Probabilistic analysis and comparison of stress-dependent rock physics models

A rock physics model attempts to account for the nonlinear stress dependence of seismic velocity by relating changes in stress and strain to changes in seismic velocity and anisotropy. Understanding and being able to model this relationship is crucial for any time-lapse geophysical or geohazard modelling scenario. In this study, we take a number of commonly used rock physics models and assess their behaviour and stability when applied to stress versus velocity measurements of a large (dry) core data set of different lithologies. We invert and calibrate each model and present a database of models for over 400 core samples. The results of which provide a useful tool for setting a priori parameter constraints for future model inversions. We observe that some models assume an increase in VP/VS ratio (hence Poisson’s ratio) with stress. A trait not seen for every sample in our data set. We demonstrate that most model parameters are well constrained. However, third-order elasticity models become ill-posed when their equations are simplified for an isotropic rock. We also find that third-order elasticity models are limited by their approximation of an exponential relationship via functions that lack an exponential term. We also argue that all models are difficult to parametrize without the availability of core data. Therefore, we derive simple relationships between model parameters, core porosity and clay content. We observe that these relationship are suitable for estimating seismic velocities of rock but poor when comes to predicting changes related to effective stress. The findings of this study emphasize the need for improvement to models if quantitatively accurate predictions of time-lapse velocity and anisotropy are to be made. Certain models appear to better fit velocity depth log data than velocity–stress core data. Thus, there is evidence to suggest a limitation in core data as a representation of the stress dependence of the subsurface. The differences in the stress dependence of the subsurface compared to that measured under laboratory conditions could potentially be significant. Although potentially difficult to investigate, its importance is of great significance if we wish to accurately interpret the stress dependence of subsurface seismic velocities

Pleistocene uplift and palaeoenvironments of Macquarie Island: evidence from palaeobeaches and sedimentary deposits

Macquarie Island (54°30'S, 159°00'E) is an emergent part of the Macquarie Ridge Complex composed of ocean-floor rocks of Miocene age now 4000 m above the ocean floor. A number of landforms, including palaeobeaches now above sea level (a.s.l.)on Macquarie Island, were formed by marine erosion during uplift of the island. During the last Pleistocene period of low sea level (c. 20 ka) the island was three times larger than now. Thermoluminescence (TL) dating of two palaeobeaches indicates Pleistocene ages: 172 ± 40 ka for one at 100 m a.s.l. and 340 ± 80 ka for another at 263 m a.s.l. Matching the altitude sequence of palaeobeaches on Macquarie Island with the pattern of peaks in world sea level determined from deep sea cores allows an independent estimate of beach ages. Comparison of the altitude and sea level sequences most plausibly places the 100 m palaeobeach in Oxygen Isotope Stage 5e (130-125 ka) and the 263 m palaeobeach in Stage 9 (340-330 ka), matching reasonably with the TL dates. Other palaeobeaches at about 50 m and 170-190 m a.s.l. then correlate with high sea levels. We calculate an average rate of uplift forthe island of 0.8 mma-I . At this rate, 4000 m of Macquarie Ridge uplift would have taken about five million years and the top of the island may first have emerged some 700 to 600 ka. During the six Pleistocene glacial-interglacial cycles since then, there has been periglacial rather than glacial activity on cold uplands, but conditions suitable for vegetation of the present type persisted close to sea level

Proceedings of the 43rd Annual Meeting, Southern Soybean Disease Workers (March 9-10, 2016, Pensacola Beach, Florida)

