Enhanced runout and erosion by overland flow at low pressure and sub-freezing conditions: Experiments and application to Mars     

Susan J. Conway  Michael P. Lamb  Martin C. Towner  John B. Murray 《Icarus》2011,211(1):443-457

We present the results of laboratory experiments to study the sediment transport and erosional capacity of water at current martian temperature and pressure. We have performed laboratory simulation experiments in which a stream of water flowed over test beds at low temperature (∼−20 °C) and low pressure (∼7 mbar). The slope angle was 14° and three sediment types were tested. We compared the erosive ability, runout and resulting morphologies to experiments performed at ambient terrestrial temperature (∼20 °C) and pressure (∼1000 mbar), and also to experiments performed under low pressure only. We observed that, as expected, water is unstable in the liquid phase at low temperature and low pressure, with boiling and freezing in competition. Despite this, our results show that water at low temperature and low pressure has an equivalent and sometimes greater erosion rate than at terrestrial temperature and pressure. Water flows faster over the sediment body under low temperature and low pressure conditions because the formation of ice below the liquid-sediment contact inhibits infiltration. Flow speed and therefore runout distance are increased. Experiments at low pressure but Earth-ambient temperature suggest that flow speeds are faster under these conditions than under Earth-ambient pressure and temperature. We hypothesise that this is due to gas bubbles, created by the boiling of the water under low atmospheric pressure, impeding liquid infiltration. We have found that both basal freezing and low pressure increase the flow propagation speed - effects not included in current models of fluvial activity on Mars. Any future modelling of water flows on Mars should consider this extra mobility and incorporate the large reduction in fluid loss through infiltration into the substrate.  相似文献   

Moisture Movement Through Cracked Clay Soil Profiles   总被引：1，自引：0，他引：1  

Delwyn G. Fredlund  Sandra L. Houston  Quan Nguyen  Murray D. Fredlund 《Geotechnical and Geological Engineering》2010,28(6):865-888

A continuum mechanics approach is used for the formulation of unsaturated hydraulic conductivity functions and the water storage functions for fractured or cracked clay soils in this parametric study. Suggested procedures are based on available research literature related to the behavior of cracked unsaturated porous media. The soil–water characteristic curve, hydraulic conductivity and water storage functions take on the character of bi-modal unsaturated soil property functions. The bimodal character arises out of the independent behavior of the cracks and the intact clay soil. Matric suction changes beneath a slab-on-grade foundation placed on a cracked clay soil profile are modeled for varied surface flux conditions using the proposed unsaturated hydraulic conductivity and water storage functions. The impact of various levels of surface cracking on soil suction distributions is discussed. Suction distribution patterns are dependent on the initial soil surface suction. In particular, the results are dependent upon whether the initial matric suction is less than or greater than the air entry of the cracked clay. There is an extremely wide range of possible conditions that could be modeled but the parametric study results presented in this paper are limited to a series of selected crack widths and densities for an exfiltration case and an infiltration case.  相似文献   

The effect of eccentricity and inclination on the motion near the Lagrangian points L4 and L5     

F. Namouni  C. D. Murray 《Celestial Mechanics and Dynamical Astronomy》2000,76(2):131-138

We study the effect of eccentricity and inclination on small amplitude librations around the equilibrium points L ₄ and L ₅ in the circular restricted three-body problem. We show that the effective libration centres can be displaced appreciably from the equilateral configuration. The secular extrema of the eccentricity as a function of the argument of pericentre are shifted by ∼25 ° This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

The outbursts of dwarf novae     

M. R. Truss  J. R. Murray  G. A. Wynn  R. G. Edgar 《Monthly notices of the Royal Astronomical Society》2000,319(2):467-476

We present a numerical scheme for the evolution of an accretion disc through a dwarf nova outburst. We introduce a time-varying artificial viscosity into an existing smoothed particle hydrodynamics code optimized for two- and three-dimensional simulations of accretion discs. The technique gives rise to coherent outbursts and can easily be adapted to include a complete treatment of thermodynamics. We apply a two-dimensional isothermal scheme to the system SS Cygni, and present a wide range of observationally testable results.  相似文献   

Accurate laboratory wavelengths of some ultraviolet lines of Cr, Zn and Ni relevant to time variations of the fine structure constant     

J. C. Pickering  A. P. Thorne  J. E. Murray  U. Litzén  S. Johansson  V. Zilio  J. K. Webb 《Monthly notices of the Royal Astronomical Society》2000,319(1):163-167

The quality of astronomical spectroscopic data now available is so high that interpretation and analysis are often limited by the uncertainties of the laboratory data base. In particular, the limit with which space–time variations in the fine structure constant α can be constrained using quasar spectra depends on the availability of more accurate laboratory rest wavelengths. We recently measured some transitions in magnesium by high-resolution Fourier transform spectroscopy for this purpose, and we now report measurements on some ultraviolet resonance lines of Zn  ii (2062 and 2026 Å), Cr  ii (2066, 2062 and 2056 Å) and Ni  ii (1751, 1741, 1709 and 1703 Å). Apart from the last line, which is very weak, the uncertainty of these measurements is 0.002 cm⁻¹ (0.08 må) for the lines around 2000 Å and 0.004 cm⁻¹ (0.12 må) for the lines around 1700 Å.  相似文献   

Hydrocarbon‐Rich Groundwater above Shale‐Gas Formations: A Karoo Basin Case Study          下载免费PDF全文

