|
Marzo de 2007
Zeolite compositions
as proxies for eruptive paleoenvironment
Authors: Joanne S. Johnson and
John L. Smellie
Link: Click here Abstract
We report chemical compositions of authigenic alteration minerals in subglacially
erupted hyaloclastite breccias from James Ross Island, Antarctica, which provide
a geochemical proxy for paleoenvironment. Analyses of zeolites (phillipsite and
chabazite) from several lava-fed deltas on the island were compared with published
compositions of zeolites from known freshwater |
and marine environments. Using values of (Na + K)/Ca = 3.0 (for phillipsites)
and 1.0 (for chabazites), the data successfully distinguish between marine and
freshwater alteration. However, our study also suggests that volcanic breccias
may not be sealed to fluids, even when the pore spaces are completely filled.
The James Ross Island data show that, at higher elevations in lava-fed deltas,
the zeolites preserve their original freshwater-influenced (i.e., glacial) compositions,
whereas at lower elevations, (Na + K)/Ca ratios are generally higher and indicate
a later marine influence. We suggest that water may penetrate several kilometers
into the volcanic pile by movement along fractures and/or grain boundaries. | Marzo
de 2007
Triggering of earthquakes during the 2000 Papua New Guinea earthquake
sequence
Authors: Sun-Cheon Park and Jim Mori
Link: Click here Abstract
A sequence of large earthquakes occurred in the New Britain/New Ireland region
of Papua New Guinea in late 2000. The sequence started with a Mw 6.8 earthquake
along the New Britain Trench on 29 October. About 20 days later a Mw 8.2 earthquake
occurred in the New Ireland region on 16 November and produced large strike-slip
surface displacements and tsunamis. Following the Mw 8.2 event, two large earthquakes
(Mw ~ 7.5) occurred along the nearby New Britain Trench | on
16 and 17 November. Furthermore, small triggered events were observed over a wide
area outside of the rupture zones of the large earthquakes, with different focal
mechanisms from those of the major events. There is likely some mechanism(s) that
triggered this remarkable sequence, and we document the details of the spatial
and temporal patterns of the events. We investigated if static stress changes
can explain the initiation of the large earthquakes and also some groups of triggered
smaller events. There are mixed results. The occurrences of the two M ~ 7.5 major
events may be explained by the static stress changes; however, there are also
some earthquakes that are not consistent with triggering by static stress changes.
There may be multiple mechanisms, including static and dynamic stress changes,
that are needed to explain the complicated sequence of earthquakes. | Marzo
de 2007
Amplitude loss of sonic waveform due to source coupling to the
medium
Authors: Myung W. Lee and William F. Waite
Link: Click here Abstract
In contrast to hydrate-free sediments, sonic waveforms acquired in gas hydrate-bearing
sediments indicate strong amplitude attenuation associated with a sonic velocity
increase. The amplitude attenuation increase has been used to quantify pore-space
hydrate content by attributing observed attenuation to the hydrate-bearing |
sediment's intrinsic attenuation. A second attenuation mechanism must be considered,
however. Theoretically, energy radiation from sources inside fluid-filled boreholes
strongly depends on the elastic parameters of materials surrounding the borehole.
It is therefore plausible to interpret amplitude loss in terms of source coupling
to the surrounding medium as well as to intrinsic attenuation. Analyses of sonic
waveforms from the Mallik 5L-38 well, Northwest Territories, Canada, indicate
a significant component of sonic waveform amplitude loss is due to source coupling.
Accordingly, all sonic waveform amplitude analyses should include the effect of
source coupling to accurately characterize a formation's intrinsic attenuation.
| Marzo
de 2007
A geostatistical framework for incorporating transport information
in estimating the distribution of a groundwater contaminant plume
Authors:
Shahar Shlomi and Anna M. Michalak
Link: Click here Abstract
The goal of groundwater contaminant plume interpolation is to provide an accurate
representation of the spatial distribution of the plume given the data limitations
associated with sparse monitoring networks with irregular geometries. Currently
available methods for plume estimation cannot fully take advantage of | prior
knowledge of flow and transport information or the location of a contaminant source.
