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Octubre de 2006
Simple modification to describe the soil water retention
curve between saturation and oven dryness
Authors: Muhammed Khlosi , Wim
M. Cornelis et al
Link: Click here
Abstract
Prediction of water and vapor flow in porous media requires
an accurate estimation of the soil water retention curve describing
the relation between matric potential and the respective soil
water content from saturation to oven dryness. In this study,
we modified the Kosugi (1999) function to represent soil water
retention at all matric potentials. This modification retains
the form of the original Kosugi function in the wet range
and transforms to an adsorption equation in the dry
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range. Following a systems
identification approach, the extended function was tested
against observed data taken from literature that cover the
complete range of water contents from saturation to almost
oven dryness with textures ranging from sand to silty clay.
The uncertainty of parameter estimates (confidence intervals)
as well as the correlation between parameters was studied.
The predictive capability of the extended model was evaluated
under two reduced sets of data that do not contain observations
below a matric potential of -1500 and -100 kPa. This evaluation
showed that the extended model successfully predicted the
water content with acceptable uncertainty. These results add
confidence into the proposed modification and suggest that
it can be used to better predict the soil water retention
curve, particularly under reduced data sets.
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Octubre de 2006
Measuring noise in magnetometers: An example using the
Mars Global Surveyor magnetometers
Authors: B. A. Hausman , F. C.
Michel et al
Link: Click here
Abstract
A recently developed data analysis tool allows direct measurement
of the total root-mean-square noise in magnetometer data.
Since the total is a combination of intrinsic instrument noise
and solar wind magnetic field fluctuations, we can put upper
limits on the instrument noise during
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periods
of relatively quiet solar wind fields. The technique involves
taking vector and scalar products of successive magnetic measurements;
the resulting scalar quantity is insensitive to slow field variations
as well as sudden ones (i.e., tangential discontinuities) and
provides a measure of total noise. We illustrate this analysis
by determining the noise levels for the Mars Global Surveyor
magnetometer. Far from Mars and any obvious waves, we find that
the magnetic field measurements noise level is ±0.05
nT. This is consistent with the ground-based instrument's measured
noise level of ±0.01 nT expected under ideal conditions.
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Octubre de 2006
Water in the lower crustal granulite xenoliths from Nushan,
eastern China
Authors: Qun-Ke Xia , Xiao-Zhi
Yang et al
Link: Click here
Abstract
Nominally anhydrous clinopyroxene (cpx), orthopyroxene (opx),
and plagioclase (pl) from 10 lower crustal granulite (two-pyroxene
granulite and hypersthene granulite) xenoliths in Cenozoic
basalts from the Nushan volcano, eastern China, have been
analyzed for their hydrogen content by microscopic Fourier
transform infrared
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spectroscopy
(Micro-FTIR). The results demonstrate that hydrogen was incorporated
in all these minerals in the manner of OH and that the content
(H2O weight) is up to 2360 ppm for cpx, 1170 ppm for opx, and
880 ppm for pl. On the basis of the water content of constitutive
minerals and their proportions, whole rock water contents of
the Nushan granulites were estimated to be 150-950 ppm. Estimated
equilibrium temperatures of the Nushan granulites are in the
range of 810-892°C, corresponding to the lowermost crust
at Nushan (about 25-30 km). Therefore this study provides direct
evidence that the lower continental crust, even the lowermost
part devoid of hydrous minerals, can contain a certain amount
of water in nominally anhydrous minerals. |
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Octubre de 2006
Non-self-similar scaling of plasma sheet and solar wind
probability distribution functions of magnetic field fluctuations
Authors: J. M. Weygand , M. G.
Kivelson et al
Link: Click here
Abstract
Solar wind magnetometer and plasma data and magnetometer data
acquired by Cluster in the magnetospheric plasma sheet are
employed to construct probability distribution functions (PDFs)
of magnetic field fluctuations over various temporal and spatial
scales. This technique, often used in analysis of laboratory
plasmas, is used to look for intermittent plasma turbulence
and non-self-similar properties in the fluctuations. We examine
the distribution of the magnetic field
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fluctuations
for a single spacecraft and between pairs of correlated spacecraft
time series data in both the plasma sheet and the solar wind
(fast and slow speed streams). We demonstrate that the plasma
sheet fluctuations have a distribution consistent with intermittent
turbulence and that they scale non-self-similarly using two
different methods (Sorriso-Valvo et al., 1999; Hnat et al.,
2002). In the solar wind both methods show that the fast solar
wind magnetic field fluctuations have a PDF consistent with
intermittent turbulence and scale non-self-similarly, but the
two methods give conflicting results for the slow solar wind
scaling properties. Finally, we use the results of the two methods
as well as kurtosis versus scale size to roughly determine the
turbulent eddy scale size in both types of solar wind (about
500 RE) and the magnetospheric plasma sheet (between 0.9 and
1.5 RE). |
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Octubre de 2006
Comparison of the Schumann resonance
parameters in horizontal magnetic and electric fields according
to observations on the Kola Peninsula
Authors: V. C. Roldugin , A.
