scholarly journals Effective coefficient of diffusion and permeability of firn at Dome C and Lock In, Antarctica – Estimates over the 100–850 kg m<sup>−3</sup> density range

2021 ◽  
Author(s):  
Neige Calonne ◽  
Alexis Burr ◽  
Armelle Philip ◽  
Frédéric Flin ◽  
Christian Geindreau
Keyword(s):  
Diffusion Coefficient ◽  
Analytical Models ◽  
Density Range ◽  
Effective Coefficient ◽  
Structural Anisotropy ◽  
Near Surface ◽  
Tomographic Images ◽  
Porosity Ratio ◽  
Lock In ◽  
Coefficient Of Diffusion

Abstract. Modeling air transport through the entire ice sheet column is needed to interpret climate archives. To this end, different regressions were proposed to estimate the effective coefficient of diffusion and permeability of firn. Such regressions are often valid for specific depth or porosity ranges and were little evaluated as data of these properties are scarce. To contribute with a new dataset, this study presents the effective coefficient of diffusion and the permeability at Dome C and Lock In, Antarctica, from the near-surface to the close-off (23 to 133 m depth). Also, microstructure is characterized based on density, specific surface area, closed porosity ratio, connectivity index and structural anisotropy through the correlation lengths. All properties were estimated based on pore-scale computations on 3D tomographic images of firn samples. Normalized diffusion coefficient ranges from 1.9 × 10−1 to 8.3 × 10−5 and permeability ranges from 1.2 × 10−9 to 1.1 × 10−12 m2, for densities between 565 and 888 kg m−3. No or little anisotropy is reported. Next, we investigate the relationship of the transport properties with density over the firn density range as well as over the entire density range encountered in ice sheets by including snow data. Classical analytical models and regressions from literature are evaluated. For firn (550–850 kg m−3), good agreements are found for permeability and diffusion coefficient with the regressions based on the open porosity of Freitag et al. (2002) and Adolph and Albert (2014), despite the rather different site conditions (Greenland). Over the entire 100–850 kg m−3 density range, permeability is accurately reproduced by the Carman-Kozeny and Self-Consistent (spherical bi-composite) model when expressed in terms of a rescaled porosity ϕres = (ϕ–ϕoff) / (1–ϕoff) to account for pore closure, with ϕoff the close-off porosity. For the normalized diffusion coefficient, none of the evaluated formulas were satisfactory so we propose a new regression based on the rescaled porosity that reads D/Dair = (ϕres)1.61.

scholarly journals Technical Note: Simple formulations and solutions of the dual-phase diffusive transport for biogeochemical modeling

2014 ◽  
Vol 11 (14) ◽  
pp. 3721-3728 ◽  
Author(s):  
J. Y. Tang ◽  
W. J. Riley
Keyword(s):  
Steady State ◽  
Tracer Diffusion ◽  
Technical Note ◽  
Analytical Models ◽  
Dual Phase ◽  
Near Surface ◽  
Solution Algorithms ◽  
Biogeochemical Models ◽  
Volume Approximation ◽  
Multi Phase

Abstract. Representation of gaseous diffusion in variably saturated near-surface soils is becoming more common in land biogeochemical models, yet the formulations and numerical solution algorithms applied vary widely. We present three different but equivalent formulations of the dual-phase (gaseous and aqueous) tracer diffusion transport problem that is relevant to a wide class of volatile tracers in land biogeochemical models. Of these three formulations (i.e., the gas-primary, aqueous-primary, and bulk-tracer-based formulations), we contend that the gas-primary formulation is the most convenient for modeling tracer dynamics in biogeochemical models. We then provide finite volume approximation to the gas-primary equation and evaluate its accuracy against three analytical models: one for steady-state soil CO2 dynamics, one for steady-state soil CH4 dynamics, and one for transient tracer diffusion from a constant point source into two different sequentially aligned medias. All evaluations demonstrated good accuracy of the numerical approximation. We expect our result will standardize an efficient mechanistic numerical method for solving relatively simple, multi-phase, one-dimensional diffusion problems in land models.

Crosscoherence-based interferometry for the retrieval of first arrivals and subsequent tomographic imaging of differential weathering

Geophysics ◽  
2019 ◽  
Vol 84 (4) ◽  
pp. Q37-Q48 ◽  
Author(s):  
Joachim Place ◽  
Deyan Draganov ◽  
Alireza Malehmir ◽  
Christopher Juhlin ◽  
Chris Wijns
Keyword(s):  
Seismic Waves ◽  
Seismic Velocity ◽  
Signal To Noise Ratio ◽  
Seismic Profile ◽  
Seismic Interferometry ◽  
Near Surface ◽  
Tomographic Images ◽  
Traveltime Tomography ◽  
Seismic Methods ◽  
Source Signal

