Three-dimensional Density Structure Research Articles

Multi-Scale Scratch Analysis in Qinghai-Tibet Plateau and its Geological Implications

Multi-scale scratch analysis on a regional gravity field is a new data processing system for depicting three-dimensional density structures and tectonic features. It comprises four modules including the spectral analysis of potential fields, multi-scale wavelet analysis, density distribution inversion, and scratch analysis. The multi-scale scratch analysis method was applied to regional gravity data to extract information about the deformation belts in the Qinghai-Tibet Plateau, which can help reveal variations of the deformation belts and plane distribution features from the upper crust to the lower crust, provide evidence for the study of three-dimensional crustal structures, and define distribution of deformation belts and mass movement. Results show the variation of deformation belts from the upper crust to the lower crust. The deformation belts vary from dense and thin in the upper crust to coarse and thick in the lower crust, demonstrating that vertical distribution of deformation belts resembles a tree with a coarse and thick trunk in the lower part and dense and thin branches at the top. The dense and thin deformation areas in the upper crust correspond to crustal shortening areas, while the thick and continuous deformation belts in the lower crust indicate the structural framework of the plateau. Additionally, the lower crustal deformation belts recognized by the multi-scale scratch analysis coincide approximately with the crustal deformation belts recognized using single-scale scratch analysis. However, deformation belts recognized by the latter are somewhat rough while multi-scale scratch analysis can provide more detailed and accurate results.

Electron density profiles probed by radio occultation of FORMOSAT-7/COSMIC-2 at 520 and 800 km altitude

Abstract. The FORMOSAT-7/COSMIC-2 (F7/C2) will ultimately place 12 satellites in orbit with two launches with 24–28.5° inclination and 520–550 km altitude in 2016 and with 72° inclination and 720–750 km altitude in 2018. It would be very useful for the community to construct the global three-dimensional electron density structure by simultaneously combining the two launch observations for studying ionospheric structure and dynamics. However, to properly construct the global electron density structure, it is essential to know and evaluate differences between the ionospheric electron densities probed by the two launches. To mimic the F7/C2 observations, we examine the electron density probed at the two satellite altitudes 500 and 800 km by means of FORMOSAT-3/COSMIC (F3/C) observations at the parking orbit 500 km altitude and mission orbit 800 km altitude, as well as a corresponding observing system simulation experiment (OSSE). Observation and OSSE results show that the sounding geometries by satellite orbiting at 500 and 800 km altitudes can cause the overall differences in the electron density, the F2 peak electron density, and the F2 peak height of about 18–24, 12–28 %, and 7–19 km, respectively. Results confirm that the discrepancies mainly result from the sounding geometry and the grid (contour) bias of the electron density.

Three‐Dimensional Density Structure of North China's Craton Crust and Its Geological Implications

AbstractOne of the goals of geological interpretation on gravity anomalies is to reveal three‐dimensional density structure of the earth's crust, for which density inversion is crucial. This paper presents multiple‐scale density inversion of gravity anomalies. Firstly wavelet transform is employed to decompose gravity anomalies. Secondly, power spectrum analysis is performed to estimate the average depth of the field source for each layer, and then the generalized density inversion method is used to calculate the depth of each layer. Finally, such multi‐scale density inversion is conducted for the North China region, resulting in the density structure of its crust. The results demonstrate that this approach is effective. The followed analysis indicates that low density anomalies in orogenic belts mainly reflect the granite bodies distributed there. In comparison with shallow density disturbance in eastern North China, depressions bearing oil and gas seem to have low‐density anomalies of small amplitude. It implies that the density information extracted by wavelet multi‐scale inversion has some guiding significance for deployment of oil and gas exploration.

