柴达木盆地盐湖环境中超铀元素的分布特征     

韩凤清  黄麒  梁青生  刘建华  刘书田 《海洋与湖沼》1998,29(5):520-526

1993年采用α谱仪和大面积屏栅电离定对柴达木盆地盐湖水、地下水、河水以及地表和两个钻孔沉积物中的U,Pu和Am进行了测定。结果表明,各类水样中239Pu（含240Pu,下同）的平均含量高出表层海水约1000倍,其239Pu／238U的活度比在0．001－0．530之间;两钻孔沉积物中239Pu的平均值分别为49±0．8mBq／g和2．4±0．5mBq／g,239Pu／238U的活度比为0．141和0．088,由此探讨了U,Pu和Am的分布特征、239Pu／238U比值的变化规律以及沉积物和水中Pu的来源。  相似文献   

Research Note: A workaround for the corner problem in numerically exact non-reflecting boundary conditions     

W.A. Mulder 《Geophysical Prospecting》2020,68(9):2857-2866

Simulations of wave propagation in the Earth usually require truncation of a larger domain to the region of interest to keep computational cost acceptable. This introduces artificial boundaries that should not generate reflected waves. Most existing boundary conditions are not able to completely suppress all the reflected energy, but suffice in practice except when modelling subtle events such as interbed multiples. Exact boundary conditions promise better performance but are usually formulated in terms of the governing wave equation and, after discretization, still may produce unwanted artefacts. Numerically exact non-reflecting boundary conditions are instead formulated in terms of the discretized wave equation. They have the property that the numerical solution computed on a given domain is the same as one on a domain enlarged to the extent that waves reflected from the boundary do not have the time to reach the original truncated domain. With a second- or higher-order finite-difference scheme for the one-dimensional wave equation, these boundary conditions follow from a recurrence relation. In its generalization to two or three dimensions, a recurrence relation was only found for a single non-reflecting boundary on one side of the domain or two of them at opposing ends. The other boundaries should then be zero Dirichlet or Neumann. If two non-reflecting boundaries meet at a corner, translation invariance is lost and a simple recurrence relation could not be found. Here, a workaround is presented that restores translation invariance by imposing classic, approximately non-reflecting boundary conditions on the other sides and numerically exact ones on the two opposing sides that otherwise would create the strongest reflected waves with the classic condition. The exact ones can also be applied independently. As a proof of principle, the method is applied to the two-dimensional acoustic wave equation, discretized on a rectangular domain with a second-order finite-difference scheme and first-order Enquist–Majda boundary conditions as approximate ones. The method is computationally costly but has the advantage that it can be reused on a sequence of problems as long as the time step and the sound speed values next to the boundary are kept fixed.  相似文献   

A comparison of seismic saltbody interpretation via neural networks at sample and pattern levels     

Haibin Di  Ghassan AlRegib 《Geophysical Prospecting》2020,68(2):521-535

Saltbodies are important subsurface structures that have significant implications for hydrocarbon accumulation and sealing in petroleum reservoirs, and accurate saltbody imaging and delineation is now greatly facilitated with the availability of three-dimensional seismic surveying. However, with the growing demand for larger survey coverage and higher imaging resolution, the size of seismic data is increasing dramatically. Correspondingly, manual saltbody interpretation fails to offer an efficient solution, particularly in exploration areas of complicated salt intrusion history. Recently, artificial intelligence is attracting great attention from geoscientists who desire to utilize the popular machine learning technologies for evolving the interpretational tools capable of mimicking an experienced interpreter's intelligence. This study first implements two popular machine learning tools, the multi-layer perceptron and the convolutional neural network, for delineating seismic saltbodies at sample and pattern levels, respectively, then compares their performance through applications to the synthetic SEAM seismic volume, and moreover tentatively investigates what contributes to the better convolutional neural network delineation. Specifically, the multi-layer perceptron scheme is capable of efficiently utilizing an interpreter's knowledge by selecting, pre-conditioning and integrating a set of seismic attributes that best highlight the target saltbodies, whereas the convolutional neural network scheme makes it possible for saltbody delineation directly from seismic amplitude and thus significantly reduces the dependency on attribute selection from interpreters. It is concluded that the better performance from the convolutional neural network scheme results from two factors. First, the convolutional neural network builds the mapping relationship between the seismic signals and the saltbodies using the original seismic amplitude instead of manually selected seismic attributes, so that the negative impact of using less representative attributes is virtually eliminated. Second and more importantly, the convolutional neural network defines, learns and identifies the saltbodies by utilizing local seismic reflection patterns, so that the seismic noises and processing artefacts of distinct patterns are effectively identified and excluded.  相似文献   

