A multivariate analytical strategy for classifying paleoenvironments

A multivariate analytical strategy is proposed for aiding the investigator in extracting maximum information from environmental data. Data are carefully coded and scaled and are tested for redundancy using R-mode cluster analysis. The samples are partitioned into environmental classes using Q-mode cluster analysis. Q-mode ordination facilitates interpretations, which usually can be verified by comparison with field relationships. Discriminant analysis serves as an identification procedure for extending the classification to unknown samples. The strategy is demonstrated by application to Cape Hatteras microorganism distributions and Devonian sedimentary facies.     

Primary ‘cut-and-fill’ channels and gravitational diagenetic features

Ore beds with fluorite and sphalerite occur in the Cave-In-Rock fluorite district, Illinois in the upper part of the Mississippian System; in the upper parts of the Fredonia and Joppa Members of the Ste. Genevieve Formation and in the upper part of the Downeys Bluff Formation. Sedimentary structures within the upper two horizons are described and discussed. Primary cut-and-fill channels usually contain broken pieces of dolomite, limestone, shale, sandstone, fluorite and sphalerite. Some of the channels show geopetal features. Horizontal stylolitic seams bend downward and their thicknesses increase while their amplitudes decrease near and along the channel slopes. Another geopetal feature consists of clastic shale fragments which have mechanically filled from above into an open crack formed in the hydroplastic carbonate mud below. A number of features known as collapsed structures prove to be cut-and-fill structures of primary origin. During the compaction, carbonate beds were reduced in thickness by at least 37% through styloitization. The relationships between primary sedimentary channels and fluorite-sphalerite ore rocks suggest that the latter may have been deposited during or after the formation of such sedimentary structures, i. e., during various stages of diagenesis.

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Zusammenfassung Die Fluorit-Zinkblende-Vorkommen des Cave-In-Rock Distrikts von Illinois treten stratigraphisch in den oberen Partien des Mississippian System, speziell in der Fredonia und Joppa Serie der Ste. Genevieve Formation und in der Downeys Bluff Formation auf. Einige sedimentäre Strukturen in diesen beiden oberen Horizonten werden beschrieben und diskutiert. Primäre cut-and-fill channels enthalten im allgemeinen Bruchstücke von Dolomit, Kalk- und Sandstein, Schieferton, Fluorit und Zinkblende. Einige channels zeigen geopetale Gefüge, d. h. horizontale stylolithische Lagen sind mit zunehmender Mächtigkeit und abnehmender Amplitude der Fältelung in der Nähe oder entlang der Flanken der channel Strukturen nach unten durchgebogen. Ein weiteres geopetales Gefüge besteht aus Schieferton-Fragmenten, die mechanisch in eine offene Spalte des darunterliegenden hydroplastischen Karbonatschlammes eingesunken sind. Eine Anzahl von Gefügen, bekannt als collapsed structures scheinen cut-and-fill und slump Strukturen von primär subaquatischer Herkunft zu sein. Während der Diagenese wurden Karbonatbänke durch Stylolithbildung um mindestens 37% der ursprünglichen Mächtigkeit reduziert. Die Beziehungen zwischen den primären sedimentären channels und dem Fluorit-Zinkblende-Erz deuten darauf hin, daß das letztere während oder nach der Bildung der sedimentären Strukturen abgelagert wurde, d. h., während verschiedener Perioden der Diagenese.

     

Red beds: DRM or CRM?

Thermal demagnetization and chemical leaching experiments show that, in some Hopewell Group sediments, both detrital (DRM) and chemical (CRM) remanent magnetizations are present. The CRM was acquired in two phases (CRM_A, CRM_B), one before and one after a field reversal. Field evidence indicates that all three magnetizations were probably acquired in less than 35 my. The magnetization process occurred over a period of time long enough to span one or more field reversals. The CRM_B and then the CRM_A can be successively removed by progressively longer immersion times in hydrochloric acid so that the DRM is uncovered. The oppositely directed CRM_A and DRM cannot be thermally separated so that accurate field directions cannot be determined from thermal demagnetization alone. It is suggested that field reversals occurringduring magnetization may have a bearing on different paleomagnetic problems. The two techniques of thermal demagnetization and chemical leaching complement each other and together they can provide new information about the behaviour of the earth's field in the past.     

A discontinuous Galerkin method for seismic soil–structure interaction analysis in the time domain

A technique for modeling transient wave propagation in unbounded media is extended and applied to seismic soil–structure interaction analysis in the time domain. The technique, based on the discontinuous Galerkin method, requires lower computational cost and less storage than the boundary element method, and the time‐stepping scheme resulting from Newmark's method in conjunction with the technique is unconditionally stable, allowing for efficient and robust time‐domain computations. To extend the technique to cases characterized by seismic excitation, the free‐field motion is used to compute effective forces, which are introduced on the boundary of the computational domain containing the structure and the soil in the vicinity of the structure. A numerical example on a dam–foundation system subjected to seismic excitation demonstrates the performance of the method. Copyright © 2003 John Wiley & Sons, Ltd.     

