scholarly journals Geographic variations in lowermost mantle structure from the ray parameters and decay constants of core-diffracted waves

2017 ◽  
Vol 122 (7) ◽  
pp. 5369-5394 ◽  
Author(s):  
Garrett G. Euler ◽  
Michael E. Wysession
Keyword(s):  
Mantle Structure ◽  
Geographic Variations ◽  
Decay Constants ◽  
Diffracted Waves ◽  
Lowermost Mantle

scholarly journals Global mantle structure from multifrequency tomography using P, PP and P-diffracted waves

2019 ◽  
Vol 220 (1) ◽  
pp. 96-141 ◽  
Author(s):  
Kasra Hosseini ◽  
Karin Sigloch ◽  
Maria Tsekhmistrenko ◽  
Afsaneh Zaheri ◽  
Tarje Nissen-Meyer ◽  
...  
Keyword(s):  
Cross Correlation ◽  
P Wave ◽  
Mantle Structure ◽  
Eastern Asia ◽  
P Waves ◽  
Data Set ◽  
Spatial Coverage ◽  
Diffracted Waves ◽  
Dominant Period ◽  
Lowermost Mantle

SUMMARY In global-scale seismic tomography, teleseismic P and PP waves mainly constrain structures in the upper two thirds of the mantle, whereas core-diffracted waves (Pdiff) constrain the lower third. This study is the first to invert a very large data set of Pdiff waves, up to the highest possible frequencies. This results in tomographic resolution matching and exceeding that of global S-wave tomographies, which have long been the models of choice for interpreting lowermost mantle structure. We present three new global tomography models of 3-D isotropic P-wave velocity in the earth’s mantle. Multifrequency cross-correlation traveltimes are measured on all phases in passbands from 30 s dominant period to the highest frequencies that produce satisfactory fits (≈3 s). Model DETOX-P1 fits ≈2.5 M traveltimes from teleseismic P waves. DETOX-P2 fits the same data, plus novel measurements of ≈1.4 M traveltimes of Pdiff waves. DETOX-P3 fits the same data as DETOX-P2, plus ≈ 1.2 M PP traveltimes. Synthetics up to 1 s dominant period are computed by full wave propagation in a spherically symmetric earth using the spectral-element method AxiSEM. Traveltimes are linked to 3-D velocity perturbations (dVP/VP) by finite-frequency Fréchet kernels, parametrized on an adaptive tetrahedral grid of ≈400 000 vertices spaced by ≈80 km in the best-sampled regions. To complete spatial coverage, the waveform cross-correlation measurements are augmented by ≈5.7 million analyst-picked, teleseismic P arrival times. P, Pdiff and PP traveltimes are jointly inverted for 3-D isotropic P-velocity anomalies in the mantle and for events corrections, by least squares solution of an explicit matrix–vector equation. Inclusion of Pdiff traveltimes (in DETOX-P2, -P3) improves the spatial sampling of the lowermost mantle 100- to 1000-fold compared to teleseismic P waves (DETOX-P1). Below ≈2400 km depth, seismically slow anomalies are clustered at southern and equatorial latitudes, in a dozen or more intensely slow patches of 600–1400 km diameter. These features had long been classed into two large low shear velocity provinces (LLVP), which now appears questionable. Instead, patches of intensely slow anomalies in the lowermost mantle seem to form a nearly continuous, globe-spanning chain beneath the southern hemisphere, according to our increased resolution of LLVP-internal subdivisions and newly imaged patches beneath South America. Our tomography also supports the existence of whole-mantle plumes beneath Iceland, Ascension, Afar, Kerguelen, Canary, Azores, Easter, Galapagos, Hawaii, French Polynesia and the Marquesas. Seismically fast structure in the lowermost mantle is imaged as narrowly elongated belts under Eastern Asia and the Americas, presumably reflecting the palaeo-trench geometries of subduction zones and arcs that assembled Eastern Asia and the American Cordilleras in Palaeozoic and early Mesozoic times. Mid-mantle structure is primarily constrained by teleseismic P waves, but Pdiff data have a stabilizing effect, for example, sharpening the geometries of subducted slabs under the Americas, Eurasia and the Northern Pacific in the upper 2000 km. PP traveltimes contribute complementary constraints in the upper and mid mantle, but they also introduce low-velocity artefacts beneath the oceans, through downward smearing of lithospheric structure. Our three new global P-wave models can be accessed and interactively visualized through the SubMachine web portal (http://submachine.earth.ox.ac.uk/).

New form of Transmission Cross Coefficient for High-Resolution Imaging

1990 ◽  
Vol 48 (1) ◽  
pp. 60-61
Author(s):  
Kazuo Ishizuka
Keyword(s):  
Electron Microscope ◽  
High Resolution ◽  
Transmission Function ◽  
Incident Beam ◽  
Optical Microscope ◽  
Strong Interactions ◽  
Small Range ◽  
Specimen Thickness ◽  
Beam Direction ◽  
Diffracted Waves

It is well known that taking into account spacial and temporal coherency of illumination as well as the wave aberration is important to interpret an image of a high-resolution electron microscope (HREM). This occues, because coherency of incident electrons restricts transmission of image information. Due to its large spherical and chromatic aberrations, the electron microscope requires higher coherency than the optical microscope. On an application of HREM for a strong scattering object, we have to estimate the contribution of the interference between the diffracted waves on an image formation. The contribution of each pair of diffracted waves may be properly represented by the transmission cross coefficients (TCC) between these waves. In this report, we will show an improved form of the TCC including second order derivatives, and compare it with the first order TCC.In the electron microscope the specimen is illuminated by quasi monochromatic electrons having a small range of illumination directions. Thus, the image intensity for each energy and each incident direction should be summed to give an intensity to be observed. However, this is a time consuming process, if the ranges of incident energy and/or illumination direction are large. To avoid this difficulty, we can use the TCC by assuming that a transmission function of the specimen does not depend on the incident beam direction. This is not always true, because dynamical scattering is important owing to strong interactions of electrons with the specimen. However, in the case of HREM, both the specimen thickness and the illumination angle should be small. Therefore we may neglect the dependency of the transmission function on the incident beam direction.

Electron Optical Study of Surface Morphologies of Crystals

1980 ◽  
Vol 38 ◽  
pp. 376-377
Author(s):  
Sumio Iijima ◽  
Tung Hsu
Keyword(s):  
Thin Film ◽  
Optical Study ◽  
Regular Array ◽  
Surface Steps ◽  
Diffracted Waves ◽  
Image Position ◽  
Surface Morphologies

Suppose the thickness of a thin film of a crystal varies periodically like a regular array of surface steps, kinematical intensities of diffracted waves from this crystal are modulated by a shape transform,

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