Three-dimensional density structure of the lithosphere beneath the North China Craton and the mechanisms of its destruction

Abstract The lithosphere beneath the North China Craton (NCC) has been thinning since the Phanerozoic. Previous geophysical studies on NCC structures have mainly focused on the results of seismic wave propagation. In this study, we obtained 3D density structures of the NCC lithosphere by using the sequential inversion of observed gravity data and P-wave travel times. Analyses of the resulting density model and discussions of the destruction of the NCC are provided. Our density model shows that distinct horizontal and vertical density heterogeneities exist throughout the lithosphere beneath the NCC; in the west of the craton, the Ordos block is characterised by a relatively uniform high-density lithospheric mantle, while laterally heterogeneous densities are observed at shallower than 100 km depth. Distinct low-density anomalies are imaged in the Cenozoic Yinchuan–Hetao and Shaanxi–Shanxi rifts surrounding the Ordos block. Comparing with the thinned lithosphere and high Poisson's ratio, the low density in the rifts may indicate partial melting induced by thermo-mechanical erosion. Predominantly low-density anomalies, which represent an upwelling of the asthenosphere, are revealed in the south of the Taihang orogen, as well as at the junction of the Yinshan–Yanshan orogen (near Datong volcano) and the Taihangshan orogen, from 100 to 200 km depth. This supports the hypothesis of thermo-mechanical erosion in these regions. High-density anomalies are visible at the base of the lithosphere or in the upper mantle beneath the central North China Basin. Combined with high velocities at 300 km depth indicated by seismic tomography, these high densities suggest that the lithospheric destruction of this region may not be explained by thermo-mechanical erosion, as there is no evidence of deep, hot upwelling that could support thermo-mechanical erosion.

Moho structure of Central America based on three-dimensional lithospheric density modelling of satellite-derived gravity data

The Central American isthmus hosts a highly variable Moho structure due to the diverse origin and composition of the crustal basement and the influence of large-scale neotectonic processes. Gravity data from the combined geopotential model EGM2008 were interpreted via forward modelling to outline the three-dimensional lithospheric density structure along the Middle American Trench, as well as the segmentation of the oceanic Cocos and Nazca plates and the overriding Caribbean plate. In this work, results for the depth of the Moho obtained from the density model are presented. The Quaternary volcanic arc correlates with a maximum Moho depth of 44 km in western Guatemala. To the south-east of the continental shelf, the Caribbean plate shows Moho depths between 20 and 12 km whereas to the north, values as shallow as 8 km are observed at the Cayman trough. For the oceanic Cocos plate, depths between 16 and 21 km are obtained for the Moho along the Cocos ridge, contrasting with values between 15 and 12 km for the seamount segment and 8 and 11 km for the segments of the crust that are not affected by the Galapagos hot-spot track.

HYPERION: an open-source parallelized three-dimensional dust continuum radiative transfer code

HYPERION is a new three-dimensional dust continuum Monte-Carlo radiative transfer code that is designed to be as generic as possible, allowing radiative transfer to be computed through a variety of three-dimensional grids. The main part of the code is problem-independent, and only requires an arbitrary three-dimensional density structure, dust properties, the position and properties of the illuminating sources, and parameters controlling the running and output of the code. HYPERION is parallelized, and is shown to scale well to thousands of processes. Two common benchmark models for protoplanetary disks were computed, and the results are found to be in excellent agreement with those from other codes. Finally, to demonstrate the capabilities of the code, dust temperatures, SEDs, and synthetic multi-wavelength images were computed for a dynamical simulation of a low-mass star formation region. HYPERION is being actively developed to include new features, and is publicly available (http://www.hyperion-rt.org).

