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Tang X.-D.,CAS Institute of Geology and Geophysics | Tang X.-D.,University of Chinese Academy of Sciences | Liu H.,CAS Institute of Geology and Geophysics | Zhang H.,MLR Key Laboratory of Marine Mineral Resources
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2015

Full-waveform inversion (FWI) can supply accurate velocity model for seismic imaging and is becoming a very important technology in seismic exploration. The frequency domain modeling is the base of frequency domain FWI and can be used for each iteration. In this case, accurate and fast frequency domain modeling is very necessary. The traditional frequency domain modeling methods can only be used in square grid and the memory and computation requirements are huge. We present a high efficient and adaptable frequency domain modeling technique in this paper. We design a 17-point scheme to perform the frequency domain modeling and use the average-derivative method to overcome the disadvantage that the traditional frequency domain modeling method can not be used in rectangular grid. Our operator can not only be applied in the condition of square grid but also rectangular grid, which is very useful in the real world situation because the rectangular grid is very popular in field work. Therefore, our operator is of greater applicability than the traditional methods. In order to reduce the dispersion and the number of grid points per shortest wavelength, we combine the mass acceleration term into the point calculation and get the optimal coefficients through simulated annealing algorithm. As the program Matlab provides the large scale optimization method, we use fmincon in this program to further optimize the coefficients. At last, the number of grid points per shortest wavelength is reduced to 2.5 comparing to 4 in 9-point method which is the most common frequency domain modeling method with phase velocity errors no larger than 1%. Therefore, the requirements of calculation amount and memory are reduced greatly. There are two main ways in solving Helmholtz equation which is the algorithm of frequency domain acoustic modeling, including direct method and iterative method. The direct method is LU decomposition while the matrix of LU decomposition in our algorithm usually is huge in 2D data and is difficult to solve using current computer. Therefore, iterative method is an option. However, in iterative method the convergent rate usually is very low, which leads to huge computation. We use the incomplete LU decomposition as the precondition and accelerate the speed of convergence. For the same test, the traditional method needs more than 20000 times to converge to a stable value while our method needs only 6 times, which reduces the amount of calculation greatly. In order to accelerate the process of frequency domain modeling, we apply the Graphics Processing Unit (GPU) because GPU has powerful computing capability. In the GPU, fair use of video memory is very crucial and can lead to higher efficiency. Using the GPU, we obtain twice speed of the traditional CPU method in modeling. There are two main methods in frequency domain acoustic modeling, including 9-point method and 25-point method. We compare our method with those of two methods in accuracy and efficiency. We use the same velocity model and the dx=dz=25 m. The dominant frequency of the Ricker wavelet is 20 Hz. In order to remove all disturbances, we do not apply any boundary condition. Because our method only needs 2.4 grids, to reach the same accuracy our operator can use relatively coarse mesh and it leads to less computing work. Meanwhile, the storage is reduced, too. For example, if the 9-point method needs nx×ny grids, the nonzero elements of coefficient matrix is 9×nx×nx×nz×nz, the 25-point method is 9.76×nx×nx×nz×nz and our method is 6.12×nx×nx×nz×nz. Therefore, our method produces a great savings of memory. In order to test our method in the complex velocity model, we applied it in the Marmousi model the sampling intervals of which are 12.5 m and 4 m. The grid of Marmousi is rectangular and our method can be used in this kind of model while 9-point and 25-point can not. We also give the result of time domain method which is second-order in time and twelfth-order in space. Comparing the result from time-domain method with the result from our technique, we find that there is no significant difference between them. Besides, the boundary condition is very important in modeling, we derive the perfectly matched layer equation based on our 17-point scheme and verify it in the Marmousi model. We propose a 17-point scheme which has the advantage of wide application and high accuracy to perform frequency domain acoustic modeling. It overcomes the disadvantage of traditional methods which can only be used in the square grid. After optimization, the number of grid points per shortest wavelength is reduced to 2.5, which reduces huge memory requirement and computing time. Because of the slow convergence, we apply the incomplete LU decomposition as the precondition in solving Helmholtz equation and accelerate the speed of convergence. As the GPU has the powerful computing capability, we use it to accelerate the process of modeling and save much time on computing. ©, 2015, Science Press. All right reserved.

Li X.,Wuhan University | Zhu P.,Wuhan University | Kusky T.M.,Wuhan University | Kusky T.M.,China Three Gorges University | And 4 more authors.
Tectonophysics | Year: 2015