N. Vasiljev et al
Link: Click here
Abstract
The measurements of horizontal electric components at extremely
low frequencies (0.1-20 Hz) have been started in the high-latitude
observatory of Lovozero in the Kola Peninsula. It is found
that the electric components are not less informative than
the horizontal magnetic ones for Schumann resonance study.
The diurnal variations in amplitude, frequency, and bandwidth
of the first Schumann resonance mode in the electric W-E and
N-S components are similar to the variations in the magnetic
H and D
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components, respectively.
The same correspondence of the components keeps for Q bursts.
The frequencies in electric and magnetic components are not
always equal: In summer, the frequency of the electric N-S
component in the diurnal variation exceeds the frequency of
the magnetic D component by 0.1 Hz. The parameters of both
magnetic and electric components have seasonal variations.
Three maxima of thunderstorm activity are observed in daily
variations of the amplitudes of electric components: the Asian
and American ones in the W-S component and the African one
in the N-S component. The width of resonance bands in the
electric components is somewhat larger than in the magnetic
ones. The calculations of ELF wave components near poorly
conducting surface are made, the results being in accordance
with the observations.
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Octubre de 2006
Investigating multi-polarization GPR wave transmission
through thin layers: Implications for vertical fracture characterization
Authors: Georgios P. Tsoflias
and Anthony Hoch
Link: Click here
Abstract
We investigate the controls governing the response of ground-penetrating
radar (GPR) wave transmission through thin layers in order
to explore the use of variable polarization GPR signals for
remote characterization of fracture aperture and fluid-fill.
We employ an experimental setting that provides controlled
observations of the effects of thin-layer properties to the
transmitted
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GPR wavefield. GPR signals of variable polarization, variable
angle of incidence, and variable frequency are transmitted through
an air- and water-filled layer of variable thickness. We observe
that at high angles of incidence, variable polarization GPR
signals display characteristic and quantifiable phase and amplitude
responses that are related to thin-layer properties. The GPR
data are in agreement to analytical solutions of plane-wave
oblique-incidence transmission through layered media. We conclude
that multi-polarization GPR observations can be exploited to
determine fracture properties. This work has implications in
the remote determination of fractured formation anisotropic
properties, such as fluid-flow. |
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Octubre de 2006
Sustainable conjunctive water management in irrigated agriculture:
Model formulation and application to the Yaqui Valley, Mexico
Authors: Gerrit Schoups , C.
Lee Addams et al
Link: Click here
Abstract
This paper investigates strategies to alleviate the effects
of droughts on the profitability and sustainability of irrigated
agriculture. These strategies include conjunctive management
of surface water and groundwater resources, and engineered
improvements such as lining of irrigation canals and addition
of regional pumping well capacity. A spatially distributed
simulation-optimization model was developed for an irrigated
system consisting of multiple surface water reservoirs and
an alluvial aquifer. The simulation model consists of an agronomic
component and simulators describing the hydrologic system.
The physical models account for storage and flow through the
reservoirs, routing through the irrigation canals, and regional
groundwater flow. The agronomic model describes crop productivity
as a function of irrigation quantity and salinity, and
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determines agricultural profit. A profit maximization problem
was formulated and solved using large-scale constrained gradient-based
optimization. The model was applied to a real-world conjunctive
surface water/groundwater management problem in the Yaqui Valley,
an irrigated agricultural region in Sonora, Mexico. The model
reproduces recorded reductions in agricultural production during
a historical drought. These reductions were caused by a decline
in surface water availability and limited installed pumping
capacity. Results indicate that the impact of the historical
8-year drought could have been significantly reduced without
affecting profit in wet years by better managing surface water
and groundwater resources. Namely, groundwater could have been
more heavily relied upon and surface water allocation capped
at a sustainable level as an operating rule. Lining the irrigation
canals would have resulted in water savings of 30% of historical
reservoir releases during wet years, which could have been used
in subsequent drier years to increase agricultural production.