A hybrid approach to compute seismic travel times in three-dimensional tetrahedral meshes     

Maher Nasr  Bernard Giroux  J. Christian Dupuis 《Geophysical Prospecting》2020,68(4):1291-1313

We propose an optimized method to compute travel times for seismic inversion problems. It is a hybrid method combining several approaches to deal with travel time computation accuracy in unstructured meshes based on tetrahedral elementary cells. As in the linear travel time interpolation method, the proposed approach computes travel times using seismic ray paths. The method operates in two sequential steps: At a first stage, travel times are computed for all nodes of the mesh using a modified version of the shortest path method. The difference with the standard version is that additional secondary nodes (called tertiary nodes) are added temporarily around seismic sources in order to improve accuracy with a reasonable increase in computational cost. During the second step, the steepest travel time gradient method is used to trace back ray paths for each source–receiver pair. Travel times at each receiver are then recomputed using slowness values at the intersection points between the ray path and the traversed cells. A number of numerical tests with an array of different velocity models, mesh resolutions and mesh topologies have been carried out. These tests showed that an average relative error in the order of 0.1% can be achieved at a computational cost that is suitable for travel time inversion.  相似文献   

Free-surface implementation in a mesh-free finite-difference method for elastic wave propagation in the frequency domain     

Junichi Takekawa  Hitoshi Mikada 《Geophysical Prospecting》2019,67(8):2104-2114

We present an original implementation of the free-surface boundary condition in a mesh-free finite-difference method for simulating elastic wave propagation in the frequency domain. For elastic wave modelling in the frequency domain, the treatment of free surfaces is a key issue which requires special consideration. In the present study, the free-surface boundary condition is directly implemented at node positions located on the free-surface. Flexible nature of the mesh-free method for nodal distribution enables us to introduce topography into numerical models in an efficient manner. We investigate the accuracy of the proposed implementation by comparing numerical results with an analytical solution. The results show that the proposed method can calculate surface wave propagation even for an inclined free surface with substantial accuracy. Next, we calculate surface wave propagation in a model with a topographic surface using our method, and compare the numerical result with that using the finite-element method. The comparison shows the excellent agreement with each other. Finally, we apply our method to the SEG foothill model to investigate the effectiveness of the proposed method. Since the mesh-free method has high flexibility of nodal distribution, the proposed implementation would deal with models of topographic surface with sufficient accuracy and efficiency.  相似文献   

海水脱硫技术现状   总被引：13，自引：0，他引：13  

郭鲁钢  王海增  朱培怡  邓培昌  邢坤 《海洋技术学报》2006,25(3):10-14

海水脱硫主要是利用海水的天然碱度脱除烟气中的二氧化硫。近年来,该技术在火电厂的应用发展迅速。文中介绍了海水脱硫技术的基本原理和工艺流程,并对其国内外应用现状进行了概述。  相似文献   

Chen-Chao Koo and the Early Numerical Weather Prediction Experiments in China     

Jianhua LU 《大气科学进展》2021,38(5):707-716

Although the first successful numerical weather prediction(NWP)project led by Charney and von Neumann is widely known,little is known by the international community about the development of NWP during the 1950s in China.Here,a detailed historical perspective on the early NWP experiments in China is provided.The leadership in NWP of the late Professor Chen-Chao Koo,a protége of C.G.Rossby at the University of Stockholm during the late 1940s and a key leader of modern meteorology(particularly of atmospheric dynamics and physics)in China during the 1950s?70s,is highlighted.The unique contributions to NWP by Koo and his students,such as the ideas of formulating NWP as an“evolution”problem,in which the past data over multiple time steps are utilized,rather than an initial-value problem,and on the cybernetic aspects of atmospheric processes,i.e.,regarding the motion of the atmosphere at various time scales as an optimal control system,are also emphasized.  相似文献   

Evidence for an inverted metamorphic gradient associated with a Precambrian suture, southern Wyoming     