This paper presents two new geostatistical tools for incorporating transport information
in estimating the spatial distribution of groundwater contaminant plumes. The
methods account for the spatial/temporal covariance of the contaminant plume in
defining a best estimate of the plume distribution and its associated uncertainty.
Overall, the methods require that the estimated plume distribution be physically
feasible given both the available concentration measurements and the flow and
transport in the affected aquifer. Sample applications in homogeneous and heterogeneous
formations are presented. Even with relatively few observations, the new methods
yield results that are superior to those obtained by kriging, with a better reproduction
of the true plume shape and lower uncertainty. | Marzo
de 2007
Is geomagnetic activity driven by solar wind turbulence?
Authors: R. D'Amicis, R. Bruno et al
Link: Click here Abstract
The Earth's magnetosphere is a highly dynamical system, which continuously exchanges
energy, mass and momentum with the solar wind and the Earth's ionosphere. Existing
literature would suggest that particular large amplitude | interplanetary
Alfvén wave trains might cause intense auroral activities known as High-Intensity
Long-Duration Continuous Auroral Activity (HILDCAAs), as a result of the magnetic
reconnection between the southward magnetic field z component and the magnetopause
magnetic fields. This paper presents a statistical analysis concerning the role
played by Alfvénic turbulence in the solar wind-magnetosphere coupling
as a function of the solar cycle. We find that Alfvénic fluctuations are
geoeffective at solar minimum while at solar maximum magnetic structures play
a role. | Marzo
de 2007
Are asperity patterns persistent? Implication from large earthquakes
in Papua New Guinea
Authors: Sun-Cheon Park, Jim Mori et al
Link:
Click here Abstract
We studied the distribution of asperities for large recent earthquakes along the
New Britain trench, Papua New Guinea, to investigate if they are the same for
repeated ruptures of the subduction boundary. We determined the slip distributions
of | two
earthquakes (Mw ~ 8) in 1971 using Pdiff waveforms, and an earthquake (Mw 7.9)
in 1995 using direct teleseismic P waves. Combining these findings with previous
results for two earthquakes (Mw ~ 7.5) in 2000, we compared the source areas and
asperity distributions for this region of the New Britain Trench. Our results
show that the locations of the asperities for the individual earthquakes did not
significantly overlap, although the same portion of the subduction zone seems
to have reruptured. This fact supports the idea that asperities are not persistent
features when portions of the New Britain subduction zone slip in large earthquakes.
| Marzo
de 2007
Volcanic activity influenced by tectonic earthquakes: Static and
dynamic stress triggering at Mt. Merapi
Authors: T. R. Walter , R. Wang
et al
Link: Click here Abstract
Mt. Merapi is one of the most dangerous volcanoes in Indonesia, located within
the tectonically active region of south-central Java. This study investigates
how Mt. Merapi affected - and was affected by - nearby tectonic earthquakes. In
2001, a Mw6.3 earthquake occurred in conjunction with an increase in | fumarole
temperature at Mt. Merapi. In 2006, another Mw6.3 earthquake took place, concomitant
with an increase of magma extrusion and pyroclastic flows. Here, we develop theoretical
models to study the amount of stress transfer between the earthquakes and the
volcano, showing that dynamic, rather than static, stress changes are likely responsible
for the temporal and spatial proximity of these events. Our examination of the
2001 and 2006 events implies that volcanic activity at Mt. Merapi is influenced
by stress changes related to remote tectonic earthquakes, a finding that is important
for volcano hazard assessment in this densely inhabited area. | Marzo
de 2007
Magnetic properties and potential field modeling of the Peculiar
Knob metamorphosed iron formation, South Australia: An analog for the source of
the intense Martian magnetic anomalies?
