Hydro-geomechanical modelling of seal behaviour in overpressured basins using discontinuous deformation analysis

A coupled hydro-geomechanical modelling environment, developed to evaluate the coupled responses of fluid flow in deforming discontinuous media, is described. A staggered computational framework is presented, where the two simulations tools, HYDRO and DDA, communicate via the mapping of an equivalent porosity (and related permeabilities) from the rock system to the fluid phase and an inverse mapping of the pressure field. Several algorithmic and modelling issues are discussed, in particular the computational procedure to map the current geometry of the discontinuous rock blocks assembly into an equivalent porosity (and permeability) field. A generic, geometrically simple, overpressured reservoir/seal system is analysed for illustration. Further examples investigate discontinuous, fractured configurations in flexure causing a degree of spatial variability in the induced stresses. Model predictions show that the combination of hydraulic and mechanical loads causes a dilational opening of some pre-existing fractures and closure of others, with strong localisation of the modified flow pattern along wider fracture openings.     

Assessing burn severity and comparing soil water repellency,Hayman Fire,Colorado

An important element of evaluating a large wildfire is to assess its effects on the soil in order to predict the potential watershed response. After the 55 000 ha Hayman Fire on the Colorado Front Range, 24 soil and vegetation variables were measured to determine the key variables that could be used for a rapid field assessment of burn severity. The percentage of exposed mineral soil and litter cover proved to be the best predictors of burn severity in this environment. Two burn severity classifications, one from a statistical classification tree and the other a Burned Area Emergency Response (BAER) burn severity map, were compared with measured ‘ground truth’ burn severity at 183 plots and were 56% and 69% accurate, respectively. This study also compared water repellency measurements made with the water drop penetration time (WDPT) test and a mini‐disk infiltrometer (MDI) test. At the soil surface, the moderate and highly burned sites had the strongest water repellency, yet were not significantly different from each other. Areas burned at moderate severity had 1·5 times more plots that were strongly water repellent at the surface than the areas burned at high severity. However, the high severity plots most likely had a deeper water repellent layer that was not detected with our surface tests. The WDPT and MDI values had an overall correlation of r = ?0·64(p < 0·0001) and appeared to be compatible methods for assessing soil water repellency in the field. Both tests represent point measurements of a soil characteristic that has large spatial variability; hence, results from both tests reflect that variability, accounting for much of the remaining variance. The MDI is easier to use, takes about 1 min to assess a strongly water repellent soil and provides two indicators of water repellency: the time to start of infiltration and a relative infiltration rate. Copyright © 2005 John Wiley & Sons, Ltd.     

Interhemispheric contrasts in the ionospheric convection response to changes in the interplanetary magnetic field and substorm activity: a case-study

Interhemispheric contrasts in the ionospheric convection response to variations of the interplanetary magnetic field (IMF) and substorm activity are examined, for an interval observed by the Polar Anglo-American Conjugate Experiment (PACE) radar system between 1600 and 2100 MLT on 4 March 1992. Representations of the ionospheric convection pattern associated with different orientations and magnitudes of the IMF and nightside driven enhancements of the auroral electrojet are employed to illustrate a possible explanation for the contrast in convection flow response observed in radar data at nominally conjugate points. Ion drift measurements from the Defence Meteorological Satellite Program (DMSP) confirm these ionospheric convection flows to be representative for the prevailing IMF orientation and magnitude. The location of the fields of view of the PACE radars with respect to these patterns suggest that the radar backscatter observed in each hemisphere is critically influenced by the position of the ionospheric convection reversal boundary (CRB) within the radar field of view and the influence it has on the generation of the irregularities required as scattering targets by high-frequency coherent radar systems. The position of the CRB in each hemisphere is strongly controlled by the relative magnitudes of the IMF B_z and B_y components, and hence so is the interhemispheric contrast in the radar observations.     

Undistorted lensed images in galaxy clusters

To date, the study of high-magnification gravitational lensing effects of galaxy clusters has focused upon the grossly distorted, luminous arc-like features formed in massive, centrally condensed clusters. We investigate the formation of a different type of image, highly magnified yet undistorted, in two widely employed cluster mass density profiles, namely an isothermal sphere with a core, and a universal dark matter halo profile derived from the numerical simulations of Navarro et al. We examine the properties of images of extended sources produced by these two cluster profiles, paying particular attention to the undistorted images. Using simple assumptions about the source and lens population, we estimate the relative frequency of the occurrence of highly magnified, undistorted images and the more commonly known giant arcs.     

Weighing a galaxy bar in the lens Q2237 +=0305

In the gravitational lens system Q2237+0305 the cruciform quasar image geometry is twisted by 10c by the lens effect of a bar in the lensing galaxy. This effect can be used to measure the mass of the bar. We construct a new lensing model for this system with a power-law elliptical bulge and a Ferrers bar. The observed ellipticity of the optical isophotes of the galaxy leads to a nearly isothermal elliptical profile for the bulge, with a total quasar magnification of 16⁺⁵₋₄. We measure a bar mass of (7.5 ∼ 1.5) −10⁸ h ⁻¹₇₅ M⊙ in the region inside the quasar images.     

Microlensing-induced spectral variability in Q 2237+0305

We present both photometry and spectra of the individual images of the quadruple gravitational lens system Q 2237+0305. Comparison of spectra obtained at two epochs, separated by ∼3 yr, shows evidence for significant changes in the emission line-to-continuum ratio of the strong ultraviolet C  IV  λ1549, C  III ] λ1909 and Mg  II  λ2798 lines. The short, ∼1 day, light-travel time differences between the sight lines to the four individual quasar images rule out any explanation based on intrinsic variability of the source. The spectroscopic differences thus represent direct detection of microlensing-induced spectroscopic differences in a quasar. The observations allow constraints to be placed on the relative spatial scales in the nucleus of the quasar, with the ultraviolet continuum arising in a region of ≲0.05 pc in extent, while the broad emission-line material is distributed on scales much greater than this.     

Fecundity of Atlantic menhaden,Brevoortia tyrannus

Since the 1950’s the population structure of the Atlantic menhaden,Brevoortia tyrannus, has undergone dramatic changes. High rates of fishery exploitation during the 1960’s resulted in a drastic decline in the breadth of the age structure and numbers of spawning aged fish. The stock recovered under continued exploitation during the 1970’s in spite of these unfavorable conditions. Hence, the length and age at first spawning, the seasons and areas of spawning, and the potential number of ova produced as a function of length were reexamined during the 1978, 1979, and 1981 fishing seasons. No changes were detected in the reproductive biology of Atlantic menhaden. The size at first spawning, areas, and season of spawning were similar to results of previous studies. The minimum fork length of potential spawners was 180 mm. Few age-1 fish were mature but most females were mature at age-2. Analysis of the ovaries collected along the coast indicated some spawning occurred in the spring, a limited amount occurred in the summer, and most spawning occurred during the fall and winter. There was high intra-and interyear variability in the relationship between length and potential number of ova produced, and regression coefficients encompassed values from two previous studies, each based on a single fishing year. Data from this and the previous studies were combined and equally weighted to generate a representative predictive equation for the potential number of ova spawned as a function of length: eggs = 2563e^0.0150(FL).