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2004 Vol. 15, No. 4

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Retrogressive Microstructures of the Eclogites from the Northern Dabie Mountains, Central China: Evidence for Rapid Exhumation
Yican Liu, Shuguang Li, Shutong Xu, Guanbao Chen
2004, 15(4): 349-354.

The ultrahigh-pressure eclogites from the northern Dabie Mountains in central China occurred as tectonic lens or blocks within granitic gneisses or meta-peridotites. Petrologic studies suggest that the studied eclogites experienced strongly retrogressive metamorphism and produced a series of characteristic retrogressive microstructures. The retrograde structures mainly include: (1) oriented needle mineral exsolution, e.g., quartz needles in Na-clinopyroxene and rutile, clinopyroxene and apatite exsolution in garnet formed under decreasing pressure conditions during exhumation; (2) symplectite, especially, two generations of symplectites developed outside the garnet grains, which are called "double symplectite" here; (3) compositional zoning of minerals such as garnet and clinopyroxene; (4) minerals with a reaction rim or retrograde rim, e.g., omphacite with a diopside rim, diopside with an amphibole rim and rutile with a rim of ilmenite. These retrograde textures, especially mineral zoning and symplectite, provide important petrologic evidence for the exhumation process and uplift of high-grade metamorphic rocks such as eclogite in the northern Dabie Mountains, indicating a rapid exhumation process.

Sorption Characteristics of Rare Earth Element Eu3+ onto Silica-Water Interfaces Studied by Fluorescence Spectroscopy
Xiulan Yin, Ping Hu, Dien Li
2004, 15(4): 355-360.

The sorption of Eu species onto nano-size silica-water interfaces was investigated using fluorescence spectroscopy for pH ranges of 1-8.5 and an initial Eu concentration (CEu) of 2×10-4 M. The sorption rate of Eu was initially low, but significantly increased at pH > 4.The sorption density of Eu species on a silica surface was ~1.58×10-7mol/m2 when the dissolved Eu species were completely sorbed onto silica-water interfaces at pH=~5.8.The sorbed Eu species at pH < 6 is aquo Eu3+, which is sorbed onto silica-water interfaces as an outer-sphere complex at pH < 5, but may be sorbed as an inner-sphere bidentate complex at 5 < pH < 6, due to the decrease of NH 2O to ~6 at pH=6.At pH=6 -8.5, Eu(OH)2+, Eu(CO3)+ and Eu(CO3)2- formed in the solutions, and Eu(CO3)+ is dominant at pH=~7.5.These ions may be sorbed onto silica-water interfaces as inner-sphere bidentate complexes.

Research on the Occurrence of Titanium in Coal-Measure Kaoline in Songyi, Hubei
Songlin Ran, Shangyue Shen, Xianzhong Cheng
2004, 15(4): 361-364.

Using X-ray diffraction (XRD) analysis and an electron-probe microanalyser (EPMA), this research found that the main independent mineral of titanium (Ti) in the kaoline of Songyi is anatase. The granularity of anatase is from 0.5 μm to 1.5 μm, and some exceed 10 μm. This research provides important scientific evidence for the exploitation of the coal-measure kaoline in the South China.

Planktonic Foraminiferal Assemblage Variations of Ontong-Java Plateau during Late Quaternary and Their Implications for Paleotemperature in the Western Pacific Warm Pool
Haiyan Jin, Zhimin Jian, Jun Tian
2004, 15(4): 365-371.

Ocean Drilling Program (ODP) Site 807A was recovered from the Ontong-Java plateau, western equatorial Pacific. Quantitative analysis of planktonic foraminifera, combined with oxygen and carbon isotope data, reveals the glacial-interglacial variations of sea-surface temperature and the upper water vertical structure in this region during the late Quaternary. Our results indicate that since 530 ka sea-surface temperature (SST) and the depth of thermocline (DOT) have changed significantly in the western Pacific warm pool (WPWP). The average glacial-interglacial annual SST difference was up to 4.2 ℃, and the DOT fluctuations could exceed more than 100 m, further suggesting the instability of the WPWP. The spectral analyses of SST and DOT reveal two dominating cyclicities—the typical 100 ka cycle and the semi-precessional cycle, which is significant in the tropical spectrum, indicating that late Quaternary paleoceanographic changes in the study area were influenced not only by a high latitude forcing but also by tropic-driving factors.

Discrimination of Natural Fractures Using Well Logging Curve Unit
Hongqi Liu, Shimi Peng, Yongyi Zhou, Yongchao Xue
2004, 15(4): 372-378.