The benefits of a greater reliance on groundwater pumping by
installing additional wells are limited due to pumping restrictions
near the coast to avoid seawater intrusion and due to increased
pumping costs. |
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Octubre de 2006
Three-dimensional finite difference time domain modeling
of the diurnal and seasonal variations in Schumann resonance
parameters
Authors: Heng Yang and Victor
P. Pasko
Link: Click here
Abstract
The diurnal and seasonal variations of Schumann resonances
(SR) have been reported in a number of experiments. In this
paper, a three-dimensional
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FDTD
model of the Earth-ionosphere cavity with a day-night asymmetric
conductivity profile is employed to study the diurnal and seasonal
variability of the power and frequency of the first Schumann
resonance (SR) mode. Comparison of the FDTD results and recent
experimental measurements shows a clear modulation in the SR
power related to the local ionospheric height and global lightning
activity. It is found that SR frequencies are not only a function
of the local time but also are controlled by the global lightning
activity changing with universal time. |
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Octubre de 2006
Three-dimensional waveform modeling of ionospheric signature
induced by the 2004 Sumatra tsunami
Authors: Giovanni Occhipinti,
Philippe Lognonné et al
Link: Click here
Abstract
The Sumatra, December 26th, 2004, tsunami produced internal
gravity waves in the neutral atmosphere and large disturbances
in the
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overlying
ionospheric plasma. To corroborate the tsunamigenic hypothesis
of these perturbations, we reproduce, with a 3D numerical modeling
of the ocean-atmosphere-ionosphere coupling, the tsunami signature
in the Total Electron Content (TEC) data measured by the Jason-1
and Topex/Poseidon satellite altimeters. The agreement between
the observed and synthetic TEC shows that ionospheric remote
sensing can provide new tools for offshore tsunami detection
and monitoring. |
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Octubre de 2006
Least squares inversion of self-potential (SP) data and
application to the shallow flow of ground water in sinkholes
Authors: A. Jardani , A. Revil
et al
Link: Click here
Abstract
We propose a least squares inversion algorithm to determine
the spatially variable depth of the water table in shallow
unconfined aquifers using self-potential signals measured
on the ground surface. Traditionally, the water table is determined
only at few locations using piezometers. Our approach relates
its shape with the distribution of the self-potential signals
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according to a Fredholm equation of the first kind. The latter
is discretized to obtain a linear matrix formulation of the
forward problem. This new formulation is very general and can
account for the resistivity distribution of the vadose zone.
It is used to setup the inverse problem using the approach of
Tarantola (1987) for a test site located in Normandy (France)
where 225 self-potential measurements were performed over an
area of 15,400 m2. Ground water flows through the loess overlying
a low permeability clay-with-flint weathered chalk, at a depth
between 1 to 7 meters, into sinkholes in chalk bedrock. The
method determines the water table with a precision of 0.4 m.
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Octubre de 2006
Cross-borehole ground-penetrating radar for monitoring
and imaging solute transport within the vadose zone
Authors: Ping-Yu Chang, David
Alumbaugh et al
Link: Click here
Abstract
A two-stage salt infiltration experiment was conducted to
simulate vadose zone contaminant transport and to investigate
mechanisms that affect the development of contaminant plumes
in the unsaturated zone. A low-concentration sodium chloride
solution (1024 ppm) was first infiltrated for 7 months through
one third of a specially designed infiltrometer at a rate
of 2.7 cm/d, with
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tap
water infiltrating through the remaining two thirds at the same
rate. This first stage was followed by 3 months of running tap
water through the entire infiltrometer to flush the system while
maintaining the same unsaturated infiltration conditions. Next,
the concentration of the sodium chloride solution was increased
to 6900 ppm, which was introduced at the same infiltration rate
over a different one third of the infiltrometer for 3 months.
This paper focuses on using cross-borehole ground-penetrating
radar attenuation tomography to image the developing plume in
a time-lapse sense. Attenuation is shown to increase with increasing
salt concentration, and the time-lapse images indicate that
regions of anomalous attenuation develop in locations where
pockets of higher moisture content exist. |
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Octubre de 2006
Tidal synchronicity of the 26 December
2004 Sumatran earthquake and its aftershocks
Authors: R. G. M. Crockett, G.
K. Gillmore et al
Link: Click here
Abstract
The frequency of earthquake incidence along the Andaman/Sunda/Java
Trench plate-boundary region has been investigated for the
ten-lunar-month period 28 October 2004-19 August 2005, encompassing
the 26 December 2004
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earthquake.