Authors: Phillip W. Schmidt ,
Suzanne A. McEnroe et al
Link: Click here Abstract
Magnetic property measurements show that the strongly metamorphosed Peculiar Knob
iron formation (IF), South Australia, is coarse-grained, high-grade hematite with
variable amounts of magnetite and maghemite. This body exhibits a relatively low
magnetic susceptibility (<0.3 SI) that cannot explain the associated intense
magnetic anomaly, 30,000 nT, in terms of induced magnetization alone. Peculiar
Knob IF possesses an extremely intense (~120 A m-1) remanence, directed steeply
upward. This ancient remanence reinforces the local Earth's field (inclination
-63°). A simple geological model, constrained by drilling | and
physical property measurements, explains both the observed magnetic and gravity
anomalies, consistent with the Poisson theorem. Koenigsberger ratios (Qs) of 10
and greater, as found here, are rare in nature. We postulate that acquisition
of a thermoremanent magnetization (TRM) by the ore during postmetamorphic cooling
from above the Curie/Néel temperature accounts for the intense remanence
and high Qs. Although the hematite is in the multidomain size range, the coercivity
is higher than expected. Also, the natural remanent magnetization (NRM) values
are less than 10% of the expected value for a saturated TRM of hematite. On the
basis of reflected light, scanning electron microscope observations, and rock
magnetism, we propose that the common fine intergrowths of a very small amount
of magnetite and/or maghemite within the hematite host are responsible for the
relatively high coercivity and contribute to the NRM. These intergrowths are not
normal exsolution lamellae and were likely present at high temperature. This study
suggests that coarse-grained hematite-rich bodies that carry TRM and have been
subjected to high-grade (>680°C) metamorphism may be possible sources for
some of the prominent Martian anomalies. | Marzo
de 2007
A wavelet investigation of possible orbital influences on past
geomagnetic field intensity
Authors: D. Heslop
Link: Click here Abstract
A number of sedimentary records of relative paleointensity have been reported
to contain modes of variability that could have been driven by changes in the
Earth's orbital or climatic state. Proposed relationships are often based on visual
correlation in the time domain or the presence of peaks in relative paleointensity
frequency spectra | close
to those of the orbital components. When discussing hypothesized connections,
it is not common for the phase relationship between paleointensity and orbital
change to be addressed, so a true mechanistic link cannot be confirmed. The existence
of a direct link between records of paleointensity and orbital/climatic variation
is tested using the cross-wavelet transform and squared wavelet coherence. It
is found that while all the records show common power at certain periods, the
geomagnetic and orbital variations do not exhibit a consistent phase relationship,
suggesting there is no direct physical link between them. This interpretation
is supported by a lack of significant correlation in the time-frequency plane
between the studied records as measured by the squared wavelet coherence. | Marzo
de 2007
Modeling of the thermal state of Mount Vesuvius from 1631 A.D.
to present and the role of CO2 degassing on the volcanic conduit closure after
the 1944 A.D. eruption
Authors: Francesca Quareni , Roberto Moretti et
al
Link: Click here Abstract
The last eruptive event at Mount Vesuvius occurred in 1944 A.D., ending a cycle
of continuous eruptive activity started with the sub-Plinian event of 1631 A.D.
