ASIA - Publications deposited by individuals
http://hdl.handle.net/1834/17753
2024-03-29T07:30:29ZLeast-Squares Reverse Time Migration in Pseudodepth Domain and Its Application
http://hdl.handle.net/1834/42447
Least-Squares Reverse Time Migration in Pseudodepth Domain and Its Application
HUSSEIN MUHAMMED, HUSSEIN ABDUELHALEIM
The Cartesian-based least squares reverse time migration (LSRTM) aims to obtain a relatively high-resolution amplitude preserving imaging by calculating and solving the Hessian matrix. In addition, conjugate gradient algorithm is proved to be an efficient iterative method, which makes the traditional LSRTM feasible in practical data processing applications. Each iteration process of LSRTM includes two main parts; The first part is the forward simulation of wavefield, and the second part is the back propagation of wavefield. However, the calculation of these two parts takes a lot of time, and oversampling effects will occur in the calculation process. The thesis develops a smooth understanding of the LSRTM scheme in pseudodepth domain and gives information about this type of wavefield extrapolation method to achieve amplitude-preserved image with low computational costs in terms of memory and time. The underground media faced by seismic exploration usually include low velocity bodies, high steep structures and pore fracture units. When simulating the seismic wavefield in these media, in order to ensure the simulation accuracy and computational stability by finite difference methods, the grid spacing needs to be very small, which leads to the oversampling problem of the traditional finite difference forward simulation method in Cartesian coordinate system. In order to overcome this problem, the pseudo depth domain algorithm is applied to the least-squares reverse time migration to improve its computational efficiency. The problem of stabilizing this Pseudodepth wavefield arises from the introduction of the mapping function and velocity and also the vertical axis operator that converts the finite difference solution partially from time into frequency domains. Stability and convergence analysis suggests that the spatial derivatives of Riemannian axis should be approximated by a mixed Fourier pseudo-spectral and ordinary finite-difference schemes methods using a special Gaussian-like impulse function to generate the vector-matrix of the complex source term within the finite-difference operator, in addition to the mapping velocity, which is a differential form of the initial input velocity model, manifestly controls the CFL conditions of the associated Riemannian-finite difference operator. Numerical and synthetic examples indicated that this approach is more stable and efficient in extrapolating a smooth Riemannian wavefield while maintaining Claerbout’s principle for locating subsurface reflectors also choosing an appropriate sampling rate for the new vertical axis is related inversely by the maximum frequency of the impulse wavelet and directly with minimum velocity value in the given model. The LSRTM wavefield extrapolation usually uses the two-dimensional constant density acoustic wave equation, by considering the change of velocity field distribution, and converts it to the corresponding pseudodepth domain, so as to solve the oversampling problem in the true depth domain. For each point in the Cartesian coordinate system, there is a corresponding point in the pseudo depth domain. Therefore, we can interpolate and remodel the reconstructed finite difference modelled wavefield in the new coordinate system through the Cartesian-to-pseudodepth mapping function. Regardless of the applied finite difference algorithm and boundary conditions, wavefield extrapolation in pseudodepth domain can ensure high accuracy and efficiency. Through the test of synthetic and actual data, compared with the traditional LSRTM results, pseudodepth domain LSRTM shows great potentiality in amplitude preserving imaging. On the other hand, pseudodepth domain LSRTM has great advantages in computational efficiency and ensures computational accuracy.
2022-06-30T00:00:00ZNew records of coral diseases in the Persian Gulf
http://hdl.handle.net/1834/41128
New records of coral diseases in the Persian Gulf
Tavakoli-Kolour, Parviz; Hazraty-Kari, Sanaz
We report first observations of three coral diseases including black band disease on Acropora, a syndrome resembling yellow band disease and red band disease on Porites colonies from depth 5–6 m at Abu-Musa Island after widespread coral bleaching in the northern part of the Persian Gulf in October 2012.