During this period variations in earthquake activity correlate
with the tidal-force cycles: maxima in earthquake activity occur
around the times of new and full moons, typically lagging by
0-3 days. This relationship is consistent with earthquake inducement
via ocean tidal loading. Also, during this period the earthquake
incidence associated with new and full moons at the western
end of the region was (a) 38% and (b) 86% higher than the period
averages for the full and declustered major-earthquake catalogs
respectively. |
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Octubre de 2006
Evidence for rainfall-triggered earthquake activity
Authors: S. Hainzl, T. Kraft
et al
Link: Click here
Abstract
Fluids are known to be of major importance for the earthquake
generation because pore pressure variations alter the strength
of faults. Thus they can initiate earthquakes if the crust
is close
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enough
to its critical state. Based on the observations of the isolated
seismicity below the densely monitored Mt. Hochstaufen, SE Germany,
we are now able to demonstrate that the crust can be so close-to-failure
that even tiny pressure variations associated with precipitation
can trigger earthquakes in a few kilometer depth. We find that
the recorded seismicity is highly correlated with the calculated
spatiotemporal pore pressure changes due to diffusing rain water
and in good agreement with the response of faults described
by the rate-state friction law. |
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Octubre de 2006
Thermal imaging on simulated faults during frictional sliding
Authors: Karen Mair, François
Renard et al
Link: Click here
Abstract
Heating during frictional sliding is a major component of
the energy budget of earthquakes and represents a potential
weakening mechanism. It is therefore important to investigate
how heat dissipates during sliding on simulated faults. We
present results from laboratory friction experiments where
a halite (NaCl) slider held under constant load is dragged
across a coarse
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substrate.
Surface evolution and frictional resistance are recorded. Heat
emission at the sliding surface is monitored using an infra-red
camera. We demonstrate a link between plastic deformations of
halite and enhanced heating characterized by transient localized
heat spots. When sand 'gouge' is added to the interface, heating
is more diffuse. Importantly, when strong asperities concentrate
deformation, significantly more heat is produced locally. In
natural faults such regions could be nucleation patches for
melt production and hence potentially initiate weakening during
earthquakes at much smaller sliding velocities or shear stress
than previously thought. |
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Octubre de 2006
Incipient axial collapse of the Main Cordillera and strain
partitioning gradient between the central and Patagonian Andes,
Lago Laja, Chile
Authors: Daniel Melnick, François
Charlet et al
Link: Click here
Abstract
Lago Laja is a late Quaternary volcanic-dammed lake located
near the drainage divide of the south central Andes. Field
observations, lake reflection seismic profiles, bathymetry,
and remote sensing data reveal an active fault system that
runs parallel to the volcanic arc along the axis of the Main
Cordillera, the Lago Laja fault system (LLFS). Normal faults
of this extensional system cut late Pleistocene volcanics,
<7.1 ka still water lacustrine sediments, 6.3 ka pyroclastic
deposits, and Holocene alluvial fans. We divide the LLFS in
three segments on the basis of fault geometry, width, and
slip magnitude. The underwater faults of the central segment
in the lake's deepest part have the maximum Holocene vertical
slip rate of >2.7 mm/yr. Since 7.1 ka, the LLFS accounts
for ~0.7% of arc-normal extension at an average
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minimum
rate of 1.2 mm/yr and strain rate of ~10-14 s-1. Seismites and
surface ruptures evidence M>6 paleoearthquakes. The Main
Cordillera at ~37°S is a large-scale pop-up structure uplifted
by thrusting along its foothills. In this light, we interpret
extension in the axial and highest part of the Andes as incipient
synorogenic gravitational collapse in response to uplift and
crustal thickening. Thermal weakening due to elevated heat flow
and postglacial lithospheric rebound and unbending have probably
contributed to the arc-limited collapse and Holocene acceleration
of deformation rates. The lack of significant strike-slip offsets
along the LLFS as well as along both foothills-thrust systems
at 37°S contrasts with the intra-arc dextral fault zone
south of 38°S. Regional structural data indicates that north
of 38°S, diffusely distributed strain reflects low partitioning
of oblique subduction, while to the south deformation is localized
in a discrete strike-slip fault zone along the volcanic arc,
reflecting a higher degree of partitioning. We relate this strain
partitioning gradient to favorable fault orientations in the
fore arc north of the Arauco Peninsula, a major seismotectonic
boundary. |
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Octubre de 2006
Distribution of slip from 11 Mw > 6 earthquakes in the
northern Chile subduction zone
Authors: M. E. Pritchard , C.
Ji et al
Link: Click here
Abstract
We use interferometric synthetic aperture radar, GPS, and
teleseismic data to constrain the relative location of coseismic
slip from 11 earthquakes on the subduction interface in northern
Chile (23°-25°S) between the years 1993 and 2000.
We invert body wave waveforms and geodetic data both jointly
and separately for the four largest earthquakes during this
time period (1993 M w 6.8; 1995 M w 8.1; 1996 M w 6.7; 1998
M w 7.1). While the location of slip in the teleseismic-only,
geodetic-only, and joint slip
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inversions
is similar for the small earthquakes, there are differences
for the 1995 M w 8.1 event, probably related to nonuniqueness
of models that fit the teleseismic data. There is a consistent
mislocation of the Harvard centroid moment tensor locations
of many of the 6 < M w < 8 earthquakes by 30-50 km toward
the trench. For all models, the teleseismic data are better
able to resolve fine details of the earthquake slip distribution.