3-D-geocoronal hydrogen density derived from TWINS Ly-α-data

Abstract. Based on Ly-α-line-of-sight measurements taken with two Ly-α detectors onboard of the satellite TWINS1 (Two Wide-angle Imaging Neutral-atom Spectrometers) density profiles of the exospheric, neutral geocoronal hydrogen were derived for the time period between summer solstice and fall equinox 2008. With the help of specifically developed inversion programs from Ly-α line of sight intensities the three-dimensional density structure of the geocoronal hydrogen at geocentric distances r>3 RE could be derived for the period mentioned characterized by very low solar 10.7 cm radiofluxes of ≈65–70 [10−22 W m−2 Hz−1]. The time-variable, solar "line-centered"-Ly-α-flux was extracted on the basis of daily (terrestrial) NGDC 10.7 cm radioflux data using the models from Barth et al. (1990) and Vidal-Madjar (1975). The results for the geocoronal H-densities are compared here both with theoretical calculations based on a Monte-Carlo model by Hodges (1994) and with density profiles obtained with the Geocoronal Imager (GEO) by Østgaard and Mende (2003). In our results we find a remarkably more pronounced day-/night-side asymmetry which clearly hints to the existence of a hydrogen geotail (i.e. a tail structure with comparatively higher hydrogen densities on the night side of the earth for geocenctric distances >4 RE), and a only weakly pronounced polar depletion. These unexpected features we try to explain by new models in the near future. The derived 3-D-H-density structures are able to explain the line-of-sight (LOS) dependent Ly-α intensity variations for all LOS seen up to now with TWINS-LAD. The presented results are valid for the region with geocentric distances 3 RE<r<7 RE and are based on the reasonable assumption of an optically thin H-exosphere with respect to resonant Ly-α-scattering allowing the use of single scattering calculations.

Numerical interpretation of the gravitational field of density heterogeneities of the Earth’s inner core using models

The geometric shape and sizes of a one-, two-and three-dimensional density structures of the Earth’s inner core have been found with sufficient accuracy using approximate values of five mass moments of homogeneous bodies of simple symmetric shapes. The depth of the mass center and four shape coefficients have been calculated by solving the inverse problem of the gravitational potential from six field elements at a single point.

Accurate assessment of 3D crustal velocity and density parameters: Application to Vesuvius data sets

The three-dimensional P-wave velocity and density structure of Mt. Vesuvius volcano has been revealed by tomographic inversion of seismic and gravity data. We apply the tomographic inversion method of sequential integrated inversion, now extended to 3D structures, to the first arrival travel time data collected during 1994 and 1996 TOMOVES experiment and to a new gravity map of the Campanian area. The approach, now significantly improved, allows us to reconstruct both velocity and density structures, while reducing traveltimes and gravity residuals. Inversion of traveltimes, modelled through a ray-tracing technique, is performed using an iterative algorithm which uses the second order Sobolev norm as the regularization term of the ill conditioned seismic inverse problem. This speeds up the optimization problem and allows us to obtain stable solutions with a weak dependence on the starting model and on the chosen discretization. In order to improve the match between seismic and gravity modelling and to avoid artificial density contrasts across the interfaces, the density model is parametrized with polyhedral bodies whose density is linearly dependent on the three coordinates. This study brings insight into the main velocity and density structural units of Mt. Vesuvius, which include a C shaped negative anomaly surrounding the flanks of the crater up to depths of 6 km. This anomaly is bordered in the south by a strip of high density gradients which run parallel to the Sorrento peninsula, and in the east by an intruding Mesozoic carbonate basement structure. We observe evidence for the thinning of the hard dolomites in the north-western and south-eastern part of the model, related to previously observed NW–SE and SW–NE collapse faults within the Plio-Pleistocene graben of the Campanian plain. Beneath the volcano edifice, we find a positive and thick intrusion anomaly which we compare to the results of previous studies. The reliability of the reconstructed models is quantified through a restoring test and the estimation of traveltimes and gravity residuals.

3GRAINS: 3D Gravity Interpretation Software and its application to density modeling of the hellenic subduction zone

3GRAINS is a program, which allows interactive modeling of a density structure with respect to given gravity anomalies. It is written in standard C ++ with the use of Qt Library. It has been compiled on the Linux platform, but it can be also compiled under the Windows environment. The program uses rectangular prisms to calculate the gravity effect of a model. Blocks used in interpretation can have different size; it is useful to model shallow layers with higher resolution than the deep ones. Gravity stations used in modeling can have arbitrary distribution. The software uses the altitude of the station to compute its anomaly. The gravity effect of each block for each station is calculated once and saved together with the modeled structure. A major objective was to reduce the amount of memory used by the program. Thanks to the applied compression algorithm, high-resolution density structures can be accommodated by an average modern computer. The absolute accuracy of modeled anomalies depends on scaling of the problem. The relative error does not exceed 0.001. A very important characteristic of the software is a simplicity in creating and changing of the models by means of the graphical user interface. The three-dimensional density structure can be browsed in 2D cross-sections on one horizontal and two perpendicular vertical planes. The program offers a number of export options. Numerical models as well as different cross-sections and layer boundaries can be saved to a file for further visualization of the results. As an example a density model of the Hellenic Subduction Zone is presented. The model reflects a very complicated tectonic situation of the region and includes main tectonic units: seawater, sediments, crust and upper mantle. Horizontal density distributions within each unit allow recognition of different types of the lithosphere.

