Measuring intergranular force in granular media

A new method is proposed to measure intergranular forces in granular geomaterial from time-lapsehigh-resolution X-ray computed tomographyimaging using a grain trackingapproachand discrete element metho

Permeability of Bituminous Coal to CH4 and CO2 Under Fixed Volume and Fixed Stress Boundary Conditions: Effects of Sorption

Permeability evolution in coal reservoirs during CO2-enhanced coalbed methane (ECBM) production is strongly influenced by swelling/shrinkage effects related to sorption and desorption of CO2 and CH4, respectively. Recent research has demonstrated fully coupled stress–strain–sorption–diffusion behavior in small samples of cleat-free coal matrix material exposed to a sorbing gas. However, it is unclear how such effects influence permeability evolution at the scale of a cleated coal seam and whether a simple fracture permeability model, such as the Walsh elastic asperity loading model, is appropriate. In this study, we performed steady-state permeability measurements, to CH4 and CO2, on a cylindrical sample of highly volatile bituminous coal (25 mm in diameter) with a clearly visible cleat system, under (near) fixed volume versus fixed stress conditions. To isolate the effect of sorption on permeability evolution, helium (non-sorbing gas) was used as a control fluid. All flow-through tests reported here were conducted under conditions of single-phase flow at 40°C, at applied Terzaghi effective confining pressures of 14–41 MPa. Permeability evolution versus effective stress data were obtained under both fixed volume and fixed stress boundary conditions, showing an exponential correlation. Importantly, permeability ((Formula presented.)) obtained at similar Terzaghi effective confining pressures showed (Formula presented.) > (Formula presented.) >> (Formula presented.), while (Formula presented.) -values measured in the fixed volume condition were higher than those in the fixed stress case. The results show that permeability to CH4 and CO2, under in situ conditions where free swelling of rock is not possible, is strongly influenced by the coupled effects of 1) self-stress generated by constrained swelling, 2) the change in effective stress coefficient upon sorption, 3) sorption-induced closure of transport paths independently of poroelastic effect, and 4) heterogeneous gas penetration and equilibration, dependent on diffusion. Our results also show that the Walsh permeability model offers a promising basis for relating permeability evolution to in situ stress evolution, using appropriate parameter values corrected for the effects of stress–strain–sorption

Goal-seeking compresses neural codes for space in the human hippocampus and orbitofrontal cortex

Humans can navigate flexibly to meet their goals. Here, we asked how the neural representation of allocentric space is distorted by goal-directed behavior. Participants navigated an agent to two successive goal locations in a grid world environment comprising four interlinked rooms, with a contextual cue indicating the conditional dependence of one goal location on another. Examining the neural geometry by which room and context were encoded in fMRI signals, we found that map-like representations of the environment emerged in both hippocampus and neocortex. Cognitive maps in hippocampus and orbitofrontal cortices were compressed so that locations cued as goals were coded together in neural state space, and these distortions predicted successful learning. This effect was captured by a computational model in which current and prospective locations are jointly encoded in a place code, providing a theory of how goals warp the neural representation of space in macroscopic neural signals

Goal-seeking compresses neural codes for space in the human hippocampus and orbitofrontal cortex

Humans can navigate flexibly to meet their goals. Here, we asked how the neural representation of allocentric space is distorted by goal-directed behavior. Participants navigated an agent to two successive goal locations in a grid world environment comprising four interlinked rooms, with a contextual cue indicating the conditional dependence of one goal location on another. Examining the neural geometry by which room and context were encoded in fMRI signals, we found that map-like representations of the environment emerged in both hippocampus and neocortex. Cognitive maps in hippocampus and orbitofrontal cortices were compressed so that locations cued as goals were coded together in neural state space, and these distortions predicted successful learning. This effect was captured by a computational model in which current and prospective locations are jointly encoded in a place code, providing a theory of how goals warp the neural representation of space in macroscopic neural signals.</p

Fault Weakening During Short Seismic Slip Pulse Experiments: The Role of Pressurized Water and Implications for Induced Earthquakes in the Groningen Gas Field

