System identification of the Vincent Thomas suspension bridge using earthquake records

The Vincent Thomas Bridge in the Los Angeles metropolitan area, is a critical artery for commercial traffic flow in and out of the Los Angeles Harbor, and is at risk in the seismically active Southern California region, particularly because it straddles the Palos Verdes fault zone. A combination of linear and non‐linear system identification techniques is employed to obtain a complete reduced‐order, multi‐input–multi‐output (MIMO) dynamic model of the Vincent Thomas Bridge based on the dynamic response of the structure to the 1987 Whittier and 1994 Northridge earthquakes. Starting with the available acceleration measurements (which consists of 15 accelerometers on the bridge structure and 10 accelerometers at various locations on its base), an efficient least‐squares‐based time‐domain identification procedure is applied to the data set to develop a reduced‐order, equivalent linear, multi‐degree‐of‐freedom model. Although not the main focus of this study, the linear system identification method is also combined with a non‐parametric identification technique, to generate a reduced‐order non‐linear mathematical model suitable for use in subsequent studies to predict, with good fidelity, the total response of the bridge under arbitrary dynamic environments. Results of this study yield measurements of the equivalent linear modal properties (frequencies, mode shapes and non‐proportional damping) as well as quantitative measures of the extent and nature of non‐linear interaction forces arising from strong ground shaking. It is shown that, for the particular subset of observations used in the identification procedure, the apparent non‐linearities in the system restoring forces are quite significant, and they contribute substantially to the improved fidelity of the model. Also shown is the potential of the identification technique under discussion to detect slight changes in the structure's influence coefficients, which may be indicators of damage and degradation in the structure being monitored. Difficulties associated with accurately estimating damping for lightly damped long‐span structures from their earthquake response are discussed. The technical issues raised in this paper indicate the need for added spatial resolution in sensor instrumentation to obtain identified mathematical models of structural systems with the broadest range of validity. Copyright © 2003 John Wiley & Sons, Ltd.     

American Trappist Monasteries and the Changing Spiritual Landscape

Spiritual landscapes arise from a dynamic relationship of spiritual beliefs, ritual practices, and embodied encounters in place. They can contain multiple spiritual and non‐spiritual elements that change over time. This paper offers an appreciation of the diverse, overlapping, and ambivalent meanings emerging from Trappist monasteries in the United States. With origins tracing back to eleventh‐century France, Trappist monasteries are Roman Catholic intentional communities belonging to the Order of Cistercians of the Strict Observance. Attempts to establish monasteries in the American scene began around the turn of the nineteenth century. Contemplation, a receptive state of interior spiritual silence, represents one significant component of Trappist spirituality. Like other aspects of the spiritual landscape, contemplation has been reprioritized as Trappist monks and nuns confront situations like political conflict, changes in monastic leadership, and economic problems. These places continue to address challenges and possibilities for reinvention as they become open to shifting social contexts.     

A low resolution map of Europa from four occultations by Io

Analysis of three occultations of JII (Europa) by JI (Io) has resulted in a preliminary reflectivity map of JII for the hemisphere centered on longitude 324°, a measurement of 1483±20 km for the radius of JII, estimates of the event impact parameters, determination of the mid event times, and a visual geometric albedo, p_ν = 0.74, for JII. A fourth occultation light curve was used after derivation of the results to confirm their validity.     

A new computational strategy for solving two‐phase flow in strongly heterogeneous poroelastic media of evolving scales

We develop a new computational methodology for solving two‐phase flow in highly heterogeneous porous media incorporating geomechanical coupling subject to uncertainty in the poromechanical parameters. Within the framework of a staggered‐in‐time coupling algorithm, the numerical method proposed herein relies on a Petrov–Galerkin postprocessing approach projected on the Raviart–Thomas space to compute the Darcy velocity of the mixture in conjunction with a locally conservative higher order finite volume discretization of the nonlinear transport equation for the saturation and an operator splitting procedure based on the difference in the time‐scales of transport and geomechanics to compute the effects of transient porosity upon saturation. Notable features of the numerical modeling proposed herein are the local conservation properties inherited by the discrete fluxes that are crucial to correctly capture the fingering patterns arising from the interaction between heterogeneity and nonlinear viscous coupling. Water flooding in a poroelastic formation subject to an overburden is simulated with the geology characterized by multiscale self‐similar permeability and Young modulus random fields with power‐law covariance structure. Statistical moments of the poromechanical unknowns are computed within the framework of a high‐resolution Monte Carlo method. Numerical results illustrate the necessity of adopting locally conservative schemes to obtain reliable predictions of secondary recovery and finger growth in strongly heterogeneous deformable reservoirs. Copyright © 2011 John Wiley & Sons, Ltd.     

