Shallow seismic AVO variations related to partial water saturation during a pumping test

This is the published version. Copyright 2007 by the American Geophysical Union. All Rights Reserved.High-resolution shallow seismic reflection experiments were conducted during and after a pumping test of an agricultural irrigation well to image the cone of depression. Although variations in the reflection time from the top of the saturated zone were not observed, amplitude-versus-offset (AVO) analysis revealed changes in reflection amplitude responses that correlate temporally and spatially to expected changes to the partially saturated zone induced by the pumping and recovery of the aquifer. The AVO responses exhibit dependence on aquifer drawdown and recovery cycles and the distance from the pumping well. We propose that near-surface soil heterogeneity and relatively rapid changes in the water table elevation during irrigation cycles caused a thickening of the partially saturated zone above the water table, which resulted in detectable changes in seismic reflection amplitudes. This study offers insights about the response of shallow seismic reflections to changes in subsurface water saturation and the potential application of seismic techniques to hydrogeophysical problems

Ultra-shallow seismic imaging of the top of the saturated zone

This is the published version. Copyright 2010 by the American Geophysical Union. All Rights Reserved.We collected ultra-shallow seismic-reflection data to image the near-surface stratigraphy of a Kansas River point bar. We were successful in identifying a discontinuous clay layer and the top of the saturated zone at depths of 0.95 and 1.4 m. Seismic walkaway data collected using various .22-caliber ammunition show that decreased source energy is necessary to generate higher frequencies and prevent clipping of critical near-offset traces needed to identify ultra-shallow reflections. The seismic reflections exhibited average normal moveout velocities of 180–195 m/s with dominant frequencies of 200–450 Hz. Coincident subsurface features were also imaged using 200-MHz ground-penetrating radar. This study presents the shallowest seismic reflection from the top of the saturated zone reported in the literature to date and further demonstrates the potential of using seismic-reflection methods for ultra-shallow imaging of the subsurface as a stand-alone tool or in conjunction with other high-resolution geophysical techniques

Numerical simulation of dynamic pore fluid-solid interaction in fully saturated non-linear porous media

In this paper, a large deformation formulation for dynamic analysis of the pore fluid-solid interaction in a fully saturated non-linear medium is presented in the framework of the Arbitrary Lagrangian-Eulerian method. This formulation is based on Biot’s theory of consolidation extended to include the momentum equations of the solid and fluid phases, large deformations and non-linear material behaviour. By including the displacements of the solid skeleton, u, and the pore fluid pressure, p, a (u-p) formulation is obtained, which is then discretised using finite elements. Time integration of the resulting highly nonlinear equations is accomplished by the generalized–α method, which assures second order accuracy as well as unconditional stability of the solution. Details of the formulation and its practical implementation in a finite element code are discussed. The formulation and its implementation are validated by solving some classical examples in geomechanics

Spitzer Space Telescope spectral observations of AGB stars in the Fornax dwarf spheroidal galaxy

We have observed five carbon-rich AGB stars in the Fornax dwarf spheroidal (dSph) galaxy, using the Infrared Spectrometer on board the Spitzer Space Telescope. The stars were selected from a near-infrared survey of Fornax and include the three reddest stars, with presumably the highest mass-loss rates, in that galaxy. Such carbon stars probably belong to the intermediate-age population (2-8 Gyr old and metallicity of [Fe/H] -1) of Fornax. The primary aim of this paper is to investigate mass-loss rate, as a function of luminosity and metallicity, by comparing AGB stars in several galaxies with different metallicities. The spectra of three stars are fitted with a radiative transfer model. We find that mass-loss rates of these three stars are 4-7x10^-6 Msun yr-1. The other two stars have mass-loss rates below 1.3x10^-6 Msun yr-1. We find no evidence that these rates depend on metallicity, although we do suggest that the gas-to-dust ratio could be higher than at solar metallicity, in the range 240 to 800. The C2H2 bands are stronger at lower metallicity because of the higher C/O ratio. In contrast, the SiC fraction is reduced at low metallicity, due to low silicon abundance. The total mass-loss rate from all known carbon-rich AGB stars into the interstellar medium of this galaxy is of the order of 2x10^-5 Msun yr-1. This is much lower than that of the dwarf irregular galaxy WLM, which has a similar visual luminosity and metallicity. The difference is attributed to the younger stellar population of WLM. The suppressed gas-return rate to the ISM accentuates the difference between the relatively gas-rich dwarf irregular and the gas-poor dwarf spheroidal galaxies. Our study will be useful to constrain gas and dust recycling processes in low metallicity galaxies.Comment: MNRAS accepte

The Peaceful Atom Comes to Campus

Youthful idealism, institutional ambition, and Cold War sensibilities all helped shape the Michigan Memorial–Phoenix Project, the University of Michigan’s tribute to fallen World War II soldiers.</jats:p