High-velocity friction experiments on simulated fault gouges sheared at high normal stress and to low displacement are particularly relevant to induced seismicity, which is becoming an important topic in fault mechanics. Using a new, improved set-up, which allows simulation of fault stress and fluid pressure ((Formula presented.)) conditions approaching in-situ reservoir values, we performed ring-shear experiments on simulated fault gouges prepared from the source-, reservoir-, and caprock-formations of the Groningen gas field. Pre-sheared gouges were subjected to a rotational slip pulse reaching ∼1.0 m/s peak velocity and 13–16 cm total displacement at effective normal stresses ((Formula presented.)) of 5–31 MPa and (Formula presented.) up to 5 MPa, using water or dry nitrogen as pore fluid. All water-saturated gouges show strong dynamic weakening within a few cm of slip, with the lowest dynamic friction (0.2–0.4) measured at the highest (Formula presented.). By contrast, the weakening was subtle in experiments using nitrogen. Our analyses focus on the high- (Formula presented.) experiments, which are more realistic and show a distinct dependence of constitutive parameters (e.g., slip-weakening rate) on (Formula presented.), in the form of empirical linear, power-law or exponential relations. The results provide much-needed constraints for numerical modeling of induced rupture propagation in the Groningen field. Based on temperature- and (Formula presented.) -measurements made in near-direct contact with the active shear band, and using “post-mortem” microstructures, we exclude previously-proposed dynamic weakening mechanisms (e.g., flash heating or thermal pressurization) and suggest that water pressurization at heated asperity or grain contacts explains the weakening seen in our high- (Formula presented.) experiments

Hydrothermal Friction Experiments on Simulated Basaltic Fault Gouge and Implications for Megathrust Earthquakes

Nucleation of earthquake slip at the plate boundary fault (décollement) in subduction zones has been widely linked to the frictional properties of subducting sedimentary facies. However, recent seismological and geological observations suggest that the décollement develops in the subducting oceanic crust in the depth range of the seismogenic zone, at least in some cases. To understand the frictional properties of oceanic crustal material and their influence on seismogenesis, we performed hydrothermal friction experiments on simulated fault gouges of altered basalt, at temperatures of 100–550°C. The friction coefficient (μ) lies around 0.6 at most temperature conditions but a low μ down to 0.3 was observed at the highest temperature and lowest velocity condition. The velocity dependence of μ, (a−b), changes with increasing temperature from positive to negative at ∼100°C and from negative to positive at ∼450°C. Compared to gouges derived from sedimentary facies, the altered basalt gouge showed potentially unstable velocity weakening over a wider temperature range. Microstructural observations and microphysical interpretation infer that competition between dilatant granular flow and viscous compaction through pressure-solution creep of albite contributed to the observed transition in (a−b). Alteration of oceanic crust during subduction produces fine grains of albite and chlorite through interactions with interstitial water, leading to reduction in its frictional strength and an increase in its seismogenic potential. Therefore, shear deformation possibly localizes within the altered oceanic crust leading to a larger potential for the nucleation of a megathrust earthquake in the depth range of the seismogenic zone

An Intelligent Propulsion Control Architecture to Enable More Autonomous Vehicle Operation

This paper describes an intelligent propulsion control architecture that coordinates with the flight control to reduce the amount of pilot intervention required to operate the vehicle. Objectives of the architecture include the ability to: automatically recognize the aircraft operating state and flight phase; configure engine control to optimize performance with knowledge of engine condition and capability; enhance aircraft performance by coordinating propulsion control with flight control; and recognize off-nominal propulsion situations and to respond to them autonomously. The hierarchical intelligent propulsion system control can be decomposed into a propulsion system level and an individual engine level. The architecture is designed to be flexible to accommodate evolving requirements, adapt to technology improvements, and maintain safety

An Intelligent Propulsion Control Architecture to Enable More Autonomous Vehicle Operation

This paper describes an intelligent propulsion control architecture that coordinates with the flight control to reduce the amount of pilot intervention required to operate the vehicle. Objectives of the architecture include the ability to: automatically recognize the aircraft operating state and flight phase; configure engine control to optimize performance with knowledge of engine condition and capability; enhance aircraft performance by coordinating propulsion control with flight control; and recognize off-nominal propulsion situations and to respond to them autonomously. The hierarchical intelligent propulsion system control can be decomposed into a propulsion system level and an individual engine level. The architecture is designed to be flexible to accommodate evolving requirements, adapt to technology improvements, and maintain safety