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November 10, 2008

Determining the Age of the Grand Canyon

The Grand Canyon's age has remained controversial since the days of John Wesley Powell and has resulted in over a century of scientific controversy. While scientists have reached general consensus that the canyon was probably carved by the Colorado River starting around six million years ago, a recent paper in the journal Science claimed to have found rocks in a cave near the western section of the canyon that proved the huge chasm was at least 17 million years old.

David Coblentz and Jolante Van Wijk of EES-17 along with collaborators at the University of New Mexico published a rebuttal paper in the journal Geology to help to resolve this debate by constraining the dynamics of the canyon incision. They developed an incision model that integrates tectonic influences and driving forces such as faulting, mantle to surface fluid interconnections, and mantle-driven dynamic uplift of the western edge of the Colorado Plateau.

The authors argue that a combination of three faults in the area and upwelling hot mantle material pushed the region's rocks upward, causing the canyon to form in segments from east to west over the last six million years (See figure below). The scientist believe that the faulting and uplift have influenced differential incision of the Grand Canyon in the past 6 million years, causing persistently different rates in the western versus eastern Grand Canyon.

These conclusions differ from the traditional view of canyon incision solely through a river passively carving the landscape. This fault evolution and tectonic driving forces are components that must be included in a viable model for the incision history of the Grand Canyon. Metaphorically, it is like cutting a layer cake with a knife. The knife indeed cuts the cake, but there is also a component of the cake rising.

Sequential restoration and model constraints for interplay between Grand Canyon incision rates (red arrows) and normal faulting (yellow arrows) in the past 6 million years. Shown here are 3 panels of a 12-panel animation.

K.E. Karlstrom, R. Crow, L.J. Crossey, D. Coblentz, and J.W. Van Wijk, "Model for Tectonically Driven Incision of the <6 Ma Grand Canyon", Geology 36, 835-838 (2008). The LANL research was sponsored mostly through Institute for Geophysics and Planetary Physics (IGPP) and support of yearly Geodynamics of the Western U.S. workshop.

November 3, 2008

Simulating the Effects of Aerosol Pollution in Clouds

The Earth's atmosphere typically contains a mixture of different types of aerosols that are produced by natural or anthropogenic processes. These aerosols play an important role in weather and climate through direct or indirect mechanisms. For example, a direct mechanism involves the ability of aerosol to influence the Earth's energy budget by either reflecting and/or absorbing radiation. However, by serving as cloud condensation nuclei, an indirect process is the impact of aerosol on cloud droplet concentration and also the mean cloud droplet radius and subsequent overall radiation budget. Because of the abundance of stratus clouds over vast stretches of the Earth's atmosphere, any changes in their mean properties via the indirect effect can have a rather large impact on the Earth's climate. Therefore it is important to understand the impact of pollution via anthropogenic aerosol on stratus clouds.

In a paper published October 10, 2008 in the Journal of Geophysical Research, five LANL scientists from three Divisions integrated models, experiments, and observations to successfully simulate the effects of aerosol pollution on clouds. The authors, M. Andrejczuk of EES-2, J. M. Reisner of EES-16, M. Dubey of EES-14, B. Henson of C-PCS, and C. Jeffery of ISR-2, developed a new high-fidelity cloud resolving model with laboratory based parameterization of cloud activation by soot containing aerosols

The model is the first to successfully simulate airborne field observations (DYACOMS-II) of polluted and clean clouds and quantify the "indirect" effect of aerosols on cloud brightness, a key uncertainty in climate assessments. The researchers assessed the impact of changes in aerosol number and composition on stratus clouds. The work resolves a scientific controversy by showing that chemical effects, as well as aerosol particle size, are important in the nucleation of cloud droplets. This new capability supports LANL's climate observations and modeling programs in the DOE Office of Biological and Environmental Research - Office of Science. The researchers are working with PNNL, the Science Focus Area lead, to incorporate the findings into global climate models.