The aim of this research is (1) to model the thermal evolution of the volcanic
system from 1631 A.D. up to the present and (2) to investigate the possible process
leading the volcano to the current state of quiescence. A finite element software
is employed to solve the time-dependent energy equation and obtain the thermal
field in the volcanic edifice and the surrounding medium. Volcanological, petrological,
and geophysical constraints are used to define the crustal structure beneath the
volcanic edifice, the magma supply system active since 1631 A.D., and the physico-chemical
conditions of magma. Thermodynamic properties of magma and wall rocks have been
evaluated from well-established thermo-chemical compilations and data from the
literature. It is shown that heat transfer due to magma degassing is required
in addition to the heat conduction in order to obtain transient depth- |
temperature fields consistent with geochemical observations, high crustal magnetization,
and rigid behavior of the shallow crust as indicated by geophysical data. Surface
data of carbon dioxide soil flux coming out from the Mount Vesuvius crater are
taken to constrain such an additional heat flux. The agreement between modeled
and measured temperatures at the crater since 1944 A.D. proves the consistency
of the model. It is concluded that the present state of quiescence of Mount Vesuvius
is mostly a consequence of the absence of magma supply from the deep reservoir
into the shallower system. This allows the cooling of residual magma left within
the volcanic conduit and the transition from continuous eruptive activity to the
condition of conduit obstruction. In this scenario, the hydrothermal system may
have developed subsequent to the cooling of the magma within the conduit. Our
findings are a direct consequence of the high concentration of CO2 in the most
mafic Vesuvian magmas: The low solubility of CO2, with respect to H2O, enables
a high mass flux of carbon dioxide through the volcanic edifice. The results of
this study are relevant for hazard assessment at Vesuvius and indicate directions
for further investigation, such as the role of the hydrothermal system on the
thermal energy budget of the volcanic system and its relationships with fluids
released by crustal structures likely to host the magmatic reservoir. In general,
the role of the high concentration of carbon dioxide in magmas should be more
questioned and investigated when studying the behavior of volcanic systems, particularly
in south Italy volcanoes. | Marzo
de 2007
Waveform inversion of volcano-seismic signals for an extended source
Authors: M. Nakano, H. Kumagai et al
Link: Click here Abstract
We propose a method to investigate the dimensions and oscillation characteristics
of the source of volcano-seismic signals based on waveform inversion for an extended
source. An extended source is realized by a set of point sources distributed on
a grid surrounding the centroid of the source in accordance with the source geometry
and orientation. The source-time functions for all point sources are estimated
simultaneously by waveform inversion carried out in the frequency domain. We apply
a smoothing constraint to suppress short-scale noisy fluctuations of source-time
functions between adjacent sources. The strength of the smoothing constraint we
select is that which minimizes the Akaike Bayesian Information Criterion (ABIC).
We perform a series of numerical tests to investigate | the
capability of our method to recover the dimensions of the source and reconstruct
its oscillation characteristics. First, we use synthesized waveforms radiated
by a kinematic source model that mimics the radiation from an oscillating crack.
Our results demonstrate almost complete recovery of the input source dimensions
and source-time function of each point source, but also point to a weaker resolution
of the higher modes of crack oscillation. Second, we use synthetic waveforms generated
by the acoustic resonance of a fluid-filled crack, and consider two sets of waveforms
dominated by the modes with wavelengths 2L/3 and 2W/3, or L and 2L/5, where W
and L are the crack width and length, respectively. Results from these tests indicate
that the oscillating signature of the 2L/3 and 2W/3 modes are successfully reconstructed.
The oscillating signature of the L mode is also well recovered, in contrast to
results obtained for a point source for which the moment tensor description is
inadequate. However, the oscillating signature of the 2L/5 mode is poorly recovered
owing to weaker resolution of short-scale crack wall motions. The triggering excitations
of the oscillating cracks are successfully reconstructed. | |
Febrero de 2007
Los primeros americanos vivieron en Chile
Autores: Michael R. Waters
and Thomas W. Stafford, Jr.
Link
1
Link
2 Resumen (www.terra.cl)
En un estudio
publicado esta semana en la revista "Science", los arqueólogos
Michael Waters y Thomas Stafford indican que las nuevas dataciones con radiocarbono
de la cultura Clovis apuntan a que tal vez sus miembros no fueron los primeros
habitantes del Nuevo Mundo y que su influencia sólo duró unos pocos
siglos.
Esa admisión también descarta la posibilidad de que
hubieran sido miembros de esa cultura norteamericana los que, hace más
de 13 000 años, colonizaron el continente en medio de la glaciación
a través de un corredor templado que unía a Norteamérica
con Sudamérica.