Exhumation of crust exposes rocks to weathering agents that weaken the rocks’ mechanical strength. Weakened rocks will have lower seismic velocity than intact rocks and can therefore be mapped using seismic methods. However, if the rocks are heavily weathered, they will attenuate controlled-source seismic waves to such a degree that the recorded wavefield would become dominated by ambient noise and/or surface waves. Therefore, we have examined the structure of differential weathering by first-break traveltime tomography over a seismic profile extending approximately 3.5 km and acquired at a mining site in Zambia using explosive sources and a source based on the swept-impact seismic technique (SIST). Seismic interferometry has been tested for the retrieval of supervirtual first arrivals masked by uncorrelated noise. However, use of crosscorrelation in the retrieval process makes the method vulnerable to changes in the source signal (explosives and SIST). Thus, we have developed a crosscoherence-based seismic-interferometry method to tackle this shortcoming. We investigate the method’s efficiency in retrieving first arrivals and, simultaneously, correctly handling variations in the source signal. Our results illustrate the superiority of the crosscoherence- over crosscorrelation-based method for retrieval of the first arrivals, especially in alleviating spurious ringyness and in terms of the signal-to-noise ratio. These benefits are observable in the greater penetration depth and the improved resolution of the tomography sections. The tomographic images indicate isolated bodies of higher velocities, which may be interpreted as fresh rocks embedded into a heavily weathered regolith, providing a conspicuous example of differential weathering. Our study advances the potential of seismic methods for providing better images of the near surface (the critical zone).

Evaluation of Uncertainties of One-Directional Analytical Model for Thermoreflectance Technique

2011 ◽  
Author(s):  
Changhu Xing ◽  
Zilong Hua ◽  
Heng Ban ◽  
David Hurley ◽  
J. Rory Kennedy
Keyword(s):  
Parametric Design ◽  
Thermal Effusivity ◽  
Analytical Models ◽  
Volumetric Heating ◽  
One Dimensional ◽  
Parametric Studies ◽  
Commercial Software Package ◽  
The Difference ◽  
Lock In ◽  
General Guidance

One dimensional (1-D) analytical models are generally used for the evaluation of thermal effusivity of film or substrate in thermoreflectance measurement. However, the range of uncertainties associated with the 1-D assumptions needs to be quantified in order to determine the applicability of 1-D models. In the current study, a two-dimensional (2-D) numerical model was created in a commercial software package, COMSOL, to investigate the difference of results obtained by 1-D or 2-D models. The analysis used a frequency lock-in strategy and the result was verified by a comparison of 1-D numerical simulation to 1-D theoretical values. Parametric studies were performed by considering film thickness, heating laser radius, volumetric heating, thermal resistance between layers, and anisotropic thermal conductivities. The results and conclusions provide a general guidance and reference for the parametric design of thermoreflectance measurements.

Seismoelectric imaging of shallow targets

Geophysics ◽  
2007 ◽  
Vol 72 (2) ◽  
pp. G9-G20 ◽  
Author(s):  
Seth S. Haines ◽  
Steven R. Pride ◽  
Simon L. Klemperer ◽  
Biondo Biondi
Keyword(s):  
Electric Fields ◽  
Field Experiments ◽  
Electrode Array ◽  
Seismic Source ◽  
Analytical Models ◽  
Synthetic Image ◽  
Response Signal ◽  
Line Geometry ◽  
Near Surface ◽  
Promising Application

We have undertaken a series of controlled field experiments to develop seismoelectric experimental methods for near-surface applications and to improve our understanding of seismoelectric phenomena. In a set of off-line geometry surveys (source separated from the receiver line), we place seismic sources and electrode array receivers on opposite sides of a man-made target (two sand-filled trenches) to record separately two previously documented seismoelectric modes: (1) the electromagnetic interface response signal created at the target and (2) the coseismic electric fields located within a compressional seismic wave. With the seismic source point in the center of a linear electrode array, we identify the previously undocumented seismoelectric direct field, and the Lorentz field of the metal hammer plate moving in the earth’s magnetic field. We place the seismic source in the center of a circular array of electrodes (radial and circumferential orientations) to analyze the source-related direct and Lorentz fields and to establish that these fields can be understood in terms of simple analytical models. Using an off-line geometry, we create a multifold, 2D image of our trenches as dipping layers, and we also produce a complementary synthetic image through numerical modeling. These images demonstrate that off-line geometry (e.g., crosswell) surveys offer a particularly promising application of the seismoelectric method because they effectively separate the interface response signal from the (generally much stronger) coseismic and source-related fields.

scholarly journals Analysis of Sensitive Parameters Affecting Unlocking Force of Finger Lock in Landing Gear

2021 ◽  
Vol 2021 ◽  
pp. 1-19
Author(s):  
Yu Hou ◽  
Ming Zhang ◽  
Hong Nie
Keyword(s):  
Mechanical Characteristics ◽  
Landing Gear ◽  
Effective Coefficient ◽  
Wear Model ◽  
Obvious Effect ◽  
Working Cycle ◽  
Inner Surface ◽  
Surface Diameter ◽  
Lock In ◽  
Sensitive Parameters