William K. Eymold  Kelley Swana  Myles T. Moore  Colin J. Whyte  Jennifer S. Harkness  Siep Talma  Ricky Murray  Joachim B. Moortgat  Jodie Miller  Avner Vengosh  Thomas H. Darrah 《Ground water》2018,56(2):204-224

Horizontal drilling and hydraulic fracturing have enhanced unconventional hydrocarbon recovery but raised environmental concerns related to water quality. Because most basins targeted for shale‐gas development in the USA have histories of both active and legacy petroleum extraction, confusion about the hydrogeological context of naturally occurring methane in shallow aquifers overlying shales remains. The Karoo Basin, located in South Africa, provides a near‐pristine setting to evaluate these processes, without a history of conventional or unconventional energy extraction. We conducted a comprehensive pre‐industrial evaluation of water quality and gas geochemistry in 22 groundwater samples across the Karoo Basin, including dissolved ions, water isotopes, hydrocarbon molecular and isotopic composition, and noble gases. Methane‐rich samples were associated with high‐salinity, NaCl‐type groundwater and elevated levels of ethane, ⁴He, and other noble gases produced by radioactive decay. This endmember displayed less negative δ¹³C‐CH₄ and evidence of mixing between thermogenic natural gases and hydrogenotrophic methane. Atmospheric noble gases in the methane‐rich samples record a history of fractionation during gas‐phase migration from source rocks to shallow aquifers. Conversely, methane‐poor samples have a paucity of ethane and ⁴He, near saturation levels of atmospheric noble gases, and more negative δ¹³C‐CH₄; methane in these samples is biogenic and produced by a mixture of hydrogenotrophic and acetoclastic sources. These geochemical observations are consistent with other basins targeted for unconventional energy extraction in the USA and contribute to a growing data base of naturally occurring methane in shallow aquifers globally, which provide a framework for evaluating environmental concerns related to unconventional energy development (e.g., stray gas).  相似文献   

Marine vibrators: the new phase of seismic exploration     

R.M. Laws  D. Halliday  J.-F. Hopperstad  D. Gerez  M. Supawala  A. Özbek  T. Murray  E. Kragh 《Geophysical Prospecting》2019,67(6):1443-1471

Marine seismic vibrators are generally considered to be less intrusive than airguns from an environmental perspective. This is because they emit their energy spread out in time, rather than in a single, high-intensity pulse. There are also significant geophysical benefits associated with marine vibrators, and they stem from the ability to specify in detail the output acoustic waveform. The phase can be specified independently at each frequency. Such detailed control cannot be achieved with conventional airgun sources, where the phase can only be modified using simple overall time delays. The vibrator phase can be employed in several different ways: it can be applied to the overall source phase in a sequence so that it varies from one source point to the next; it can be applied to the individual vibrators within the source array so the source directivity is changed; it can be applied to the overall source phase of each source in a simultaneous source acquisition. Carefully designed phase sequences can attenuate the residual source noise, and this in turn allows extra source points to be interleaved between the conventional ones. For these extra source points, the relative phase of the vibrators within the array can be chosen to create a transverse gradient source, which illuminates the earth predominantly in directions out of the plane of the sail line without left/right ambiguity. If seismic vibrator data are acquired using interleaved conventional and transverse gradient sweeps, more information is collected per kilometre of vessel travel than is the case in conventional acquisition. This richer data acquisition leads to the possibility of acquiring all the necessary seismic data in a shorter time. Three-dimensional reconstruction techniques are used to recover the same image quality that would have been obtained using the conventional, more time-consuming acquisition. For a marine vibrator to be suitable for these techniques it must, in general terms, have ‘high fidelity’. The precise device specifications are defined through realistic end-to-end simulations of the physical systems and the processing. The specifications are somewhat more onerous than for a conventional vibrator, but they are achievable. A prototype vibrator that satisfies these requirements has been built. In a simulated case study of a three-dimensional deep-water ocean bottom node survey, the seismic data could have been acquired using marine vibrators in one third of the time that it would have taken using airguns.  相似文献   

Outflows at the Edges of an Active Region in a Coronal Hole: A Signature of Active Region Expansion?     

M. J. Murray  D. Baker  L. van Driel-Gesztelyi  J. Sun 《Solar physics》2010,261(2):253-269

Outflows of plasma at the edges of active regions surrounded by quiet Sun are now a common observation with the Hinode satellite. While there is observational evidence to suggest that the outflows are originating in the magnetic field surrounding the active regions, there is no conclusive evidence that reveals how they are driven. Motivated by observations of outflows at the periphery of a mature active region embedded in a coronal hole, we have used a three-dimensional simulation to emulate the active region’s development in order to investigate the origin and driver of these outflows. We find that outflows are accelerated from a site in the coronal hole magnetic field immediately surrounding the active region and are channelled along the coronal hole field as they rise through the atmosphere. The plasma is accelerated simply as a result of the active region expanding horizontally as it develops. Many of the characteristics of the outflows generated in the simulation are consistent with those of observed outflows: velocities up to 45 km s⁻¹, properties akin to the coronal hole, proximity to the active region’s draining loops, expansion with height, and projection over monopolar photospheric magnetic concentrations. Although the horizontal expansion occurs as a consequence of the active region’s development in the simulation, expansion is also a general feature of established active regions. Hence, it is entirely possible and plausible that the expansion acceleration mechanism displayed in the simulation is occurring in active regions on the Sun and, in addition to reconnection, is driving the outflows observed at their edges.  相似文献