Stafford y Waters indican en su estudio que una reevaluación
de la datación y de los registros establece que la antigüedad de Clovis
fue en realidad de 11.050 a 10.800 años.
Según manifiestan Waters
y Stafford en su informe publicado en "Science", en "Sudamérica,
los seres humanos parecen haber estado presentes hace 12.500 años en Monte
Verde", en Chile, es decir un milenio antes que en Clovis.
Entre 1977 y 1985 el arqueólogo Tom Dillehay, de la Universidad de Kentucky,
excavó el sitio de Monte Verde y allí encontró residuos de
fogatas, restos de chozas y herramientas filosas, además de restos de llamas
del Pleistoceno, pequeños mamíferos, pescados y moluscos.
"Los
datos arqueológicos muestran ahora que Clovis no representa a los primeros
habitantes de las Américas y que se necesita un nuevo modelo para explicar
el origen de la presencia humana en el continente", según reconocen
Waters y Stafford. |
Febrero de 2007
Redefining the Age
of Clovis: Implications for the Peopling of the Americas
Authors: Michael
R. Waters and Thomas W. Stafford, Jr.
Link: Click here 
Science
23 February 2007:
Vol. 315. no. 5815, pp. 1122 - 1126
DOI: 10.1126/science.1137166 Abstract
The Clovis complex is considered to be the oldest unequivocal evidence of humans
in the Americas, dating between 11,500 and 10,900 radiocarbon years before the
present (14C yr B.P.). Adjusted 14C dates and a reevaluation of the existing Clovis
date record revise the Clovis time range to 11,050 to 10,800 14C yr B.P. In as
few as 200 calendar years, Clovis technology originated and spread throughout
North America. The revised age range for Clovis overlaps non-Clovis sites in North
and South America. This and other evidence imply that humans already lived in
the Americas before Clovis.
On page 1122 of this week's issue of Science,
researchers use new radiocarbon data to argue that Clovis culture, once thought
to be the progenitor of all later Native American societies, may have flourished
for as little as 2 centuries around 13,000 years ago. The dates put the Clovis
technology a bit later than thought, making it harder to accept that it was the
first in the Americas. | |
Febrero de 2007
Effects of elasticity
on the Rayleigh-Taylor instability: implications for large-scale geodynamics
Authors: Boris J. P. Kaus and Thorsten W. Becker
Link: Click here Abstract
Although parts of the lithosphere may be expected to behave elastically over certain
timescales, this effect is commonly ignored in models of large-scale mantle dynamics.
Recently it has been demonstrated that elasticity, and in particular viscoelasticity,
may have a significant effect on the buckling instability and on the creation
of lithospheric-scale shearzones. It is, however, less clear whether elasticity
also has an effect on mantle convection and density-driven lithospheric instabilities.
The focus of this work is, therefore, to study the effects of elasticity on the
two-layer Rayleigh-Taylor (RT) instability, consisting of a Maxwell viscoelastic
layer overlying a viscous layer of lower density. We analyse this problem by performing
systematic numerical simulations that are compared with newly derived analytical | solutions.