The 1995 earthquake did not rupture to the maximum depth of
the seismogenic zone (as defined by the other earthquakes).
In addition to the above events, we use only teleseismic data
to determine the rupture characteristics of four other M w >
6 earthquakes, as well as three M w > 7 events from the 1980s.
All of these earthquakes appear to rupture different portions
of the fault interface and do not rerupture a limited number
of asperities. |
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Octubre de 2006
Effect of massive gas hydrate formation on the water isotopic
fractionation of the gas hydrate system at Hydrate Ridge,
Cascadia margin, offshore Oregon
Authors: Hitoshi Tomaru , Marta
E. Torres et al
Link: Click here
Abstract
Because gas hydrate is preferentially enriched in the heavy
water isotopes, the d 18O and dD values of pore waters collected
from gas hydrate-bearing sediment can provide information
on the abundance and mechanisms of gas hydrate formation.
Pore waters sampled from deep-seated (40 to 125 mbsf) gas
hydrate deposits in Hydrate Ridge during ODP Leg 204 show
depletion in dissolved Cl- and enrichments in 18O and D due
to gas hydrate destabilization during core recovery. The oxygen
and hydrogen isotopic fractionation factors (a O = 1.0025
and a H =
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1.022)
estimated from an extensive data set (n = 30 samples) correspond
to experimentally determined values. In contrast, pore waters
from shallow samples (<25 mbsf) at the ridge summit (n =
32) are highly enriched in dissolved Cl- and depleted in 18O
and D, consistent with formation of massive gas hydrate deposits
at rates faster than those at which these anomalies would be
removed by advection or diffusion. The water isotopic fractionation
factors in the brine are significantly lower than those experimentally
determined, with a O of 1.0010 (average value of 1.0012) and
a H of 1.008 (average value of 1.008). We discuss several factors
that may be causing this anomalous fractionation and suggest
that low gas occupancy in hydrate lattice (high hydration number)
may be responsible for the observed small fractionation. If
this were the case, the oxygen and hydrogen fractionation may
serve as an indicator of hydration number during formation of
gas hydrate in natural systems. |
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Octubre de 2006
Applying artificial neural network to derive long-term
foF2 trends in the Asia/Pacific sector from ionosonde observations
Authors: Xinan Yue, Weixing Wan
et al
Link: Click here
Abstract
An artificial neural network (ANN) method is first used for
deriving long-term trends of the F2-layer critical frequency
(foF2) at 19 ionospheric stations in the Asia/Pacific sector.
It is found that the ANN method can eliminate the geomagnetic
activity effect on foF2 more effectively than usual regression
methods. Of the selected 19 stations,
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there
are significant long-term trends corresponding to a confidence
level =90% at 14 stations and 12 of these stations present negative
trends. An average trend of -0.05% per year in the selected
area can be obtained if the 12 stations with significant negative
long-term trends be considered. No pronounced diurnal and latitudinal
effects in trends and no uniform pattern of seasonal variation
in most stations are detected. The long-term trends for low
latitude and equatorial stations differ from other stations
suggest that some special dynamical processes may take effects
in the equatorial anomaly region. Many factors which can influence
ionosphere, such as the greenhouse effect, solar and geomagnetic
activity, and neutral background gas, might contribute to the
trend. |
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Octubre de 2006
Self-consistent modeling of magnetic fields and plasmas
in the inner magnetosphere: Application to a geomagnetic storm
Authors: Sorin Zaharia, V. K.
Jordanova et al
Link: Click here
Abstract
A geomagnetic storm model needs to take into account the coupling
between magnetic field and plasma, as the storm-time field
in the inner nightside magnetosphere can be very depressed
compared to that of the Earth dipole, thus significantly modifying
plasma transport. In this paper we extend our previous "one-way"
coupling between a kinetic ring current model and a magnetospheric
force-balance model to a fully magnetically self-consistent
approach, in which the force-balanced fields are fed back
into the
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kinetic
model to guide its evolution. The approach is applied to simulating
the 21-23 April 2001 "GEM Storm Challenge" event.
We use boundary and initial conditions for the kinetic model
from several spacecraft, and magnetic flux boundaries for the
equilibrium code from an empirical magnetic field model. We
find significant differences in the self-consistent results
compared to those obtained from the kinetic model with a dipolar
background field (with the same particle boundary conditions
and electric fields), due mainly to changes in the particle
drifts. In addition to large depressions in the nightside magnetic
field values compared to a dipolar field, we also find significantly
lower particle density and perpendicular plasma pressure in
the inner magnetosphere in the self-consistent case, as well
as local, narrow pressure peaks and strongly enhanced plasma
ß p in localized regions on the nightside. |
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Octubre de 2006
Studies of ELF propagation in the spherical shell cavity
using a field decomposition method based on asymmetry of Schumann
resonance curves
Authors: A. Kulak, J. Mlynarczyk
et al
Link: Click here
Abstract
Asymmetric resonance curves are observed in various resonance
systems occurring in nature. The reason for such a shape of
the resonance curves is an interaction of the standing waves
field in the resonator with the field of traveling waves which
transmit energy from sources to the resonator. This behavior
can be observed in strongly damped electromagnetic resonators.