The Evolution of Coronal Mass Ejection Density Structures

We present a discussion of the time evolution of the mass and energy of a model coronal mass ejection (CME), analyzing both synthetic coronograph images and three-dimensional data of the numerical ideal magnetohydrodynamics (MHD) simulation. Our global steady state coronal model possesses high-latitude coronal holes and a helmet streamer structure with a current sheet near the equator, reminiscent of near solar minimum conditions. Within this model system, we drive a CME to erupt by the introduction of a Gibson-Low magnetic flux rope that is embedded in the helmet streamer in an initial state of force imbalance. The flux rope rapidly expands and is ejected from the corona with maximum speeds in excess of 1000 km s-1 driving a fast-mode shock that propagates from the inner corona to a distance of 1 AU. We study the mass and energetics of the CME inferred from the three-dimensional results of the simulation, as well as calculated from synthetic coronograph images produced at different times. The two-dimensional plane-of-sky density structure of a CME is discussed for wide-angle coronographs, such as the Heliospheric Imager (HI-2) on board STEREO (Solar Terrestrial Relations Observatory), and compared with the three-dimensional density structure. We found that the CME mass derived from the synthetic coronographic images is an underestimate by about 50% of the total mass of the CME according to the three-dimensional data. Two main reasons can be invoked: the poor assumption that all the mass of the CME is in the plane of the sky and the wrapping of the front shock around the density-depleted cavity, which leads to an apparent decrease in brightness of the front shock.

Controlled experiments on self-organization of ordered structures in a pure electron plasma

We review our recent studies on structure formation in pure electron plasmas confined in magneto-electric traps. Two-dimensional dynamics of strongly magnetized plasmas is studied in terms of a two-dimensional vortex composed of high-density clumps and a low-density distribution in the background (BG). By controlling the initial distributions we examine in detail the fundamental processes where the clumps form highly ordered geometrical configurations (vortex crystals). Experimental studies of the formation of three-dimensional density structures have been initiated with a newly built strong-magnetic-field (2.2 T) trap operatable with various potential profiles including a square-well potential (Malmberg trap) and harmonic potential (Mohri trap). In the Mohri trap, Penning-type distributions are observed to form as a result of interaction of string-shaped distributions of electrons. The density distributions observed in the relaxation process are quite different from those observed in a square-well potential most probably due to three-dimensional deformations and merger of the string distributions. The equilibrium distributions generally consist of sets of spheroidal distributions, a core distribution plus halo distributions. Observationally the halo distributions appear to be associated with the amount of excessive angular momentum provided through the injection of off-axial electron strings. Like the BG vorticity distribution in the two-dimensional merger of clumps, the halo may act as the absorber of momentum in the course of the merging process of strings to create the core distribution. In the Penning-like distributions we have also observed the accumulation of negative ions replacing the electrons in time scales much longer than thermal relaxation.

Observations of the latitudinal structure of plasmaspheric convection plumes by IMAGE‐RPI and EUV

Recent IMAGE Extreme Ultraviolet Imager (EUV) observations showed the first global images of plasmaspheric convection plumes, which have been interpreted as the plasmaspheric tails predicted theoretically 3 decades earlier. Using observations by the IMAGE Radio Plasma Imager (RPI), we show that these convection plumes have large latitudinal extent. These results complement those recently made by others in correlating IMAGE EUV data with measurements of total electron content in the ionosphere. By correlating in situ RPI density measurements with global plasmaspheric EUV images, we have shown that apparently detached plasma structures, as appear in RPI dynamic spectrograms, are in many cases plasmaspheric convection plumes. The temporal separation between the RPI and EUV observations help constrain the interpretation of one data set in the context of the other, thereby enabling an examination of the three‐dimensional plasma density structures outside the core plasmasphere. The data sets are mutually reinforcing because the data are collected within a few hours of one another. We used the EUV data to provide unambiguous identification of density enhancements in the region outside the plasmasphere and used the RPI data to obtain accurate number densities and extend information from the EUV data set by measuring densities below the EUV sensitivity threshold.