Identification of base‐excited structures using output‐only parameter estimation

This paper presents a new identification technique for the extraction of modal parameters of structural systems subjected to base excitation. The technique uses output‐only measurements of the structural response. A combined subspace‐maximum likelihood algorithm is developed and applied to a three‐degree‐of‐freedom simulation model. Five ensembles of synthetically generated input signals, representing varying input characteristics, are employed in Monte Carlo simulations to illustrate the applicability of the method. The technique is able to circumvent some of the difficulties arising from short data sets by employing the Expectation Maximization (EM) algorithm to refine the subspace state estimates. This approach is motivated by successful application by previous authors on speech signals. Results indicate that, for certain system characteristics, more accurate pole estimates can be identified using the combined subspace‐EM formulation. In general, the damping ratios of the system are difficult to identify accurately due to limitations on data set length. The applicability of the technique to structural vibration signals is illustrated through the identification of seismic response data from the Vincent Thomas Bridge. Copyright © 2003 John Wiley & Sons, Ltd.     

Parameter estimation for multiple post-wildfire hydrologic models

Predictions of post-wildfire flooding and debris flows are needed, typically with short lead times. Measurements of soil-hydraulic properties necessary for model parameterization are, however, seldom available. This study quantified soil-hydraulic properties, soil-water retention, and selected soil physical properties within the perimeter of the 2017 Thomas Fire in California. The Thomas Fire burn scar produced catastrophic debris flows in January 2018, highlighting the need for improved prediction capability. Soil-hydraulic properties were also indirectly estimated using relations tied to soil-water retention. These measurements and estimates are examined in the context of parameterizing post-wildfire hydrologic models. Tension infiltrometer measurements showed significant decreases (p < .05) in field-saturated hydraulic conductivity (K_fs) and sorptivity (S) in burned areas relative to unburned areas. Wildfire effects on soil water-retention were dominated by significant decreases in saturated soil-water content (θ_S). The van Genuchten parameters α, N, and residual water content did not show significant wildfire effects. The impacts of the wildfire on hydraulic and physical soil properties were greatest in the top 1 cm, emphasizing that measurements of post-fire soil properties should focus on the near-surface. Reductions in K_fs, θ_s, and soil-water retention in burned soils were attributed to fire-induced decreases in soil structure evidenced by increases in dry bulk density. Sorptivity reductions in burned soils were attributed to increases in soil-water repellency. Rapid post-fire assessments of flash flood and debris flow hazards using physically-based hydrologic models are facilitated by similarities between K_fs, S, and the Green–Ampt wetting front potential (ψ_f) with measurements at other southern CA burned sites. We suggest that ratios of burned to unburned K_fs (0.37), S (0.36), and ψ_f (0.66) could be used to scale unburned values for model parameterization. Alternatively, typical burned values (K_fs = 20 mm hr⁻¹; S = 6 mm hr^−0.5; ψ_f = 1.6 mm) could be used for model parameterization.     

Numerical approximation of water–air two-phase flow by the mixed finite element method

A new model for two-phase flow of water and air in soil is presented. This leads to a system of two mass balance equations and two equations representing conservation of momentum of fluid and gas, respectively. This paper is concerned with the verification of this model for the special case of a rigid soil skeleton by computational experiments. Its numerical treatment is based on the Raviart–Thomas mixed finite element method combined with an implicit Euler time discretization. The feasibility of the method is illustrated for some test examples of one- and two-dimensional two-phase flow problems.     

THOMAS JEFFERSON,AMERICAN GEOGRAPHERS,AND THE USES OF GEOGRAPHY*

ABSTRACT. Thomas Jefferson (1743–1826) had a lifelong interest in geography. Except for his role in the Lewis and Clark Expedition and a few references to his Notes on the State of Virginia, however, geographers have taken a relatively slight interest in this aspect of his thought, despite his having sometimes been referred to as “one of the greatest American geographers.” This essay suggests that we need to reexamine Jefferson as a geographical thinker. Reviewing some of the more important literature thus far, it suggests where such topics may profitably be extended and points to some aspects of his geographical interests not yet incorporated into the geographical literature.     

Abiogenic Origin of Hydrocarbons: An Historical Overview

Abstract. The two theories of abiogenic formation of hydrocarbons, the Russian-Ukrainian theory of deep, abiotic petroleum origins and Thomas Gold's deep gas theory, have been considered in some detail. Whilst the Russian-Ukrainian theory was portrayed as being scientifically rigorous in contrast to the biogenic theory which was thought to be littered with invalid assumptions, this applies only to the formation of the higher hydrocarbons from methane in the upper mantle. In most other aspects, in particular the influence of the oxidation state of the mantle on the abundance of methane, this rigour is lacking especially when judged against modern criteria as opposed to the level of understanding in the 1950s to 1980s when this theory was at its peak. Thomas Gold's theory involves degassing of methane from the mantle and the formation of higher hydrocarbons from methane in the upper layers of the Earth's crust. However, formation of higher hydrocarbons in the upper layers of the Earth's crust occurs only as a result of Fischer-Tropsch-type reactions in the presence of hydrogen gas but is otherwise not possible on thermodynamic grounds. This theory is therefore invalid. Both theories have been overtaken by the increasingly sophisticated understanding of the modes of formation of hydrocarbon deposits in nature.