The ELF wave propagation inside the Earth-ionosphere cavity
is a good example of the simultaneous occurrence of resonance
and transmission phenomena. In this paper we show that the
transmission field component depends on the attenuation rate
of the Earth-ionosphere cavity and the observer-source distance.
Besides,
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the
resonance curve asymmetry causes an evident diurnal variability
of the resonance frequencies. The superposition of the two components
at any point of the resonator makes the analysis of the Schumann
resonance (SR) difficult. Here we suggest a field decomposition
method that allows separating the resonance component from the
transmission one. Owing to the decomposition, having a single
measurement of the E or B field, it is possible, independently
of the observer position, to study the physical properties of
the resonator as well as to determine the localization and the
intensity of sources. The field decomposition permits defining
new resonator parameters such as reduced resonance frequencies
and reduced quality factors, independent of the observer position
inside the cavity. We believe that the application of the decomposition
method in the analysis of the ELF observations yields a possibility
of improving the accuracy of the determination of both the distances
and intensities of the sources exciting the Earth-ionosphere
resonator, as well as its own parameters. |
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Octubre de 2006
Deformation and permeability of aggregated soft earth materials
Authors: C. G. Eggers, M. Berli
et al
Link: Click here
Abstract
This study develops a framework for modeling deformation of
individual pores in elastoviscoplastic earth material accounting
for the effects of evolving pore size and shape on material
hydraulic permeability. We describe the velocity field of
a fluid within deforming pores of hypotrochoidal cross-sectional
areas as a function of remote stress or deformation and elastoviscoplastic
material properties using finite element analysis. We find
that pore permeability decreases with increasing stress and
deformation. Pore cross-sectional areas are
|
mainly
reduced in size while the shape remains constant. Under stress-controlled
conditions, change in permeability depends on matrix constitutive
laws and loading time while there is no such dependency for
controlled strain. Permeability estimates based on the hydraulic
radius, Saint-Venant, and Aissen approximations were in good
agreement with numerical calculations for a deforming hypotrochoidal
pore. We also show that permeability of a deforming hypotrochoidal
pore can be modeled using a pore with equivalent elliptical
cross-sectional area (equal initial permeability and size),
providing the ellipse has the correct orientation. The study
shows that fluid flow in deforming elastoviscoplastic earth
material can be modeled on the pore scale knowing the evolution
of pore size and shape employing rather simple relations between
pore cross-sectional geometry and permeability. |
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Octubre de 2006
Predicting Solar Cycle 24 and beyond
Authors: Mark A. Clilverd, Ellen
Clarke et al
Link: Click here
Abstract
We use a model for sunspot number using low-frequency solar
oscillations, with periods 22, 53, 88, 106, 213, and 420 years
modulating the 11-year Schwabe cycle, to predict the peak
sunspot number of cycle 24 and for future cycles, including
the period around 2100 A.D. We extend the earlier work of
Damon and Jirikowic (1992) by adding a further long-period
component of 420 years. Typically, the standard deviation
between the model and the peak sunspot number in each solar
cycle from 1750 to 1970 is ±34. The peak
|
sunspot
prediction for cycles 21, 22, and 23 agree with the observed
sunspot activity levels within the error estimate. Our peak
sunspot prediction for cycle 24 is significantly smaller than
cycle 23, with peak sunspot numbers predicted to be 42 ±
34. These predictions suggest that a period of quiet solar activity
is expected, lasting until ~2030, with less disruption to satellite
orbits, satellite lifetimes, and power distribution grids and
lower risk of spacecraft failures and radiation dose to astronauts.
Our model also predicts a recovery during the middle of the
century to more typical solar activity cycles with peak sunspot
numbers around 120. Eventually, the superposition of the minimum
phase of the 105- and 420-year cycles just after 2100 leads
to another period of significantly quieter solar conditions.
This lends some support to the prediction of low solar activity
in 2100 made by Clilverd et al. (2003). |
|
Octubre de 2006
Gravity changes from a stress evolution earthquake simulation
of California
Authors: T. J. Hayes, K. F. Tiampo
et al
Link: Click here
Abstract
The gravity signal contains information regarding changes
in density at all depths and can be used as a proxy for the
strain accumulation in fault networks. A stress evolution
time-dependent model was used to create simulated slip histories
over the San Andreas Fault network in California.