2019-11-26T00:00:00ZQuantifying recent trends in seagrass cover and biomass in a stressed environment, Pulau Semakau, Singapore
http://hdl.handle.net/1834/40240
Quantifying recent trends in seagrass cover and biomass in a stressed environment, Pulau Semakau, Singapore
Bramante, James F.
Globally, seagrass habitats have experienced sharp declines over the past century, with an annual loss of seagrass cover of 7%yr-1 since 1990. Despite the attention to seagrass this decline has brought, little research has been directed towards trends of seagrass habitats in Singapore. The research presented here developed and applied remote sensing methods to partially fill this gap, provide tools for more extensive monitoring in the future, and contribute to the global body of seagrass research. Satellite images from four different satellite sensors were used to estimate seagrass bed extent in Singapore’s second largest seagrass meadow, at Pulau Semakau, from 2001 to 2013. Statistical estimates of image signal-to-noise ratios were used to screen images for quality. Validation data collected in 2013 were used to estimate error for supervised classifications produced from each sensor. A novel method was explored to account for macroalgae blooms in the study area, but the resulting correction could not be validated and did not affect the overall trends in seagrass bed extent. In addition to the classification analysis, an empirical model linking remote sensing reflectance to above-ground biomass was constructed to examine the distribution of seagrass within the meadow. Applied to WV2 images from 2011 and 2013, this model produced estimates of above-ground biomass with root mean squared error (RMSE) of 54 gm-2 and 44.7 gm-2, respectively, within ranges of 0-288 gm-2 and 0-229 gm-2, respectively. A novel index to measure seagrass density non-destructively was developed to help conservation and monitoring efforts. This index, normalized canopy index (NCI), was estimated from satellite imagery more precisely than above-ground biomass, producing estimates from the 2013 WV2 image corresponding to field data with an R2 of 0.71 relative to the R2 of 0.39 produced by the above-ground biomass model. This index may be a promising, non-destructive alternative to above-ground biomass for remote sensing studies and should be pursued further in future research. Based on the time-series classification analysis, seagrass bed extent at Pulau Semakau declined from over 44.6 ha in April 2002 to 25.3 ha in June 2013. This decline occurred at an average of 5.1%yr-1 from 2001 to 2013, although this rate of decline slowed to 3.7%yr-1 in 2012. These declines are likely representative of other seagrass habitats in Singapore. Broader monitoring is required to determine to what extent Singapore’s seagrasses are disappearing. Although seagrass bed extent declined by 17% from April 2011 to June 2013, over the same time period total above-ground biomass in the seagrass meadow declined only 5%, from 41.6 Mg to 39.6 Mg. Two acute sedimentation events recorded over this time period corresponded to a large and permanent decrease in bed extent captured by WV2 imagery and a small and temporary decrease in bed extent captured by ALI imagery. I hypothesize that the discrepancy in decreases in extent and biomass, coupled with an increase in median biomass, is attributable to preferential survival and recolonization of dense-biomass seagrass species during these sedimentation events. Measurements of seagrass species abundance during this time period provide support for this hypothesis. This exercise demonstrates the advantages and limitations of monitoring seagrass bed extent and above-ground biomass. Bed extent provides a measure of overall viability of a seagrass meadow, but above-ground biomass provides a better index of spatially variable health and internal change. Coupled, these two measurements provide greater insight into complex seagrass bed processes and seagrass response to disturbance.
2014-11-20T00:00:00ZÉtude morphologique et physiologique du sang, de l'immunité naturelle et acquise chez quelques poissons Indochinois
http://hdl.handle.net/1834/35926
Étude morphologique et physiologique du sang, de l'immunité naturelle et acquise chez quelques poissons Indochinois
Durand, J.
This paper deals with a morphological study on blood of some Indochinese freshwater fishes. The study has been split in two parts: one dedicated to compare some fishes' blood and the second part focused on reaction of cells and natural immunity.
1950-01-01T00:00:00Z