It is demonstrated that elasticity can be important for certain parameter combinations;
it leads to a speedup of the RT instability. The cause for this speedup is that
the RT instability is only sensitive to the viscous fraction of deformation in
the viscoelastic layer. Elasticity reduces the viscous fraction of deformation
at timescales shorter than the Maxwell relaxation time tM(tM = µ/G, where
µ is the viscosity and G the elastic shear module). For plate tectonics
on Earth, the parameters are such that the effect of elasticity on instability
growth is negligible for most boundary conditions. Whereas elasticity does not
(or only slightly) change the timescales for lithospheric detachment of the upper
mantle, it does significantly alter the response and stress build-up in the overlying
crust. Numerical simulations illustrate this effect for lithospheric detachment
and show that peak stresses in a viscoelastic crust are smaller than stresses
that develop in a viscous crust. Moreover, if the timescale for delamination of
the mantle lithosphere is equal or smaller than the Maxwell relaxation time of
the crust, the topography of the crust is increased compared to viscous models. |
|
Febrero de 2007
Imaging of buried
3D objects by using electrical profiling methods with GPR and 3D geoelectrical
measurements
Authors: Zsuzsanna Nyári and Ali Ismet Kanli
Link:
Click here Abstract
Detection of near-surface buried objects is usually carried out along one or more
survey lines and processed by 2D inversion algorithms. Problems arise if the target
is three dimensional. The effect of the 3D objects near the profile strongly depends
on the type of the electrode array, and the three-dimensional object located beneath
the survey line is also mapped differently to the 2D plane. The indications of
3D objects on a 2D resistivity section depend on each electrode configuration.
Theoretical calculations above 3D objects have been carried out in order to examine
the problems arising from 2D inversion in the cases of four different electrode
arrays (pole-pole, dipole-dipole, equatorial dipole, Wenner alpha). The results
of the modelling processes give some useful information which can make it possible
to rank the examined arrays from the point of sensitivity for 3D effects and the
quality of imaging the object. Test measurements have been carried out in order
to prove the conclusions of the theoretical investigations. Two perpendicular
survey lines have been measured above a buried cellar using dipole-dipole, pole-pole
and Wenner arrays. The 2D pseudosections were compared with the results of the
GPR and 3D geoelectric measurements. | |
Febrero de 2007
Mechanism of deep
low frequency earthquakes: Further evidence that deep non-volcanic tremor is generated
by shear slip on the plate interface
Authors: Satoshi Ide, David R. Shelly
et al
Link: Click here Abstract
We study the mechanism of low frequency earthquakes (LFEs) in the Nankai Trough
in western Shikoku, Japan. Precise locations have previously suggested that they
represent shear slip on the plate boundary. In this paper we examine the mechanism
of these events. Due to the low signal-to-noise ratio, we analyze stacked LFE
waveforms and compare them with the waveforms of nearby earthquakes of known mechanism
within the subducting Philippine Sea Plate. Analysis of both the focal mechanism
using P-wave first-motions and the moment tensor using S waveforms indicates that
LFEs represent shear slip on a low-angle thrust fault dipping to the northwest,
namely the plate interface. Together with reports that deep tremor consists of
a swarm of LFEs, our results suggest that deep tremor is generated directly by
shear slip on the plate interface, and as such represents a seismic signature
of the accompanying slow slip events. | |
Febrero de 2007
Energy: That's oil,
folks...
Author: Alexandra Witze
Link: Click here Abstract
Will world oil production flow freely for decades yet, or are we tapping the bottom
of the barrel already? In the 'peak oil' debate there's no middle ground. A steady
stream of 'authoritative' reports has put the opposing cases eloquently and there
is little sign of a consensus.
Optimists see oil gushing for decades; pessimists
see the planet's energy future already drying up. |
| Febrero
de 2007
Spatial coupling of nitrogen inputs and losses in the ocean
Authors: Curtis Deutsch, Jorge L. Sarmiento et al
Link: Click here Abstract
Nitrogen fixation is crucial for maintaining biological productivity in the oceans,
as it replaces biologically available nitrogen lost through denitrification, in
which nitrites and nitrates are converted to nitrogen gas. The factors that control
this process remain uncertain, but there is a widely held view that the rate of
supply of iron to nitrogen-fixing bacteria is rate limiting. A new approach to
the problem, which derives fixation rates from nutrient distributions in an ocean
circulation model, has arrived at a surprising finding: the nitrogen fixation
rate is highest in the Pacific Ocean, where the supply of iron from the atmosphere
is low and denitrification rates high, and lowest in the Atlantic, where iron
is more plentiful. This suggests that oceanic nitrogen fixation is not tied to
iron levels, but is stimulated by denitrification, helping to stabilize the oceanic
reservoir of fixed nitrogen.