E.M. DUEBENDORFER 《Journal of Metamorphic Geology》1988,6(1):41-63

Abstract An inverted metamorphic gradient associated with the northern mylonite zone of the Cheyenne belt, a deeply eroded Precambrian suture in southern Wyoming, has been documented within metasedimentary rocks of the Early Proterozoic Snowy Pass Supergroup. Metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite, garnet, and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone. A minimum temperature increase of approximately 100° C over a km-wide zone is required for this transition. Parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the Snowy Pass Supergroup. Field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite-grade rocks over rocks of the Snowy Pass Supergroup along the northern mylonite zone. Mineral equilibria and garnet-biotite geothermometry on synkinematic mineral assemblages within the Snowy Pass metasedimentary rocks indicate deformation at minimum temperatures of 480° C and pressures of 350–400 MPa (3°5–4°0 kbar). This implies tectonic burial or upper plate thickness of 13–15 km. The narrow character of metamorphic zonation and microtextures within the Snowy Pass Supergroup which indicate late synkine-matic growth of garnet and staurolite, preclude rotation of pre-existing isograds by folding as a mechanism for development of the inverted gradient. Conductive transport of heat from the upper into the lower plate across the originally low-angle thrust is insufficient to produce the necessary temperatures in the lower plate. Shear heating is considered insufficient to produce the observed metamorphic transition unless high shear stresses are postulated. Up-dip advection of metamorphic fluids is a feasible, but unproven, mechanism for heat transport. The possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot be evaluated at present.  相似文献   

Some aspects of South Asia's groundwater irrigation economy: analyses from a survey in India, Pakistan, Nepal Terai and Bangladesh   总被引：4，自引：2，他引：2  

Tushaar Shah  O. P. Singh  Aditi Mukherji 《Hydrogeology Journal》2006,14(3):286-309

Since 1960, South Asia has emerged as the largest user of groundwater in irrigation in the world. Yet, little is known about this burgeoning economy, now the mainstay of the region's agriculture, food security and livelihoods. Results from the first socio-economic survey of its kind, involving 2,629 well-owners from 278 villages from India, Pakistan, Nepal Terai and Bangladesh, show that groundwater is used in over 75% of the irrigated areas in the sample villages, far more than secondary estimates suggest. Thanks to the pervasive use of groundwater in irrigation, rain-fed farming regions are a rarity although rain-fed plots within villages abound. Groundwater irrigation is quintessentially supplemental and used mostly on water-economical inferior cereals and pulses, while a water-intensive wheat and rice system dominates canal areas. Subsidies on electricity and canal irrigation shape the sub-continental irrigation economy, but it is the diesel pump that drives it. Pervasive markets in tubewell irrigation services enhance irrigation access to the poor. Most farmers interviewed reported resource depletion and deterioration, but expressed more concern over the high cost and poor reliability of energy supply for groundwater irrigation, which has become the fulcrum of their survival strategy.

Tushaar ShahEmail: Phone: +91-2692-229311-13Fax: +91-2692-229310

  相似文献   

Tracer tests, hydrochemical and microbiological investigations as a basis for groundwater protection in a remote tropical mountainous karst area, Vietnam   总被引：2，自引：1，他引：2  

Vu Thi Minh Nguyet  Nico Goldscheider 《Hydrogeology Journal》2006,14(7):1147-1159

The Tam Duong karst area in NW Vietnam is among the poorest and remotest regions in the country. The local population largely depends on water from two main karst springs. Due to agricultural activity and untreated domestic wastewaters, the spring water is often microbiologically contaminated. In order to provide a scientific basis for groundwater protection in the area, different field methods have been applied including hydrogeological framework investigations, tracer tests, and hydrochemical and microbiological sampling and analyses. All methods had to be adapted to the conditions of a poor and remote area. These adaptations included, amongst other measures, the use of a portable microbiological water_testing kit and the involvement of the local population in the sampling campaign. The tracer tests showed simple and direct connections between two important swallow holes and the two main springs, and made it possible to determine the linear groundwater flow velocities, which are extremely high (up to 875 m/h). The hydrochemical and microbiological data confirmed the strong impact of the streams sinking into the swallow holes on the spring water quality. Future groundwater source protection strategies should consequently focus on the reduction of polluting activities near the sinking streams and within their catchment areas.  相似文献