The Key Role of Heavy Precipitation Events in Climate Model Disagreements of Future Annual Precipitation Changes in California

Climate model simulations disagree on whether future precipitation will increase or decrease over California, which has impeded efforts to anticipate and adapt to human-induced climate change. This disagreement is explored in terms of daily precipitation frequency and intensity. It is found that divergent model projections of changes in the incidence of rare heavy (\u3e60 mm day−1) daily precipitation events explain much of the model disagreement on annual time scales, yet represent only 0.3% of precipitating days and 9% of annual precipitation volume. Of the 25 downscaled model projections examined here, 21 agree that precipitation frequency will decrease by the 2060s, with a mean reduction of 6–14 days yr−1. This reduces California\u27s mean annual precipitation by about 5.7%. Partly offsetting this, 16 of the 25 projections agree that daily precipitation intensity will increase, which accounts for a model average 5.3% increase in annual precipitation. Between these conflicting tendencies, 12 projections show drier annual conditions by the 2060s and 13 show wetter. These results are obtained from 16 global general circulation models downscaled with different combinations of dynamical methods [Weather Research and Forecasting (WRF), Regional Spectral Model (RSM), and version 3 of the Regional Climate Model (RegCM3)] and statistical methods [bias correction with spatial disaggregation (BCSD) and bias correction with constructed analogs (BCCA)], although not all downscaling methods were applied to each global model. Model disagreements in the projected change in occurrence of the heaviest precipitation days (\u3e60 mm day−1) account for the majority of disagreement in the projected change in annual precipitation, and occur preferentially over the Sierra Nevada and Northern California. When such events are excluded, nearly twice as many projections show drier future conditions

Probabilistic estimates of future changes in California temperature and precipitation usingstatistical and dynamical downscaling

Sixteen global general circulation models were used to develop probabilistic projections of temperature (T) and precipitation (P) changes over California by the 2060s. The global models were downscaled with two statistical techniques and three nested dynamical regional climate models, although not all global models were downscaled with all techniques. Both monthly and daily timescale changes in T and P are addressed, the latter being important for a range of applications in energy use, water management, and agriculture. The T changes tend to agree more across downscaling techniques than the P changes. Year-to-year natural internal climate variability is roughly of similar magnitude to the projected T changes. In the monthly average, July temperatures shift enough that that the hottest July found in any simulation over the historical period becomes a modestly cool July in the future period. Januarys as cold as any found in the historical period are still found in the 2060s, but the median and maximum monthly average temperatures increase notably. Annual and seasonal P changes are small compared to interannual or intermodel variability. However, the annual change is composed of seasonally varying changes that are themselves much larger, but tend to cancel in the annual mean. Winters show modestly wetter conditions in the North of the state, while spring and autumn show less precipitation. The dynamical downscaling techniques project increasing precipitation in the Southeastern part of the state, which is influenced by the North American monsoon, a feature that is not captured by the statistical downscaling

Population Structure Influences Sexual Conflict in Wild Populations of Water Striders.

In sexual conflict, aggressive males frequently diminish the long-term reproductive success of females in efforts to gain a short-term advantage over rival males. This short-term advantage can selectively favour high-exploitation males. However, just as the over-exploitation of resources can lead to local extinction, the over-exploitation of females in the form of harassment by aggressive males can yield similar consequences resulting in reduced female fecundity, increased female mortality and overall decline in mating activity. This outcome may often be prevented by selection acting at multiple levels of biological organization. Directional selection favouring aggressive exploitation within groups can be balanced by directional selection amongst groups opposing exploitation. Such between-group selection has recently been demonstrated in laboratory studies of water striders, where the conditional dispersal of individuals increased variation amongst groups and influenced the balance of selection toward reduced male aggression. This multilevel selection (MLS) framework also provides predictive value when investigating natural populations differing in their relative strength of selection within versus among groups. For water striders, the consequences of local exploitation cause fitness differences between groups, favouring less aggressive males. Inconsistently flowing ephemeral streams consist of isolated pools that prevent aggressive male water striders from escaping the consequences of local exploitation. We, therefore, predicted that inconsistently flowing ephemeral streams would favour the evolution of less aggressive males than would perennial streams, which allow aggressive males to move more freely and to escape the group-level costs of their aggression. Comparing two neighbouring streams during the mating season, we found that males dispersed naturally between pools at much higher rates in the perennial stream than in the ephemeral stream. As predicted, we found that males from the perennial stream were significantly more aggressive than those from the ephemeral stream. We also found that dispersers were significantly more aggressive than non-dispersers within each stream. These field results illustrate the relevance of the MLS framework in our understanding of the evolution of sexual conflict