Some of our suppositions about the marine nitrogen
cycle may be wrong. An innovative analysis of nutrients at the ocean's surface
reveals a feedback mechanism that might hold the whole cycle in balance. |
|
Febrero de 2007
Space geodesy validation
of the global lithospheric flow
Authors: M. Crespi, M. Cuffaro et al
Link: Click here Abstract
Space geodesy data are used to verify whether plates move chaotically or rather
follow a sort of tectonic mainstream. While independent lines of geological evidence
support the existence of a global ordered flow of plate motions that is westerly
polarized, the Terrestrial Reference Frame (TRF) presents limitations in describing
absolute plate motions relative to the mantle. For these reasons we jointly estimated
a new plate motions model and three different solutions of net lithospheric rotation.
Considering the six major plate boundaries and variable source depths of the main
Pacific hotspots, we adapted the TRF plate kinematics by global space geodesy
to absolute plate motions models with respect to the mantle. All three reconstructions
confirm (i) the tectonic mainstream and (ii) the net rotation of the lithosphere.
We still do not know the precise trend of this tectonic flow and the velocity
of the differential rotation. However, our results show that assuming faster Pacific
motions, as the asthenospheric source of the hotspots would allow, the best lithospheric
net rotation estimate is 13.4 ± 0.7 cm yr 1. This superfast solution seems
in contradiction with present knowledge on the lithosphere decoupling, but it
matches remarkably better with the geological constraints than those retrieved
with slower Pacific motion and net rotation estimates. Assuming faster Pacific
motion, it is shown that all plates move orderly 'westward' along the tectonic
mainstream at different velocities and the equator of the lithospheric net rotation
lies inside the corresponding tectonic mainstream latitude band (˜ ±7°),
defined by the 1s confidence intervals. | |
Febrero de 2007
Adaptive wavelets
for analyzing dispersive seismic waves
Authors: A. Kritski, A. P. Vincent
et al
Link: Click here Abstract
Our primary objective is to develop an efficient and accurate method for analyzing
time series with a multiscale character. Our motivation stems from the studies
of the physical properties of marine sediment (stiffness and density) derived
from seismic acoustic records of surface/interface waves along the water-seabed
boundary. These studies depend on the dispersive characteristics of water-sediment
surface waves. To obtain a reliable retrieval of the shear-wave velocities, we
need a very accurate time-frequency record of the surface waves. Such a time-frequency
analysis is best carried out by a wavelet-transform of the seismic records. We
have employed the wavelet crosscorrelation technique for estimating the shear-wave
propagational parameters as a function of depth and horizontal distance. For achieving
a greatly improved resolution in time-frequency space, we have developed a new
set of adaptive wavelets, which are driven by the data. This approach is based
on a Karhunen-Loeve (KL) decomposition of the seismograms. This KL decomposition
allows us to obtain a set of wavelet functions that are naturally adapted to the
scales of the surface-wave modes. We demonstrate the superiority of these adaptive
wavelets over standard wavelets in their ability to simultaneously discriminate
the different surface-wave modes. The results can also be useful for imaging and
statistical data analysis in exploration geophysics and in other disciplines in
the environmental sciences. | |
Febrero de 2007
Development
of a software package for visualization of three-dimensional mass distributions
and forward gravity modelling
Author: Johannes Fellner
Link: Click
here Abstract
This paper describes the development of a software package, called GEOCOM, for
the visualization of three-dimensional mass distributions and forward gravity
modelling. The developed software package is intended for use in geophysical and
geodetic fields and represents a powerful tool for scientific research within
the scope of forward gravity modelling. The development process was based on the
high-level programming language MATLAB®. In order to guarantee an efficient
handling of numerous functions, the whole software program is based on several
graphical user interfaces (GUIs), which provide an interactive environment for
data visualization, three-dimensional mass modelling and numeric gravity computation.