Spatial distribution of activated cloud droplets from initial aerosol particles for (a) polluted and (b) clean conditions. The size of the droplets is indicated by colors r < 3 mm (red), 3 mm < r < 9 mm (green), 6 mm > r >9 mm (blue), and r 9 > mm (yellow). It is evident that polluted clouds with hydrophilic aerosols (a) have smaller drops than cleaner clouds and (b) the indirect effect of aerosols on clouds.

Reference: "The Potential Impacts of Pollution on a Nondrizzling Stratus Deck: Does Aerosol Number Matter More than Type?", Journal of Geophysical Research 113, D19204 (2008). The LANL-LDRD-DR, "Resolving the Aerosol-climate-water Puzzle" (PI-M. Dubey), funded the work.

October 27, 2008

New Method for Improving Confidence to Monitor a Comprehensive Test Ban Treaty

Howard Patton of LANL's EES-17 and a a collaborator developed a new method to study source characteristics of regional phases including explosion-generated S-waves (shear waves). This method uses differences between spectrograms of two closely located explosions recorded at a common station. Relative source effects of one explosion with respect to another explosion are isolated in the resultant difference spectrogram because path and receiver site effects cancel. This gives a global view of the relative frequency content.

Difference spectrograms for Nevada Test Site explosion pairs Rousanne/Techado and Baseball/Borrego provide information about the spectral content of regional phases related to source effects which can be used to gain insight into source generation processes and improve the confidence to monitor effectively at low yields.

Click thumbnail to view a larger image.

Spectrograms of Baseball (a) and Techado (b) explosions and the difference spectrogram [(a) - (b)] in (c). Regional-distance seismograms used in the analysis are plotted above the spectrograms. Cool blue and green colors indicate low amplitudes. Hot colors represent high amplitudes. A signal/noise threshold screens unreliable data, plotted in white. Spectral modulations are revealed by high amplitudes at frequencies below ~0.5 Hz, relatively low amplitudes between 0.5 and 0.7 Hz, high amplitudes between 0.7 and 1 Hz, and generally low amplitudes above 1 Hz. Between 0.5 and 0.7 Hz, the difference spectrogram has a "blotchy" appearance as a function of time down the record, possibly due to scattered arrivals of Rg-derived energy.

I.N. Gupta and H.J. Patton, "Difference Spectrograms: A New Method for Studying S-Wave Generation from Explosions," Bulletin of Seismological Society of America 98, 2460-2468 (2008).

LANL Receives First Medical Imaging Grant from Breast Cancer Research Program

The Department of Defense Breast Cancer Research Program (BCRP) awarded LANL scientist Lianjie Huang of EES-17 a breast cancer imaging grant. This is a 3-year BCRP Idea Award in collaboration with physicians T. Liu and E. Pile-Spellman of Columbia University Medical Center. The BCRP Idea Award supports highly innovative, high-risk/high-reward research that could ultimately lead to critical discoveries or major advancements that will accelerate the eradication of breast cancer. The proposed research will bring super-resolution ultrasound imaging to clinical ultrasound applications for detection of breast microcalcifications, the first sign of breast cancer in more than half of all cases. This is the first LANL medical imaging grant received from the DoD's BCRP.

The DoD-BCRP proposal response demonstrated not only the ability of LANL scientists to propose comprehensive research to early detection of breast cancer but also the value of the TT Division's Sponsored Research Office. In particular, the Proposal and Grant Administration office (proposals@lanl.gov) was invaluable in navigating the complex requirements inherent in a $740K proposal requiring administrative support. These resources are available to all LANL PIs in responding to funding opportunities.

October 14 , 2008

Model for Dissolution Driven Convection in Geological Sequestration of Carbon Dioxide (CO₂)

Sequestration of anthropogenic CO₂ emissions in saline aquifers is one of the most promising ways of mitigating emissions in the short term. CO₂ is a supercritical fluid with a density that is about 30% lower than that of the surrounding brine under the thermodynamic injection conditions. The CO₂ rises to the top of the domain under the low-permeability caprock at the top of the aquifer and begins to migrate laterally, which can lead to leakage into fractures or other wells (below). The CO₂ slowly dissolves into the underlying brine. The competition between lateral migration and dissolution plays a key role in determining the leakage risks for an aquifer. Brine with dissolved CO₂ is denser than the brine alone, resulting in a gravitational instability. The instability leads to a convective process where fingers of CO₂-rich brine penetrate the aquifer and increase the dissolution rate.

