Comparison of waveform inversion, part 3: amplitude approach     

Sukjoon Pyun  Changsoo Shin  J. B. Bednar 《Geophysical Prospecting》2007,55(4):477-485

In the second paper of this three part series, we studied the case of conventional and logarithmic phase‐only approaches to full‐waveform inversion. Here, we concentrate on deriving amplitude‐only approaches for both conventional‐ and logarithmic‐based methods. We define two amplitude‐only objective functions by simply assuming that the phase of the modelled wavefield is equal to that of the observed wavefield. We do this for both the conventional least‐squares approach and the logarithmic approach of Shin and Min. We show that these functions can be optimized using the same reverse‐time propagation algorithm of the full conventional methodology. Although the residuals in this case are not really residual wavefields, they can both be considered and utilized in that sense. In contrast to the case for our phase‐only algorithms, we show through numerical tests that the conventional amplitude‐only inversion is better than the logarithmic method.  相似文献   

Improved amplitude preservation for prestack depth migration by inverse scattering theory   总被引：6，自引：1，他引：6  

Changsoo Shin  Seonghyung Jang  Dong-Joo Min 《Geophysical Prospecting》2001,49(5):592-606

A prestack reverse time-migration image is not properly scaled with increasing depth. The main reason for the image being unscaled is the geometric spreading of the wavefield arising during the back-propagation of the measured data and the generation of the forward-modelled wavefields. This unscaled image can be enhanced by multiplying the inverse of the approximate Hessian appearing in the Gauss–Newton optimization technique. However, since the approximate Hessian is usually too expensive to compute for the general geological model, it can be used only for the simple background velocity model.We show that the pseudo-Hessian matrix can be used as a substitute for the approximate Hessian to enhance the faint images appearing at a later time in the 2D prestack reverse time-migration sections. We can construct the pseudo-Hessian matrix using the forward-modelled wavefields (which are used as virtual sources in the reverse time migration), by exploiting the uncorrelated structure of the forward-modelled wavefields and the impulse response function for the estimated diagonal of the approximate Hessian. Although it is also impossible to calculate directly the inverse of the pseudo-Hessian, when using the reciprocal of the pseudo-Hessian we can easily obtain the inverse of the pseudo-Hessian. As examples supporting our assertion, we present the results obtained by applying our method to 2D synthetic and real data collected on the Korean continental shelf.  相似文献   

Evaluation of density in layer compaction using SASW method   总被引：1，自引：0，他引：1  

Dong-Soo Kim  Min-Kyun Shin  Hyung-Choon Park   《Soil Dynamics and Earthquake Engineering》2001,21(1)

SASW test, which is non-intrusive and rapid in the field application, was used to evaluate the layer density in the roller compaction without performing the complicated inversion process. The concept of normalized shear wave velocity was introduced to minimize the effect of confinement in the density evaluation. SASW test was performed to determine the shear wave velocity of the layer, and the free–free resonant column (FF–RC) test was adopted to determine the correlation between the normalized shear wave velocity and density of the site, which is almost unique independent of confinement. Testing and data reduction procedures of both tests were briefly discussed and an evaluation procedure of the field density was proposed by effectively combining in-situ shear wave velocity determined by the SASW test with the correlation between the normalized shear wave velocity and the density determined by the FF–RC test. Finally, the feasibility of the proposed method was verified by performing a field case study at Hoengsung road construction site. Field densities determined by the proposed method matched well with those determined by sand cone tests, showing the potential of applying the proposed method in the field density evaluation.  相似文献   

Radiatively driven mass accretion on to galactic nuclei by circumnuclear starbursts     

Masayuki Umemura  Jun Fukue  & Shin Mineshige 《Monthly notices of the Royal Astronomical Society》1998,299(4):1123-1131