A review and comparison of the tectonic history and lithospheric structure of the Yangtze craton and North China craton (NCC) are made to test whether the Yangtze craton has lost part of its lithospheric root like the NCC has, and if so, is there a common driving tectonic force for root-loss. The results show that many similarities and some significant differences exist between these two cratons. The similarities include the following: (1) The eastern parts of the NCC and Yangtze craton have both been destroyed. The destroyed areas are bounded on the west by the North-South Gravity Lineament. (2) Fast velocity bodies located above depths between 200 and 400 km are detected beneath the NCC and Yangtze craton, implying that delamination may be an important mechanism of cratonic destruction for these two cratons. (3) The destruction of the NCC and Yangtze craton can be divided into several stages. The differences mainly lie in the tectonic evolution during Neoproterozoic-early Mesozoic. The tectonothermal events which are beneficial for the lithospheric weakening of the NCC dominantly affected the marginal regions. However, for the Yangtze craton, they are not only limited to the marginal regions, but also extend to the eastern and middle parts of the craton. This may be a key factor why the Yangtze craton shows more continuous slow velocity anomalies.We also make a study on the time and dynamic mechanism of the destruction of the Yangtze craton. The results show that the initial destruction started in the Neoproterozoic and may be caused by the aggregation and breakup of Rodinia. The large scale destruction occurred mostly in the Mesozoic (190-90. Ma) with a peak time of 130-120. Ma. The subduction and rollback of the Paleo-Pacific (Izanagi) plate may play a vital role in the destruction of the Yangtze craton. © 2015 Elsevier B.V.

Gai S.H.,China University of Petroleum - Beijing | Liu H.Q.,China University of Petroleum - Beijing | He S.L.,China University of Petroleum - Beijing | Lei G.,China University of Petroleum - Beijing | And 4 more authors.
Geosystem Engineering | Year: 2016

Stress-dependent permeability and porosity of porous media is very important for the development of oil reservoirs, especially in low-permeability reservoirs. Porous structure can change in response to an increase in the effective stress to induce the decline of permeability and porosity, which ultimately affects the fluid flow property. In this study, a capillary module, considering the arrangement and the deformation of particles, has been established to express the stress sensitivity of porous media based on the particle packing model. The predictions of variation of permeability by the proposed model have been validated by comparing with the experimental data. The predictions by the current model show the same variation trend with the experimental data and are consistent with the compression experiments. The proposed variation of permeability with the increase in effective stress depends on the factors such as the arrangement of particles (contact angle), the deformation of particles (elastic module and Poisson’s ratio) and effective stress. The effects of these parameters on variation of permeability are discussed in detail. © 2015 The Korean Society of Mineral and Energy Resources Engineers (KSMER).

Xi L.,MLR Key Laboratory of Marine Mineral Resources | Li H.,South University of Science and Technology of China | Xia Y.,Changjiang River Scientific Research Institute | Qu W.,Water Resources University
Environmental Earth Sciences | Year: 2016

A mangrove transect (M–M′) and a bald beach transect (B–B′) were selected in Dongzhaigang National Nature Reserve, China for conducting a comparative study about groundwater chemical characteristics. It was found that the average concentration of Cl− in groundwater sampled from M–M′ was much lower than that of B–B′. However, concentrations of heavy metals Ba, Mn, Pb, Sb in groundwater of M–M′ were significantly higher than those of B–B′. The average concentrations of Ba, Mn, Pb, Sb were 0.13, 0.83, 0.25 and 0.32 mg/L in M–M′, respectively, and 0.09, 0.27, 0.19 and 0.21 mg/L in B–B′, respectively. Both transects were affected by the human activities such as aquacultures. The high concentration of Sb in the groundwater of transect M–M′ may be mainly due to high concentration of Sb in the Yanfeng River. The higher concentrations of Ba, Mn, Pb in groundwater in M–M′ might be attributed to the accumulating and enriching effects of the mangrove systems on heavy metals. The mechanism of accumulating and enriching need to be further investigated in the future. © 2016, Springer-Verlag Berlin Heidelberg.

Wang F.L.,MLR Key Laboratory of Marine Mineral Resources | Wang F.L.,Sun Yat Sen University | Zhao T.P.,CAS Guangzhou Institute of Geochemistry | Wang Y.,CAS Guangzhou Institute of Geochemistry
Yanshi Xuebao/Acta Petrologica Sinica | Year: 2015

Syenitic dikes are widespread in the Panxi district and some of them host Nb and Ta deposits. The mineralized dikes intruded into gabbro intrusions and are spatially and temporally associated with basalts and felsic plutons (granite and syenite plutons) in the Panxi district, which are part of the ∼260Ma Emeishan large igneous province. These dikes are recently dated to be 258 ∼ 256Ma by LA-ICP-MS zircon U-Pb dating technique, contemporaneous with the gabbro and felsic plutons. The Sr-Nd isotopes of the mineralized syenitic dikes, barren syenitic dikes and syenitic plutons of Luku and Baicao Nb-Ta deposits in the Panxi district show similar and uniform initial εNd(t) values (-0.2 to +0.2 for mineralized syenitic dikes, -0.3 to +0.7 for barren syenitic dikes and -0.3 to +0.4 for syenitic plutons) but they have highly variable (87Sr/86Sr)i ratios (0.7049 to 0.7091 for the mineralized syenitic dikes and 0.7044 ∼ 0.7064 for barren syenitic dikes and 0.7032 to 0.7090 for barren syenitic plutons), comparable to the contemporaneous Hongge mafic/ultramafic layered intrusion εNd(t) = -2.7 to +1.0 and (86Sr/87Sr)i = 0.7058 to 0.7064) in the same region, suggesting that these lithologies may have evolved from a common basaltic magmas. According to the whole rock geochemistry and Sr-Nd isotopic characters, We consider that the syenitic plutons and dikes may be the products of low degree (5% ∼ 10%) partial remelting of underplated gabbro cumulates.

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