Schematic of CO₂ injection, migration, and leakage during geologic storage.

Rajesh Pawar and Philip Stauffer (both in EES-16) and collaborators developed highly accurate theoretical and computational models to understand the onset of the convective process. They used a theoretical approach called "Non-modal Stability Analysis" to study the growth of infinitesimal perturbations over the dissolution process. Previous techniques required the geometric structure of the initial perturbations and used arbitrarily chosen initial conditions as an input. In contrast, Non-modal stability theory is a powerful mathematical tool that can predict the maximum amplification over the entire set of initial perturbations. Scientists obtained, for the first time, the structure of the most-amplified initial condition. They also developed a three-dimensional pseudo-spectral solver for the governing equations to verify the results of the analysis. In the figure below the amplification computed from the spectral solver is compared to that predicted by the stability theory. The growth of perturbations is well predicted by non-modal theory as long as the non-linear terms remain weak. After a certain amount of magnification, the non-linear terms, which were ignored in the non-modal analysis, become dominant. The system transitions into a convective state. Then convection begins to dominate over the slow diffusive process. Non-modal stability calculations are extremely fast compared to full simulations because the computation only involves solving ordinary differential equations and computing singular values of matrices. This approach can be used in conjunction with a "threshold" model to predict the onset of convection. In the plot, convection is expected to begin once the perturbations have amplified by a factor of 1000. The time at which the perturbations saturate (and the process of convection begins) is strongly related to the strength of the initial perturbations.

Comparison of the amplification of perturbations computed from non-modal theory (green)
and spectral simulations (orange) as a function of non-dimensional time.

This theory also predicts the length scale of the early finger-like structures as shown below. A more rigorous approach would only need the initial strength of perturbations, which could be obtained from suitably designed experiments. Reference: S. Rapaka, S. Chen, R.J. Pawar, P.H. Stauffer and D. Zhang, "Non-modal Growth of Perturbations in Density-driven Convection in Porous Media," Journal of Fluid Mechanics 609, 285-303 (2008). DOE Zero Emissions Research Technology (ZERT) funded the research.

Isosurfaces of the concentration of carbon dioxide for a simulation show the growth
of "fingering" convection pattern from the spectral simulations as a function of time.
The isosurfaces are plotted at (a) t = 1.59 x 10^-3, (b) t = 2 x 10^-3, (c) t = 3 x 10^-3 & (d) t = 4 x 10^-3.

Students Selected for the DOE Science and Energy Research Challenge

LANL students were selected to participate in the SERCh, to be held November 9-10, at ORNL. The DOE Office of Science sponsors the program and covers all travel expenses for the student and adviser. This prestigious and rigorous National poster competition is open to all undergraduate students who conducted DOE-related science research. Twelve of the 100 students DOE selected are from LANL. The Challenge includes a poster competition in six main categories: life science, energy, computational science, engineering, environmental science, and physical science. The students will compete for scholarships in each category in addition to a single, overall grand prize scholarship of $10,000. LANL SERCh participants are listed below.

Joseph Koby's (EES-17) experimental work with mentor Jim TenCate was to look at repeated stress-strain curves of various sandstones and limestones and study the time and rate dependence of the hysteresis loops, essentially observing and quantifying an effect known since the 1940s called elastic aftereffect (analogous to magnetic after-effect) for typical oil and gas bearing rocks. Joseph's work is included in a Geophysical Research Letter to be submitted by the end of this month. The work was supported by an LDRD-ER with TenCate co-PIs with Donatella Pasqualini (EES-16) and Salman Habib (T-2).

Other Archived Highlights