We examine the physical processes of radiatively driven mass accretion on to galactic nuclei, owing to intensive radiation from circumnuclear starbursts. The radiation from a starburst not only causes the inner gas disc to contract via radition flux force, but also extracts angular momentum owing to relativistic radiation drag, thereby inducing an avalanche of the surface layer of the disc. To analyse such a mechanism, the radiation–hydrodynamical equations are solved, including the effects of the radiation drag force as well as the radiation flux force. As a result, it is found that the mass accretion rate owing to the radiative avalanche is given by M ˙ ( r )= η ( L _*/ c ²)( r / R )² (Δ R / R )(1 −  e ^−τ) at radius r , where the efficiency η ranges from 0.2 up to 1, L _* and R are respectively the bolometric luminosity and the radius of the starburst ring, Δ R is the extent of the emission regions, and τ is the face-on optical depth of the disc. In an optically thick regime, the rate depends upon neither the optical depth nor the surface mass density distribution of the disc. The present radiatively driven mass accretion may provide a physical mechanism which enables mass accretion from 100-pc scales down to ∼ parsec scales, and it may eventually be linked to advection-dominated viscous accretion on to a massive black hole. The radiation–hydrodynamical and self-gravitational instabilities of the disc are briefly discussed. In particular, the radiative acceleration possibly builds up a dusty wall, which 'shades' the nucleus in edge-on views. This provides another version of the model for the formation of an obscuring torus.  相似文献   

Delineation of spatial redox zones using discriminant analysis and geochemical modelling in arsenic‐affected alluvial aquifers     

Jin‐Jing Lee  Cheng‐Shin Jang  Sheng‐Wei Wang  Chin‐Ping Liang  Chen‐Wuing Liu 《水文研究》2008,22(16):3029-3041

This study characterized the redox conditions in arsenic‐affected groundwater aquifers of the Lanyang plain, Taiwan. Discriminant analysis was adopted to delineate three redox zones (oxidative, transitional and reductive zones) in different aquifers and yielded 92·3% correctness on groundwater quality data. Arsenic is mainly distributed in the reductive zone, and arsenic distribution in the shallow aquifer is mainly affected by surface activities. According to PHREEQC modelling results, possible mechanisms for arsenic release to groundwater in Lanyang plain are explored. Arsenic released to groundwater in the oxidative zone (zone 1) is primarily caused by the oxidations of arsenic‐bearing pyrite minerals, and arsenate is the predominant species. While the reductive dissolution of Fe‐oxides are responsible for the high arsenic concentration found in the transitional and reductive zones (zones 2 and 3), arsenite is the predominant species. The reduction potential of groundwater rises as the depths and zones increase. Some sulphates may be reduced to form sulphide ions, which then react with arsenic to form arseno‐sulphide deposits (such as realgar, orpiment) and then slightly lower groundwater arsenic concentrations. A conceptual diagram which summarized the possible release processes of arsenic in different redox zones along groundwater flow in Lanyang plain is postulated. Arsenic‐bearing pyrite and arsenopyrite (FeAsS) are oxidized as they are exposed to the infiltrated oxygenated rainwater, releasing soluble arsenate Fe(II) and SO₄^2? into zone 1. The dissolution of arsenic‐rich Fe‐oxides due to the onset of reducing conditions in zones 2 and 3 is responsible for the mobility of arsenic and likely to be the primary mechanism of arsenic release to groundwater in the Lanyang plain Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Molecular and isotopic characteristics of gas hydrate-bound hydrocarbons in southern and central Lake Baikal   总被引：4，自引：4，他引：0  

Akihiro Hachikubo  Oleg Khlystov  Alexey Krylov  Hirotoshi Sakagami  Hirotsugu Minami  Yutaka Nunokawa  Satoshi Yamashita  Nobuo Takahashi  Hitoshi Shoji  Shin’ya Nishio  Masato Kida  Takao Ebinuma  Gennadiy Kalmychkov  Jeffrey Poort 《Geo-Marine Letters》2010,30(3-4):321-329

