionally low reversal rate of the magnetic pole and are associated with a low core mantle boundary (CMB) heat flux. Hyperactive periods are defined by a high reversal rate and have a high CMB heat flux.</p><p>Here we investigate whether the occurrence of these two regimes is related to radial variations in inner core seismic structure. Using seismic body-wave observations of compressional PKIKP-waves (Irving & Deuss 2011, Waszek & Deuss 2011, Lythgoe et al. 2013)., we construct a model of inner core anisotropy by comparing the difference between travel times for polar and equatorial rays. Anisotropy is the directional dependence of wave velocity and is determined by the structure of iron crystals in the inner core, hence changes in seismic anisotropy are due to changes in inner core crystal texture. We invert for radial changes in anisotropy while allowing for lateral variations and find that a model of the inner core containing five layers best fits our data. The model contains an isotropic uppermost inner core and four deeper layers with varying degrees of anisotropy.</p><p>Texture differences of the inner core iron crystals have been linked to changes in the solidification process of the inner core (Bergman et al. 2005), i.e. the motor of outer core flow. Therefore, the observed anisotropy variation, caused by variations of inner core solidification, might be related to changes in the behaviour of the magnetic field. Using an inner core growth model (Buffett et al. 1996) we convert depth to time for a range of inner core nucleation ages between 3.0 and 0.5 Ga (Olsen 2016). We find a remarkable correlation between the solidification time of the seismically observed layers and the occurrence of the magnetic regimes for two inner core ages; one with a nucleation at 1.4 Ga and one at 0.6 Ga, corresponding to an average CMB heat flux of 7.6 TW and 16.7 TW respectively.</p><p>Although we currently cannot differentiate between these two inner core ages considering our results alone, they do show that a relation between inner core structure and the behaviour of the magnetic field is possible, and suggest that seismic observations of inner core structure might provide new and independent insights into the magnetic field and its history.</p>

Intensity and Structure Changes during Hurricane Eyewall Replacement Cycles

2011 ◽  
Vol 139 (12) ◽  
pp. 3829-3847 ◽  
Author(s):  
Matthew Sitkowski ◽  
James P. Kossin ◽  
Christopher M. Rozoff
Keyword(s):  
Wind Field ◽  
Inner Core ◽  
Core Structure ◽  
Wind Maximum ◽  
Temporal Sampling ◽  
Structure Changes ◽  
Level Data ◽  
Flight Level ◽  
The Times ◽  
Eyewall Replacement Cycles

Abstract A flight-level aircraft dataset consisting of 79 Atlantic basin hurricanes from 1977 to 2007 was used to develop an unprecedented climatology of inner-core intensity and structure changes associated with eyewall replacement cycles (ERCs). During an ERC, the inner-core structure was found to undergo dramatic changes that result in an intensity oscillation and rapid broadening of the wind field. Concentrated temporal sampling by reconnaissance aircraft in 14 of the 79 hurricanes captured virtually the entire evolution of 24 ERC events. The analysis of this large dataset extends the phenomenological paradigm of ERCs described in previous observational case studies by identifying and exploring three distinct phases of ERCs: intensification, weakening, and reintensification. In general, hurricanes intensify, sometimes rapidly, when outer wind maxima are first encountered by aircraft. The mean locations of the inner and outer wind maximum at the start of an ERC are 35 and 106 km from storm center, respectively. The intensification rate of the inner wind maximum begins to slow and the storm ultimately weakens as the inner-core structure begins to organize into concentric rings. On average, the inner wind maximum weakens 10 m s−1 before the outer wind maximum surpasses the inner wind maximum as it continues to intensify. This reintensification can be quite dramatic and often brings the storm to its maximum lifetime intensity. The entire ERC lasts 36 h on average. Comparison of flight-level data and microwave imagery reveals that the first appearance of an outer wind maximum, often associated with a spiral rainband, typically precedes the weakening of the storm by roughly 9 h, but the weakening is already well under way by the time a secondary convective ring with a well-defined moat appears in microwave imagery. The data also show that winds beyond the outer wind maximum remain elevated even after the outer wind maximum contracts inward. Additionally, the contraction of the outer wind maximum usually ceases at a radius larger than the location of the inner wind maximum at the start of the ERC. The combination of a larger primary eyewall and expanded outer wind field increase the integrated kinetic energy by an average of 28% over the course of a complete ERC despite little change in the maximum intensity between the times of onset and completion of the event.