The Structure and Evolution of a Sigmoidal Active Region

Solar coronal sigmoidal active regions have been shown to be precursors to some coronal mass ejections. Sigmoids, or S-shaped structures, may be indicators of twisted or helical magnetic structures, having an increased likelihood of eruption. We present here an analysis of a sigmoidal region's three-dimensional structure and how it evolves in relation to its eruptive dynamics. We use data taken during a recent study of a sigmoidal active region passing across the solar disk (an element of the third Whole Sun Month campaign). While S-shaped structures are generally observed in soft X-ray (SXR) emission, the observations that we present demonstrate their visibility at a range of wavelengths including those showing an associated sigmoidal filament. We examine the relationship between the S-shaped structures seen in SXR and those seen in cooler lines in order to probe the sigmoidal region's three-dimensional density and temperature structure. We also consider magnetic field observations and extrapolations in relation to these coronal structures. We present an interpretation of the disk passage of the sigmoidal region, in terms of a twisted magnetic flux rope that emerges into and equilibrates with overlying coronal magnetic field structures, which explains many of the key observed aspects of the region's structure and evolution. In particular, the evolving flux rope interpretation provides insight into why and how the region moves between active and quiescent phases, how the region's sigmoidicity is maintained during its evolution, and under what circumstances sigmoidal structures are apparent at a range of wavelengths.

Detecting Planets in Protoplanetary Disks: A Prospective Study

We investigate the possibility of finding evidence for planets in circumstellar disks by infrared and submillimeter interferometry. We present simulations of a circumstellar disk around a solar-type star with an embedded planet of 1 Jupiter mass. The three-dimensional density structure of the disk results from hydrodynamical simulations. On the basis of three-dimensional radiative transfer simulations, images of this system were calculated. The intensity maps provide the basis for the simulation of the Very Large Telescope Interferometer (VLTI; equipped with the mid-infrared instrument MIDI) and the Atacama Large Millimeter Array (ALMA). While MIDI/VLTI will not provide the possibility to distinguish between disks with or without a gap on the basis of visibility measurements, ALMA will provide the necessary basis for a direct gap detection.

Dynamics of pancake-like vortices in a stratified fluid: experiments, model and numerical simulations

The dynamics and the three-dimensional structure of vortices in a linearly stratified, non-rotating fluid are investigated by means of laboratory experiments, an analytical model and through numerical simulations. The laboratory experiments show that such vortices have a thin pancake-like appearance. Due to vertical diffusion of momentum the strength of these vortices decreases rapidly and their thickness increases in time. Also it is found that inside a vortex the linear ambient density profile becomes perturbed, resulting in a local steepening of the density gradient. Based on the assumption of a quasi-two-dimensional axisymmetric flow (i.e. with zero vertical velocity) a model is derived from the Boussinesq equations that illustrates that the velocity field of the vortex decays due to diffusion and that the vortex is in so-called cyclostrophic balance. This means that the centrifugal force inside the vortex is balanced by a pressure gradient force that is provided by a perturbation of the density profile in a way that is observed in the experiments. Numerical simulations are performed, using a finite difference method in a cylindrical coordinate system. As an initial condition the three-dimensional vorticity and density structure of the vortex, found with the diffusion model, are used. The influence of the Froude number, Schmidt number and Reynolds number, as well as the initial thickness of the vortex, on the evolution of the flow are investigated. For a specific combination of flow parameters it is found that during the decay of the vortex the relaxation of the isopycnals back to their undisturbed positions can result in a stretching of the vortex. Potential energy of the perturbed isopycnals is then converted into kinetic energy of the vortex. However, when the stratification is strong enough (i.e. for small Froude numbers), the evolution of the vortex can be described almost perfectly by the diffusion model alone.