We investigated the molecular composition (methane, ethane, and propane) and stable isotope composition (methane and ethane) of hydrate-bound gas in sediments of Lake Baikal. Hydrate-bearing sediment cores were retrieved from eight gas seep sites, located in the southern and central Baikal basins. Empirical classification of the methane stable isotopes (δ¹³C and δD) for all the seep sites indicated the dominant microbial origin of methane via methyl-type fermentation; however, a mixture of thermogenic and microbial gases resulted in relatively high methane δ¹³C signatures at two sites where ethane δ¹³C indicated a typical thermogenic origin. At one of the sites in the southern Baikal basin, we found gas hydrates of enclathrated microbial ethane in which ¹³C and deuterium were both highly depleted (mean δ¹³C and δD of –61.6‰ V-PDB and –285.4‰ V-SMOW, respectively). To the best of our knowledge, this is the first report of C₂ δ¹³C–δD classification for hydrate-bound gas in either freshwater or marine environments.  相似文献   

Outflow Propagation in Collapsars: Collimated Jets and Expanding Outflows     

Akira Mizuta  Tatsuya Yamasaki  Shigehiro Nagataki  Shin Mineshige 《Astrophysics and Space Science》2007,307(1-3):23-27

We investigate the outflow propagation in the collapsar in the context of gamma-ray bursts (GRBs) with 2D relativistic hydrodynamic simulations. We vary the specific internal energy and bulk Lorentz factor of the injected outflow from non-relativistic regime to relativistic one, fixing the power of the outflow to be 10⁵¹erg s⁻¹. We observed the collimated outflow, when the Lorentz factor of the injected outflow is roughly greater than 2. To the contrary, when the velocity of the injected outflow is slower, the expanding outflow is observed. The transition from collimated jet to expanding outflow continuously occurs by decreasing the injected velocity. Different features of the dynamics of the outflows would cause the difference between the GRBs and similar phenomena, such as, X-ray flashes.  相似文献   

Effects of flexibility on propulsive force acting on a heaving foil     

Sangmook Shin  Sung Yong Bae  In Chul Kim  Yong Jig Kim 《Ocean Engineering》2009,36(3-4):285-294

The hybrid Cartesian/immersed boundary method is applied to simulate effects of flexibility on propulsive force acting on a heaving foil in a viscous flow. Immersed boundary nodes are distributed inside an instantaneous fluid domain. Velocity vector is reconstructed at the immersed boundary node based on an interpolation along a local normal line. Using the staggered/non-staggered grid method, the demand for pressure at boundary nodes is removed. Elastic deformation of the flexible foil is modelled based on the dynamic thin-plate mechanics. The developed code is validated through comparisons with other computations on flow fields around a flapping foil. The generation of the reverse Karman vortex street is investigated. Forces acting on heaving foils are compared for flexible and rigid cases and the increased thrust of the flexible foil is attributed to the deformed configuration near the tip. The flexibility of the heaving foil decreases vertical force and improves propulsion efficiency. The variations of force and deformation are investigated according to bending stiffness of the foil.  相似文献   

Effect of abalone farming on sediment geochemistry in the Shallow Sea near Wando,South Korea     

Jeongwon Kang  Yeon Gyu Lee  Da Un Jeong  Jung Sick Lee  Yang Ho Choi  Yun Kyung Shin 《Ocean Science Journal》2015,50(4):669-682

  相似文献   

Numerical forecasting of the time interval between successive M8 earthquakes along the Nankai Trough,southwest Japan,using ocean bottom cable network data     

Takane Hori  Mamoru Hyodo  Shin’ichi Miyazaki  Yoshiyuki Kaneda 《Marine Geophysical Researches》2014,35(3):285-294

One possible approach to estimating the time interval between large-scale Tōnankai (Tōkai) and Nankai earthquakes on the Japan arc is sequential assimilation of crustal deformation data. We conducted numerical modeling of sequential assimilation using surface deformation calculated from earthquake generation cycle simulations along the Nankai Trough. To account for observation noise, we used measured ocean bottom pressure gauge data, excluding tidal modulation, from a station on the ocean bottom cable network Dense Oceanfloor Network System for Earthquakes and Tsunamis in the Kumano basin. We used sequential importance sampling as our data assimilation method. We found that as the amount of data increased, the estimated time interval between the Tōnankai and Nankai earthquakes approached the “true” observed interval. In addition, the noise in the pressure gauge data was sufficiently small that simulated crustal deformation patterns could be distinguished for different time intervals.  相似文献