scholarly journals Antigenic Potential of a Highly Conserved Neisseria meningitidis Lipopolysaccharide Inner Core Structure Defined by Chemical Synthesis

2015 ◽  
Vol 22 (1) ◽  
pp. 38-49 ◽  
Author(s):  
Anika Reinhardt ◽  
You Yang ◽  
Heike Claus ◽  
Claney L. Pereira ◽  
Andrew D. Cox ◽  
...  
Keyword(s):  
Chemical Synthesis ◽  
Neisseria Meningitidis ◽  
Inner Core ◽  
Core Structure

The effect of D″ on PKP(AB−DF) travel time residuals and possible implications for inner core structure

2000 ◽  
Vol 175 (1-2) ◽  
pp. 133-143 ◽  
Author(s):  
Ludovic Bréger ◽  
Hrvoje Tkalčić ◽  
Barbara Romanowicz
Keyword(s):  
Travel Time ◽  
Inner Core ◽  
Core Structure

scholarly journals Inner core structure behind the PKP core phase triplication

2015 ◽  
Vol 201 (3) ◽  
pp. 1657-1665 ◽  
Author(s):  
Nienke A. Blom ◽  
Arwen Deuss ◽  
Hanneke Paulssen ◽  
Lauren Waszek
Keyword(s):  
Inner Core ◽  
Core Structure ◽  
Core Phase

Frequency-dependent seismic anisotropy due to fractures: Fluid flow versus scattering

Geophysics ◽  
2013 ◽  
Vol 78 (2) ◽  
pp. WA111-WA122 ◽  
Author(s):  
Alan F. Baird ◽  
J.-Michael Kendall ◽  
Doug A. Angus
Keyword(s):  
Fluid Flow ◽  
Seismic Anisotropy ◽  
Fractured Rock ◽  
High Concentration ◽  
Frequency Dependent ◽  
Similar Frequency ◽  
Seismic Properties ◽  
Microseismic Data ◽  
Fracture Parameters ◽  
Mineral Scale

Anisotropy is a useful attribute for the detection and characterization of aligned fracture sets in petroleum reservoirs. Unfortunately, many of the traditional effective medium theories for modeling the seismic properties of fractured rock are insensitive to the size of the constituent fractures. For example, the same pattern of anisotropy may be produced by a high concentration of small, stiff cracks or by a lower concentration of large, compliant fractures. The distinction between these models is important for assessing permeability anisotropy because fluid flow is dominated by the largest fractures. One method to gain further insight is through the analysis of frequency-dependent shear-wave splitting in microseismic data because fracture compliance is frequency dependent, and microseismic data are relatively rich in frequency content. We compared two potential mechanisms causing frequency-dependent compliance of fractures: (1) squirt flow in fractured porous rock and (2) wave scattering over rough fractures. Both models showed a sensitivity to average fracture size or compliance of the constituent fractures, and thus they provide a potential means to differentiate between anisotropy produced by small cracks or large fractures. We used both mechanisms to model frequency-dependent anisotropy data obtained from a fractured gas reservoir and invert for fracture parameters. Under certain conditions, the squirt-flow mechanism can cause significant frequency dependence in the microseismic band. However, the model is highly sensitive to the empirically derived mineral-scale relaxation time, which is poorly known and requires laboratory measurements to constrain. Conversely, producing a similar frequency response using the scattering model requires implausible fracture parameters; therefore, the squirt-flow model appears to be the most likely mechanism for microseismic applications. At higher frequencies, however, scattering may become more significant. Care should be taken when upscaling ultrasonic laboratory results for field-scale problems because different mechanisms may be at play within different frequency bands.

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