It is very difficult to discriminate natural fractures using conventional well log data, especially for most of the matured oilfields in China, because the raw data were acquired with relatively obsolete tools.The raw data include only GR and SP curves, indicative of lithology, AC curves, used to calculate the porosity of the formation, and a set of logging curves from various electrode length resistivity by laterolog.On the other hand, these oilfields usually have a large amount of core data which directly display the characteristics of the formation, and enough information of injection and production. This paper describes an approach through which logging curves are calibrated in terms of the raw data, and then a prototype model of natural fractures is established based on the investigation of core data from 43 wells, totaling 4000 m in length.A computer program has been developed according to this method.Through analysis and comparison of the features of logging curves, this paper proposes a new concept, the well logging curve unit.By strictly depicting its shape through mathematical methods, the natural facture can be discriminated.This work also suggests an equation to estimate the probability of fracture occurrence, and finally other fracture parameters are calculated using some experimental expressions.With this methodology, logging curves from 100 wells were interpreted, the results of which agree with core data and field information.

Edge Effect Correction in the S-A Method for Geochemical Anomaly Separation
Yong Ge, Qiuming Cheng, Shenyuan Zhang
2004, 15(4): 379-387.

Anomaly separation using geochemical data often involves operations in the frequency domain, such as filtering and reducing noise/signal ratios.Unfortunately, the abrupt edge truncation of an image along edges and holes (with missing data) often causes frequency distribution distortion in the frequency domain.For example, bright strips are commonly seen in frequency distribution when using a Fourier transform.Such edge effect distortion may affect information extraction results; sometimes severely, depending on the edge abruptness of the image.Traditionally, edge effects are reduced by smoothing the image boundary prior to applying a Fourier transform.Zero-padding is one of the most commonly used smoothing methods.This simple method can reduce the edge effect to some degree but still distorts the image in some cases.Moreover, due to the complexity of geoscience images, which can include irregular shapes and holes with missing data, zero-padding does not always give satisfactory results.This paper proposes the use of decay functions to handle edge effects when extracting information from geoscience images.As an application, this method has been used in a newly developed multifractal method (S-A) for separating geochemical anomalies from background patterns.A geochemical dataset chosen from a mineral district in Nova Scotia, Canada was used to validate the method. KEY WORDS: edge effect correction, fractal modeling, spatial information extraction, zero-padding, decay functions.

THM Coupled Modeling in Near Field of an Assumed HLW Deep Geological Disposal Repository
Zhenyao Shen, Guoding Li, Shushen Li
2004, 15(4): 388-394.

One of the most suitable ways under study for the disposal of high-level radioactive waste (HLW) is isolation in deep geological repositories.It is very important to research the thermo-hydromechanical (THM) coupled processes associated with an HLW disposal repository.Non-linear coupled equations, which are used to describe the THM coupled process and are suited to saturated-unsaturated porous media, are presented in this paper.A numerical method to solve these equations is put forward, and a finite element code is developed.This code is suited to the plane strain or axis-symmetry problem. Then this code is used to simulate the THM coupled process in the near field of an ideal disposal repository.The temperature vs.time, hydraulic head vs.time and stress vs.time results show that, in this assumed condition, the impact of temperature is very long (over 10000 a) and the impact of the water head is short (about 90 d).Since the stress is induced by temperature and hydraulic head in this condition, the impact time of stress is the same as that of temperature.The results show that THM coupled processes are very important in the safety analysis of an HLW deep geological disposal repository.

Formation Mechanism and Stability Assessment of the Colluvial Deposit Slope in Zuoyituo
Wenxing Jian, Yihu Zhang, Hongmei Yin
2004, 15(4): 395-400.

The basic features of the colluvial deposit slope in Zuoyituo such as geological conditions, dimensions, slip surfaces and groundwater conditions are described concisely in this paper.The formation mechanism of the slope is discussed.It is considered that the formation of the colluvial deposit slope in Zuoyituo has undergone accumulation, slip, load, deformation and failure.The effects of rainfall on slope stability are categorized systematically based on existing methodology, and ways to determine the effects quantitatively are presented.The remained slip force method is improved by the addition of quantitative relations to the existing formulae and programs.The parameters of the colluvial deposit slope are determined through experimentation and the method of back-analysis.The safety factors of the slope are calculated with the improved remained slip force method and the Sarma method.The results show that rainfall and water level in the Yangtze River have a significant effect on the stability of the colluvial deposit slope in Zuoyituo.The hazards caused by the instability of the slope are assessed, and prevention methods are put forward.

An Effective Method for Free Vibration of Plate on Elastic Half Space
Yuanhan Wang, Wenhui Gong
2004, 15(4): 401-408.

The vibration analysis of a plate on an elastic foundation is an important problem in engineering.It is the interaction of a plate with the three-dimensional half space and the plate is usually loaded from both the upper and lower surfaces.The contact pressure from the soil can not be predefined.According to Lamb's solution for a single oscillating force acting on a point on the surface of an elastic half space, and the relevant approximation formulae, a relation between the local pressure and the deflection of the plate has been proposed.Based on this analysis, the reaction of the soil can be represented as the deformation of the plate.Therefore, the plate can be separated from the soil and only needs to be divided by a number of elements in the analysis.The following procedure is the same as the standard finite element method.This is a semi-analytical and semi-numerical method.It has been applied to the dynamic analysis of circular or rectangular plates on the elastic half space, at low or high frequency vibration, and on rigid, soft or flexible foundations.The results show that this method is versatile and highly accurate.

