Quantifying Rock Fracture Compliance from Elastic Wave Velocities

The purpose of this study is to test the reliability of various methods to quantify fracture compliance with elastic wave measurements. Fracture compliance is the inverse of fracture stiffness and controls many characteristics of fractures that are important for geoengineering, e.g. strength, seismic visibility and hydraulic properties. We present ultrasonic through-transmission laboratory measurements for the compliance of smooth fractures in Westerly Granite samples that were exposed to a range of uniaxial loading pressures. The influence of sample width and source transducer on the measurements are constrained with numerical discrete lattice simulations. The results of this study confirm a recently established unique relationship between phase delay and fracture compliance for fracture systems that obey linear-slip theory. We suggest that this confirmation opens the potential for a wider application of time delay based compliance quantification that was previously limited by a non-unique relationship. In some circumstances precise phase delay measurements can be difficult to achieve. We show that in such cases employing first break arrival time measurements in conjunction with numerical simulations are an effective alternative. The application of the proposed method to multiply fractured media and to larger scales at sonic and seismic frequencies is also considered

Fast atomic charge calculation for implementation into a polarizable force field and application to an ion channel protein

Polarization of atoms plays a substantial role in molecular interactions. Class I and II force fields mostly calculate with fixed atomic charges which can cause inadequate descriptions for highly charged molecules, for example, ion channels or metalloproteins. Changes in charge distributions can be included into molecular mechanics calculations by various methods. Here, we present a very fast computational quantum mechanical method, the Bond Polarization Theory (BPT). Atomic charges are obtained via a charge calculation method that depend on the 3D structure of the system in a similar way as atomic charges of ab initio calculations. Different methods of population analysis and charge calculation methods and their dependence on the basis set were investigated. A refined parameterization yielded excellent correlation of R=0.9967. The method was implemented in the force field COSMOS-NMR and applied to the histidine-tryptophan-complex of the transmembrane domain of the M2 protein channel of influenza A virus. Our calculations show that moderate changes of side chain torsion angle χ1 and small variations of χ2 of Trp-41 are necessary to switch from the inactivated into the activated state; and a rough two-side jump model of His-37 is supported for proton gating in accordance with a flipping mechanism

An investigation of the controls on Irish precipitation δ18O values on monthly and event timescales.

This two-year study investigates the relative influence of meteorological variables (precipitation amount and temperature), atmospheric circulation, air mass history, and moisture source region on Irish precipitation oxygen isotopes (δ18Op) on event and monthly timescales. Single predictor correlations reveal that on the event scale, 20% of δ18Op variability is attributable to the amount effect and 7% to the temperature effect while on the monthly timescale the North Atlantic Oscillation accounts for up to 20% of δ18Op variability and the amount and temperature effects are not significant. In comparison, multivariate linear regression reveals that the interaction of temperature and precipitation amount explains up to 40% of δ18Op variance at event and monthly timescales. Five-day kinematic back trajectories suggest that the amount-weighted mean δ18Op value of southerly- and northerly-derived events are lower by 2‰ relative to events derived from the west. Because air mass history and atmospheric circulation appear to influence δ18Op in Ireland, Irish paleo-δ18Op proxy records are best interpreted as reflecting a combination of parameters, not just paleotemperature or paleorainfall