The mechanical characteristics of the unlocking force of the landing gear finger lock were studied in this paper, the influence of its diameter, fingertip angle, wear, and other factors on the unlocking force in one complete working cycle was analyzed, and the sensitive parameters that affect the unlocking force were obtained. Firstly, the unlocking force and wear of finger lock were calculated theoretically, and the changing rule of the unlocking force and wear with each parameter was obtained. Then, the validity of the correlation coefficient and model was verified by experiment. Finally, combined with the effective coefficient obtained from the experiment, the Archard wear model was used to simulate the change rule of lock force. The results show that in one complete working cycle, the inner surface diameter is negatively related to the unlocking force, fingertip diameter has little effect on the unlocking force, fingertip angle is negatively related to the unlocking force, and wear is positively related to the unlocking force; friction coefficient and fingertip angle are high sensitive parameters that affect the unlocking force, which have obvious effect on the unlocking force. The inner surface diameter, fingertip diameter of finger lock, and wear are the low sensitive parameters that affect the unlocking force, and the influence on the unlocking force is weak.

scholarly journals Pore morphology of polar firn around closure revealed by X-ray tomography

2018 ◽  
Author(s):  
Alexis Burr ◽  
Clément Ballot ◽  
Pierre Lhuissier ◽  
Patricia Martinerie ◽  
Christophe L. Martin ◽  
...  
Keyword(s):  
Sample Size ◽  
Morphological Evolution ◽  
Total Porosity ◽  
Pore Network ◽  
Geometrical Parameters ◽  
Age Difference ◽  
X Ray ◽  
Porosity Ratio ◽  
Pore Closure ◽  
Lock In

Abstract. Understanding the slow densification process of polar firn into ice is essential in order to constrain the age difference between the ice matrix and entrapped gases. The progressive microstructure evolution of the firn column with depth leads to pore closure and gas entrapment. Air transport models in the firn usually include a closed porosity profile based on available data. Pycnometry or melting-refreezing techniques have been used to obtain the ratio of closed to total porosity and air content in closed pores, respectively. X-ray computed tomography is complementary to these methods, as it enables to obtain the full pore network in 3D. This study takes advantage of this non-destructive technique to discuss the morphological evolution of pores on four different Antarctic sites. The computation of refined geometrical parameters for the very cold polar sites Dome C and Lock In (the two Antarctic plateau sites studied here) provides new information that could be used in further studies. The comparison of these two sites shows a more tortuous pore network at Lock In than at Dome C which should result in older gas ages in deep firn. A comprehensive estimation of the different errors related to X-ray tomography and to the sample variability has been performed. The procedure described here may be used as a guideline for further experimental characterization on firn samples. We show that the closed to total porosity ratio, which is classically used for the detection of pore closure, is strongly affected by the sample size, the image reconstruction and by spatial heterogeneities. In this work, we introduce an alternative parameter, the connectivity index, which is practically independent on sample size and image acquisition conditions, and that accurately predicts the close-off depth and density. Its strength also lies in its simple computation, without any assumption on the pore status (open or close). The close-off prediction is obtained for Dome C and Lock In, without any further numerical simulations on images (e.g. by permeability or diffusivity calculations).

scholarly journals Dispersion in deep polar firn driven by synoptic-scale surface pressure variability

2016 ◽  
Author(s):  
Christo Buizert ◽  
Jeffrey P. Severinghaus
Keyword(s):  
Surface Pressure ◽  
Propagation Model ◽  
Air Pressure ◽  
Air Transport ◽  
Synoptic Scale ◽  
Near Surface ◽  
Barometric Pumping ◽  
Lock In ◽  
Dispersive Mixing ◽  
Scale Surface

Abstract. Commonly, three mechanisms of firn air transport are distinguished: molecular diffusion, advection, and near-surface convective mixing. Here we identify and describe a fourth mechanism, namely dispersion driven by synoptic-scale surface pressure variability (or barometric pumping). We use published gas chromatography experiments on firn samples to derive the along-flow dispersivity of firn, and combine this dispersivity with a dynamical air pressure propagation model forced by surface air pressure time series to estimate the magnitude of dispersive mixing in the firn. We show that dispersion dominates mixing within the firn lock-in zone. Trace gas concentrations measured in firn air samples from various polar sites confirm that dispersive mixing occurs. Including dispersive mixing in a firn air transport model suggests that our theoretical estimates have the correct order of magnitude, yet may overestimate the true dispersion. We further show that strong barometric pumping, such as at the Law Dome site, may reduce the gravitational enrichment δ15N-N2 and other tracers below gravitational equilibrium, questioning the traditional definition of the lock-in depth as the depth where δ15N enrichment ceases. Last, we propose that 86Kr excess may act as a proxy for past synoptic activity (or paleo-storminess) at the site.

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