Hybrid seismic protection of cable‐stayed bridges

This paper proposes a hybrid control strategy combining passive and semi‐active control systems for seismic protection of cable‐stayed bridges. The efficacy of this control strategy is verified by examining the ASCE first‐generation benchmark problem for a seismically excited cable‐stayed bridge, which employs a three‐dimensional linearized evaluation bridge model as a testbed structure. Herein, conventional lead–rubber bearings are introduced as base isolation devices, and semi‐active dampers (e.g., variable orifice damper, controllable fluid damper, etc.) are considered as supplemental damping devices. For the semi‐active dampers, a clipped‐optimal control algorithm, shown to perform well in previous studies involving controllable dampers, is considered. Because the semi‐active damper is a controllable energy‐dissipation device that cannot add mechanical energy to the structural system, the proposed hybrid control strategy is fail‐safe in that the bounded‐input, bounded‐output stability of the controlled structure is guaranteed. Numerical simulation results show that the performance of the proposed hybrid control strategy is quite effective in protecting seismically excited cable‐stayed bridges. Copyright © 2004 John Wiley & Sons, Ltd.     

Simulation of the seismic performance of the Bolu Viaduct subjected to near‐fault ground motions

The seismic performance of the Bolu Viaduct in the Duzce, Turkey, earthquake of November 1999 was studied via a non‐linear, time‐history analysis of a multi‐degree of freedom model. The viaduct had a seismic isolation system consisting of yielding‐steel energy dissipation units and sliding pot bearings. The Duzce earthquake caused a surface rupture across the viaduct, which resulted in excessive superstructure movement and widespread failure of the seismic isolation system. The effect of the rupture was modeled by a static, differential ground displacement in the fault‐parallel direction across the rupture. The ground motions used in the analysis contain common near‐fault features including a directivity pulse in the fault‐normal direction and a fling step in the fault‐parallel direction. The analysis used a finite element package capable of modeling the mechanical behavior of the seismic isolation system and focused on the structural response of a 10‐span module of the viaduct. This analysis showed that the displacement of the superstructure relative to the piers exceeded the capacity of the bearings at an early stage of the earthquake, causing damage to the bearings as well as to the energy dissipation units. The analysis also indicated that shear keys, both longitudinal and transverse, played a critical role in preventing collapse of the deck spans. Published in 2004 by John Wiley & Sons, Ltd.     

Active sensing using impedance‐based ARX models and extreme value statistics for damage detection

In this paper, the applicability of an auto‐regressive model with exogenous inputs (ARX) in the frequency domain to structural health monitoring (SHM) is established. Damage sensitive features that explicitly consider non‐linear system input/output relationships are extracted from the ARX model. Furthermore, because of the non‐Gaussian nature of the extracted features, Extreme Value Statistics (EVS) is employed to develop a robust damage classifier. EVS provides superior performance to standard statistical methods because the data of interest are in the tails (extremes) of the damage sensitive feature distribution. The suitability of the ARX model, combined with EVS, to non‐linear damage detection is demonstrated using vibration data obtained from a laboratory experiment of a three‐story building model. It is found that the vibration‐based method, while able to discern when damage is present in the structure, is unable to localize the damage to a particular joint. An impedance‐based active sensing method using piezoelectric (PZT) material as both an actuator and a sensor is then investigated as an alternative solution to the problem of damage localization. Copyright © 2005 John Wiley & Sons, Ltd.     

Evaluation of seismic site factors in the Mississippi Embayment. II. Probabilistic seismic hazard analysis with nonlinear site effects

An integrated probabilistic seismic hazard analysis procedure that incorporates nonlinear site effects, PSHA-NL, is developed and used to characterize the influence of thick deposits of the upper Mississippi Embayment (ME) on seismic site coefficients. PSHA-NL follows the methodology of the 2002 USGS hazard maps and generates a compatible set of ground motion records. The motions are propagated using nonlinear and equivalent linear site response analyses and ME properties developed in a companion paper and used to derive surface uniform hazard response spectra. A set of generic site coefficients are derived and summarized in a format similar to NEHRP site coefficients, with an added dimension of ME deposits thickness to the Paleozoic rock, a physically meaningful impedance boundary. These coefficients compare well with NEHRP site coefficients for 30 m profiles. For thicker soil profiles, developed site coefficients are lower at short periods and higher at long periods than NEHRP site coefficients.     

Hydrochemistry of urban groundwater in Seoul, South Korea: effects of land-use and pollutant recharge

The ionic and isotopic compositions (δD, δ¹⁸O, and ³H) of urban groundwaters have been monitored in Seoul to examine the water quality in relation to land-use. High tritium contents (6.1–12.0 TU) and the absence of spatial/seasonal change of O–H isotope data indicate that groundwaters are well mixed within aquifers with recently recharged waters of high contamination susceptibility. Statistical analyses show a spatial variation of major ions in relation to land-use type. The major ion concentrations tend to increase with anthropogenic contamination, due to the local pollutants recharge. The TDS concentration appears to be a useful contamination indicator, as it generally increases by the order of forested green zone (average 151 mg/l), agricultural area, residential area, traffic area, and industrialized area (average 585 mg/l). With the increased anthropogenic contamination, the groundwater chemistry changes from a Ca–HCO₃ type toward a Ca–Cl(+NO₃) type. The source and behavior of major ions are discussed and the hydrochemical backgrounds are proposed as the basis of a groundwater management plan.