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Using a linear sum of the gravity signals from each fault segment
in the model, via coseismic gravity Green's functions, a time-dependent
gravity model was created. The steady state gravity from the
long-term plate motion generates a signal over 5 years with
magnitudes of ±~2 µGal; the current limit of portable
instrument observations. Moderate to large events generate signal
magnitudes in the range of ~10 to ~80 µGal, well within
the range of ground-based observations. The complex fault network
geometry of California significantly affects the spatial extent
of the gravity signal from the three events studied. |
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Octubre de 2006
Wavelet analysis of the seismograms of the 2004 Sumatra-Andaman
earthquake and its application to tsunami early warning
Authors: Oliver G. Lockwood,
Hiroo Kanamori et al
Link: Click here
Abstract
We applied the wavelet transform in an attempt to detect long-period
components early in a seismogram. We analyzed the displacement
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seismograms
of the 26 December 2004 Sumatra-Andaman earthquake (Mw = 9.2)
and the 28 March 2005 Nias earthquake (Mw = 8.7). Wavelet analysis
is able to clearly distinguish the amplitudes of the long-period
W phase between the seismograms of the two earthquakes before
the S wave reaches the station. It shows that the 2004 earthquake
generates a W phase of significantly greater amplitude. This
facility has potential application to the rapid identification
of truly great earthquakes with high tsunami potential. |
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Septiembre de 2006
Mars water vapor abundance from SPICAM IR spectrometer:
Seasonal and geographic distributions
Authors: Anna Fedorova, Oleg
Korablev et al
Link: Click here
Abstract
The near-IR channel of SPICAM experiment on Mars Express spacecraft
is a 800-g acousto-optic tunable filter (AOTF)-based spectrometer
operating in the spectral range of 1-1.7 µm with resolving
power of ~2000. It was put aboard as an auxiliary channel
dedicated to nadir H2O measurements in the 1.37-µm spectral
band. This primary scientific goal of the experiment is achieved
though successful water vapor
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retrievals,
resulting in spatial and seasonal distributions of H2O. We present
the results of H2O retrieval from January 2004 (Ls = 330°)
to December 2005 (Ls = 340°), covering the entire Martian
year. The seasonal trend of water vapor obtained by SPICAM IR
is consistent with TES results and reveals disagreement with
MAWD results related to south pole maximum. The main feature
of SPICAM measurements is globally smaller water vapor abundance
for all seasons and locations including polar regions, as compared
to other data. The maximum abundance is 50-55 precipitable microns
at the north pole and 13-16 precipitable microns (pr µm)
at the south pole. The northern tropical maximum amounts to
12-15 pr µm. Possible reasons for the disagreements are
discussed. |
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Septiembre de 2006
An experimental analogue for convection and phase separation
in hydrothermal systems
Authors: Simon Emmanuel and Brian
Berkowitz
Link: Click here
Abstract
Experiments were conducted to explore the behavior of convection
and heat transfer during phase separation in porous media.
Phase separation is considered to be an important process
in many mid-ocean ridge hydrothermal systems in which a dense
brine separates from a lighter, less saline vapor phase at
supercritical temperatures. As this process occurs at high
temperatures and pressures in seawater systems, a binary fluid
(H2O-2-butoxyethanol) with
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a lower consolute point of 48.5°C, was used as a proxy for
the hydrothermal fluid. In the experiments, a cell containing
glass beads saturated with the binary fluid was heated from
below and cooled from above. It was demonstrated that chemically
differentiated regions can exist as part of a steady state convective
regime, with the denser fluid phase separating and accumulating
in a stagnant bottom layer. Furthermore, in upwelling regions
phase separating fluids can become entrained. Phase separation
was also found to lower the efficiency of convective thermal
transfer, indicated by the deviation from the calculated single-phase
Rayleigh-Nusselt curve. It is proposed that the reduction of
convective heat transfer at supercritical conditions may be
a crucial factor in controlling heat transfer through the oceanic
crust. |
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Septiembre de 2006
Three-dimensional finite difference time domain modeling
of the Schumann resonance parameters on Titan, Venus, and
Mars
Authors: Heng Yang, Victor P.