Study of Nonlinear Seismic Response and TMD Primary Control of the Cable-Stayed Bridge Section of the Third Macao-Taipa Bridge
Xiaohong Long, Li Li
2004, 15(4): 409-415.

The practical design of the cable-stayed bridge of the 3 rd Macao-Taipa bridge is investigated by the finite element analysis program ANSYS, and3-D elements BEAM188 and BEAM4 are adopted to create a dynamic calculation model.In order to analyze the material nonlinear seismic response of the cable-stayed bridge, the nonlinear behaviors of the ductile plastic hinges of the bridge towers are taken into account by employing the nonlinear rotational spring element COMBIN40.To simulate a major earthquake, three earthquake records were chosen using a wave-choosing program and input into the bridge structure along longitudinal and transversal directions.Comparisons of the linear and nonlinear seismic responses of the cable-stayed bridge are performed.In addition, a study of TMD primary control is carried out using element MASS21 and element COMBIN14, and it is indicated that the effects of mitigation monitoring are evident.

Serpentinized Peridotite as Source of Aeromagnetic Anomalies
Qingsheng Liu, Qingsong Liu, Zeming Zhang, Tao Yang, Yuanyuan Fu
2004, 15(4): 416-419.

The source of long-wavelength aeromagnetic anomalies appears to originate from the earth's deep crust. Constrained by previous studies on geochemical, petrologic analysis, the eclogite and serpentinized peridotite samples from drill hole ZK703 at Donghai in the western Sulu ultrahigh-pressure (UHP) terrane, East China, were unambiguously exhumed from the lower crust and the upper mantle, providing significant information about the magnetic properties of rocks at a deeper part of the crust.Resultsshow that the serpentinization process favors the neoformation of nearly stoichiometric magnetite, resulting in the enhancement of its magnetization up to 8.6 A/m, which is sufficient enough to contribute to some magnetic anomalies. In contrast, eclogite samples have only weaker magnetization (generally less than 0.05 A/m) compared to serpentinized peridotite. Nevertheless, experiments under the lower crustal conditions are necessary to further support these conclusions.

Nonlinear Inversion of Potential-Field Data Using an Improved Genetic Algorithm
Gangding Feng, Chao Chen
2004, 15(4): 420-424.

The genetic algorithm is useful for solving an inversion of complex nonlinear geophysical equations. The multi-point search of the genetic algorithm makes it easier to find a globally optimal solution and avoid falling into a local extremum. The search efficiency of the genetic algorithm is a key to producing successful solutions in a huge multi-parameter model space. The encoding mechanism of the genetic algorithm affects the searching processes in the evolution. Not all genetic operations perform perfectly in a search under either a binary or decimal encoding system. As such, a standard genetic algorithm (SGA) is sometimes unable to resolve an optimization problem such as a simple geophysical inversion. With the binary encoding system the operation of the crossover may produce more new individuals. The decimal encoding system, on the other hand, makes the mutation generate more new genes. This paper discusses approaches of exploiting the search potentials of genetic operations with different encoding systems and presents a hybrid-encoding mechanism for the genetic algorithm. This is referred to as the hybrid-encoding genetic algorithm (HEGA). The method is based on the routine in which the mutation operation is executed in decimal code and other operations in binary code. HEGA guarantees the birth of better genes by mutation processing with a high probability, so that it is beneficial for resolving the inversions of complicated problems. Synthetic and real-world examples demonstrate the advantages of using HEGA in the inversion of potential-field data.

Trajectory Control: Directional MWD Inversely New Wellbore Positioning Accuracy Prediction Method
Abd Alaziz Ibrahim Ahmed, Ahmed Musa Tagwa
2004, 15(4): 425-433.

The deviation control of directional drilling is essentially the controlling of two angles of the wellbore actually drilled, namely, the inclination and azimuth.In directional drilling the bit trajectory never coincides exactly with the planned path, which is usually a plane curve with straight, building, holding, and dropping sections in succession.The drilling direction is of course dependant on the direction of the resultant forces acting on the bit and it is quite a tough job to hit the optimum target at the hole bottom as required.The traditional passive methods for correcting the drilling path have not met the demand to improve the techniques of deviation control.A method for combining wellbore surveys to obtain a composite, more accurate well position relies on accepting the position of the well from the most accurate survey instrument used in a given section of the wellbore.The error in each position measurement is the sum of many independent root sources of error effects.The relationship between surveys and other influential factors is considered, along with an analysis of different points of view.The collaborative work describes, establishes a common starting point of wellbore position uncertainty model, definition of what constitutes an error model, mathematics of position uncertainty calculation and an error model for basic directional service.