On the density structure of far-wake vortices in a stratified fluid

An experimental investigation of the three-dimensional density structure of far-wake vortices generated by moving a sphere in a linear saline stratification has been carried out via conductivity measurements. These measurements were performed in the vortex cores along vertical and horizontal profiles to capture the three-dimensional nature of the vortices. We first present a simple model of an isolated vortex, the calculated internal density field of which has been confirmed by the conductivity measurements performed in both turbulent and laminar wakes. The time-dependent density structure is also described, as well as the decay of the density peak within the vortices. An identical density structure corresponding to an intensification of the background stratification has been identified for far-wake vortices originating from either laminar or turbulent near-wakes. This suggests that these vortices exhibit universal features of quasi two-component structures in stratified fluids, such as large-scale vortices commonly found in the ocean.

Comprehensive Seismic Survey Database for Developing Three-Dimensional Models of the Earth's Crust

INTRODUCTION In recent years, data from geophysical and laboratory physics gravity experiments suggested the existence of an anomalous gravitational or “fifth” force (Stacey et al. , 1987; Boynton et al. , 1987; Thieberger, 1987; Fischbach and Talmadge, 1992). However, it appears that these data are better explained by improving the models of the Earth's gravitational field instead of making major adjustments in the basic theories of physics (Kim, 1988; Kim, 1989; Braswell, 1992). Detailed modeling of the Earth's gravitational field requires a knowledge of variations in density within the Earth as well as a knowledge of the shape of the Earth (Chulick et al. , 1990). This in part is facilitated by the inclusion of a detailed model of the three-dimensional density structure of the Earth's crust. Such a model can be constructed by using the velocity and layer thickness data collected from various types of seismic surveys (Christiansen and Mooney, 1995). This...

The density profile of the elliptical planetary nebula NGC 3242

We present the three-dimensional density structure of the elliptical planetary nebula NGC 3242, deconvolved from its H-alpha image. Using the simplistic assumptions that each mass element preserves its original velocity, which is radial and depends only on latitude, we deduce from this density profile the variation of mass-loss rate from the progenitor of NGC 3242 with latitude and time. The resulting somewhat qualitative mass-loss geometry and history are used to constrain models for the formation of the elliptical structure of NGC 3242. We argue that a triple system, with a very close brown dwarf companion and a more massive distant tertiary star, is compatible with the morphology of NGC 3242. In this model the brown dwarf, of about 0.01 solar mass, shared a common envelope with the progenitor star, and spun up the envelope through deposition of angular momentum. The oblate rotating envelope blew an axisymmetrical wind. We suggest that the presence of a third star, with a mass of about 1 solar mass and an orbital period of about 4000 years, could have caused the large scale deviation from axial symmetry seen in the density structure.

Radiative transfer of X-rays in the solar corona

view Abstract Citations (45) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiative transfer of X-rays in the solar corona. Acton, L. W. Abstract The problem of resonance scattering of X-ray emission lines in the solar corona is investigated. For the resonance lines of some helium-like ions, significant optical depths are reached over distances small compared with the size of typical coronal features. A general integral equation for the transfer of resonance-line radiation under solar coronal conditions is derived. This expression is in a form useful for modeling the complex three-dimensional temperature and density structure of coronal active regions. The transfer equation is then cast in a form illustrating the terms which give rise to the attenuation or enhancement of the resonance-line intensity. The source function for helium-like oxygen (O VII) under coronal conditions is computed and discussed in terms of the relative importance of scattering. Publication: The Astrophysical Journal Pub Date: November 1978 DOI: 10.1086/156575 Bibcode: 1978ApJ...225.1069A Keywords: Optical Thickness; Radiative Transfer; Solar Corona; Solar X-Rays; X Ray Scattering; Line Spectra; Resonance Scattering; X Ray Spectra; Solar Physics; Radiative Transfer:Solar X Rays; Solar Corona:X Rays full text sources ADS |