Pasko et al
Link: Click here
Abstract
The conducting ionosphere and conducting surface of Titan,
Venus, and Mars form a concentric resonator, which would support
the possibility of the existence of global electromagnetic
resonances. On Earth, such resonances are commonly referred
to as Schumann resonances and are excited by
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lightning
discharges. The detection of such resonances on other planets
would give a support for the existence of the electrical discharges
in the lower atmosphere on these planets. In this paper, a three-dimensional
finite difference time domain modeling for the extremely low
frequency propagation is employed to study the Schumann resonance
problems on Titan, Venus, and Mars. The atmospheric conductivity
profiles for these studies are derived from the previously reported
ionospheric models for these planets. The Schumann resonance
frequencies and Q factors on these planets are calculated and
are critically compared with those obtained from the previously
published models. |
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Septiembre de 2006
Comparison of grid-based algorithms for computing upslope
contributing area
Authors: Robert H. Erskine ,
Timothy R. Green et al
Link: Click here
Abstract
Terrain attributes based on upslope contributing area, A,
are used widely in distributed hydrologic models. Several
grid-based algorithms are available for estimating A. In this
study, five algorithms (D8, ?8, MFD, DEMON, and D8) were compared
quantitatively on two undulating
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agricultural
fields (63 and 109 ha) in northeastern Colorado. Global positioning
system (GPS) data (0.02-m accuracy) were used to generate grid
digital elevation models (DEMs) at 5-, 10-, and 30-m cell sizes.
Relative differences between A values estimated using single-
and multiple-direction algorithms increased with decreasing
grid cell size. Relative differences were greatest along ridges
and side slopes, and differences decreased where the terrain
became more convergent. Multiple-direction algorithms (MFD,
DEMON, and D8), allowing for flow divergence, are recommended
on these undulating terrains for 5- and 10-m grids where A is
most sensitive to the algorithm selection. |
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Septiembre de 2006
Self-potential signals associated with
preferential groundwater flow pathways in sinkholes
Authors: A. Jardani, J. P. Dupont
et al
Link: Click here
Abstract
Two self-potential (SP) surveys have been conducted at a test
site located in Normandy, in the northwest of France, in a
chalk karst in spring and summer 2005. The spring survey showed
circular negative SP signals associated with the position
of sinkholes and crypto-sinkholes, while the survey conducted
during the summer showed fewer anomalies with lesser magnitudes.
The negative SP anomalies observed in the spring survey were
several tens of millivolts less than a reference located outside
the ridge along which the sinkholes are located. In addition
to the SP surveys, we also performed a DC electrical resistivity
survey. The electrical resistivity
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tomogram
shows the position of the interfaces between the chalk and the
overlying clay with flint and loess covers and the position
of the sinkholes. A linear relationship is observed between
the SP signals and the thickness of the loess layer. In addition,
large negative SP anomalies are associated with the sinkholes
themselves. These SP signals are explained by solving the boundary
value problem for the coupled hydroelectric problem. We fixed
the boundary condition at the ground surface in terms of hydraulic
flux associated with the infiltration of the meteoric water
and a no-flow condition on the vertical boundaries. A finite
element numerical simulation of this coupling is performed,
and a fairly good agreement is obtained between the simulation
and the observed SP signals. We show for the first time how
the SP signals reflect mainly the topography of the clay with
flint/loess interface and help to identify the location of sinkholes.
Three-dimensional tomography algorithms are proposed to locate
the source of these self-potential signals in the ground. |
Septiembre
de 2006
Multicriterion decision analysis approach
to assess the utility of watershed modeling for management decisions
Author: Amin Elshorbagy
Link: Click here
Abstract
This paper employs the multicriterion decision analysis (MCDA)
technique in a novel way to address the efficiency of watershed
instrumentation programs and the efficacy of watershed modeling.
A case study of reconstructed watersheds in northern Alberta,
Canada, is used to illustrate the proposed usage of the MCDA
technique. The watersheds have been disturbed as a result
of oil sands mining activities. Assessing the performance
of the reconstructed watersheds with regard to restoring the
hydrology of the disturbed watershed is a crucial issue for
both the mining industry and other stakeholders. The problem
is formulated in a multicriterion context. A payoff matrix
containing seven evaluation criteria and three different soil
covers as feasible alternatives is constructed. The system
dynamics watershed (SDW) model is
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used to simulate the
reconstructed watersheds over a period of 61 years using historical
meteorological records. Accordingly, 61 payoff matrices that
are populated using the results of the SDW model are evaluated.
A multicriterion decision analysis framework is implemented
to evaluate the different alternatives with respect to the
chosen set of criteria. The three alternatives are ranked
every year, and accordingly, the probability that a certain
alternative dominates others is estimated. The alternative
that has the highest probability of occupying the top rank
over the period of analysis is indicated as the best alternative.
The probability value is called the probability of making
the right decision (PMRD). Various types of uncertainty analyses
are conducted to evaluate the sensitivity of the final decision
to changes in the scores of the evaluation matrices. An index,
named the confidence in the PMRD, is developed to quantify
the reliability of the results of the watershed model. The
results highlight the utility of modeling as a possible alternative
to some components of the intensive instrumentation program.
Moreover, areas of deficiency and inaccuracies in the watershed
model are identified for further improvements.
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