Contents Schedule Southern United States Soybean Disease Loss Estimates for 2015. TW Allen, CA Bradley, JP Damicone, NS Dufault, TR Faske, CA Hollier, T Isakeit, RC Kemerait, NM Kleczewski, SR Koenning, HL Mehl, JD Mueller, C Overstreet, PP Price, EJ Sikora, TN Spurlock, and H Young Soilborne Disease Symposium (Tom Allen, moderator) Integrated management of sudden death syndrome. D Mueller, L Leandro, Y Kandel, C Bradley, M Chilvers, A Tenuta, and K Wise Field screening for stem canker: an outdated exercise or a necessary service for soybean growers? K Rowe and T Kirkpatrick Nematode associated diseases in soybean. C Overstreet, Edward C McGawley, D Xavier-Mis, and M Kularathna Characterization of taproot decline in southern soybean. M Tomaso-Peterson, T Allen, P Price, R Singh, and T Spurlock Frontline tactics to manage Phytophthora stem and root rot. A Robertson Graduate student research competition (Travis Faske and Terry Spurlock, moderators) Causative agents for the green stem disorder of soybeans in Louisiana. B Ward, C Robertson, and R Schneider Solubilization of cercosporin and its use for reproducing symptoms of Cercospora leaf blight of soybean. E Silva, M Liu, J Zhang, C Robertson, Z Liu, and R Schneider Effect of droplet size on foliar fungicide application in soybean. S Butler, H Kelly, T Mueller, and G Kruger Virulence assessment of strobilurin-sensitive and -resistant Cercospora sojina, the causal agent of frogeye leaf spot in soybean. N Brochard, M Tomaso-Peterson, T Allen, and R Melanson Application thresholds in controlling Cercospora sojina, the causal agent of frogeye leaf spot. J Jordan and H Kelly Assessment of ILeVO for management of root-knot nematodes in soybean. C Jackson, T Faske, M Emerson, and K Hurd Evaluating the physiological impacts of fungicide phytotoxicity in Mississippi soybean. J Mansour, M Tomaso-Peterson, A Henn., J Bond, T Irby, and T Allen Benefit of secondary nutrition in reducing Macrophomina phaseolina colonization in Mississippi soybean T. Wilkerson, M. Tomaso-Peterson. B Golden, S Lu, A Johnson, and T Allen Detection of a mycovirus from soybean rust and mycoviruses from other biotrophic fungi using a practical method for the extraction of viral dsRNA. R Herschlag, S Khankhum, and R Valverde Effect of Macrophomina phaseolina inoculation, irrigation and cultivar on soybean yield. M Zaccaron and J Rupe Contributed papers (Eduardo Silva, moderator) Phenotypic characterization of Cercospora sojina isolates collected from wide geographical areas. A Mengistu, J Ray, J Smith, and H Kelly Phenotypic characterization of Cercospora sojina isolates collected from wide geographical areas. A Mengistu, J Ray, J Smith, and H Kelly Effect of flower and pod removal on soybean senescence and comparison to green bean syndrome. J Rupe, B Holland, and A Steger Competition studies of QoI resistant and sensitive Cercospora sojina isolates, the causal agent of frogeye leaf spot. B Lin, H Kelly, H Yu, and A Mengistu Frequency and distribution of QoI resistant Cercospora sojina in Virginia. H Mehl and T Zhou A survey of Arkansas soybean nematodes, 2014-2015. K Sullivan, J Robinson, and T Kirkpatrick Nuclear proteins controlling soybean rust resistance B. Cooper Nuts, bolts, frogeye leaf spot, and the UUOT. T Allen, T Faske, C Hollier, P Price, T Spurlock, and H Young Proceedings of the Southern Soybean Disease Workers are published annually by the Southern Soybean Disease Workers. Text, references, figures, and tables are reproduced as they were submitted by authors. The opinions expressed by the participants at this conference are their own and do not necessarily represent those of the Southern Soybean Workers. Mention of a trademark or proprietary products in this publication does not constitute a guarantee, warranty, or endorsement of that product by the Southern Soybean Disease Workers

Cryptic variation in an ecological indicator organism: mitochondrial and nuclear DNA sequence data confirm distinct lineages of Baetis harrisoni Barnard (Ephemeroptera: Baetidae) in southern Africa

Baetis harrisoni Barnard is a mayfly frequently encountered in river studies across Africa, but the external morphological features used for identifying nymphs have been observed to vary subtly between different geographic locations. It has been associated with a wide range of ecological conditions, including pH extremes of pH 2.9–10.0 in polluted waters. We present a molecular study of the genetic variation within B. harrisoni across 21 rivers in its distribution range in southern Africa

Are oxygen and sulfur atoms structurally equivalent in organic crystals?

New guidelines for the design of structurally equivalent molecular crystals were derived from structural analyses of new cocrystals and polymorphs of saccharin and thiosaccharin, aided by a computational study. The study shows that isostructural crystals may be obtained through an exchange of >C?O with >C?S in the molecular components of the solids, but only if the exchanged atom is not involved in hydrogen bonding

Decreasing intensity of open-ocean convection in the Greenland and Iceland seas

The air–sea transfer of heat and fresh water plays a critical role in the global climate system. This is particularly true for the Greenland and Iceland seas, where these fluxes drive ocean convection that contributes to Denmark Strait overflow water, the densest component of the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). Here we show that the wintertime retreat of sea ice in the region, combined with different rates of warming for the atmosphere and sea surface of the Greenland and Iceland seas, has resulted in statistically significant reductions of approximately 20% in the magnitude of the winter air–sea heat fluxes since 1979. We also show that modes of climate variability other than the North Atlantic Oscillation (NAO) are required to fully characterize the regional air–sea interaction. Mixed-layer model simulations imply that further decreases in atmospheric forcing will exceed a threshold for the Greenland Sea whereby convection will become depth limited, reducing the ventilation of mid-depth waters in the Nordic seas. In the Iceland Sea, further reductions have the potential to decrease the supply of the densest overflow waters to the AMOC

The elastic constants of MgSiO3 perovskite at pressures and temperatures of the Earth's mantle

The temperature anomalies in the Earth's mantle associated with thermal convection1 can be inferred from seismic tomography, provided that the elastic properties of mantle minerals are known as a function of temperature at mantle pressures. At present, however, such information is difficult to obtain directly through laboratory experiments. We have therefore taken advantage of recent advances in computer technology, and have performed finite-temperature ab initio molecular dynamics simulations of the elastic properties of MgSiO3 perovskite, the major mineral of the lower mantle, at relevant thermodynamic conditions. When combined with the results from tomographic images of the mantle, our results indicate that the lower mantle is either significantly anelastic or compositionally heterogeneous on large scales. We found the temperature contrast between the coldest and hottest regions of the mantle, at a given depth, to be about 800K at 1000 km, 1500K at 2000 km, and possibly over 2000K at the core-mantle boundary.Comment: Published in: Nature 411, 934-937 (2001