The density application enables user-defined topographical as well as isostatic
density models based on a layer-oriented input scheme. The subdivision of the
density model into volumes of discrete elements (right rectangular prisms) permits
the usage of an analytical solution with respect to the calculation of the gravitational
attraction of each element. Focusing on several numerical case studies, which
demonstrate topographic, as well as topographic-isostatic density models, this
paper shows possible software applications using the developed software package
GEOCOM. | |
Febrero de 2007
Magnetic probability
tomography for environmental purposes: test measurements and field applications
Authors: D Chianese and V Lapenna
Link: Click here Abstract
A new tomographic technique for magnetic data inversion in near-surface geophysical
investigations is presented and discussed. It represents a powerful tool for analysing
magnetic profiles and/or maps obtained during geophysical surveys carried out
for environmental and engineering applications. The inversion technique is based
on the cross-correlation integral between the magnetic field measured on earth
surface and the theoretical magnetic field produced by an unit scanner dipole
positioned in the subsoil at the nodes of a regular grid. The cross-correlation
values are then plotted in a 2D map, giving as a result a probability distribution
of finding the magnetic dipoles at the selected depth, named dipolar occurrence
probability (DOP). The skill of the tomographic technique has been evaluated using
theoretical simulations, laboratory experiments with controlled magnetic sources
and field measurements for environmental monitoring. Our findings demonstrated
that the DOP probabilistic function allows us to localize the presence of metallic
buried bodies (hunks), or objects characterized by high magnetic susceptibility
contrasts. | |
Febrero de 2007
2D joint inversion
of dc and scalar audio-magnetotelluric data in the evaluation of low enthalpy
geothermal fields
Authors: Fernando A Monteiro Santos, António
R Andrade Afonso et al
Link: Click here Abstract
Audio-magnetotelluric (AMT) and resistivity (dc) surveys are often used in environmental,
hydrological and geothermal evaluation. The separate interpretation of those geophysical
data sets assuming two-dimensional models frequently produces ambiguous results.
The joint inversion of AMT and dc data is advocated by several authors as an efficient
method for reducing the ambiguity inherent to each of those methods. This paper
presents results obtained from the two-dimensional joint inversion of dipole-dipole
and scalar AMT data acquired in a low enthalpy geothermal field situated in a
graben. The joint inverted models show a better definition of shallow and deep
structures. The results show that the extension of the benefits using joint inversion
depends on the number and spacing of the AMT sites. The models obtained from experimental
data display a low resistivity zone (<20 O m) in the central part of the graben
that was correlated with the geothermal reservoir. The resistivity distribution
models were used to estimate the distribution of the porosity in the geothermal
reservoir applying two different approaches and considering the clay minerals
effect. The results suggest that the maximum porosity of the reservoir is not
uniform and might be in the range of 12% to 24%. | |
Febrero de 2007
An improved nearly
analytical discrete method: an efficient tool to simulate the seismic response
of 2-D porous structures
Authors: Dinghui Yang, Guojie Song et al
Link: Click here Abstract
The nearly analytic discrete method (NADM) for acoustic and elastic waves in porous
elastic media is a perturbation method recently proposed by Yang et al (2006a
Commun. Comput. Phys. 1 528-47). This method uses the truncated Taylor series
expansion to approximate the time derivatives and the local high-order interpolation
to approximate the spatial high-order derivatives by simultaneously using the
displacements and its gradients and the velocity. As a result, it can suppress
effectively numerical dispersions caused by the discretizing the wave equations
when too-coarse grids are used. In this paper, we present an improved nearly-analytic
discrete method (INADM) for the porous case. We compare numerically the error
of the INADM with those of the original NADM and the so-called Lax-Wendroff correction
(LWC) schemes for 1-D and 2-D cases, and give the wave-field modelling in 2-D
porous isotropic and anisotropic media. We show that, compared with the original
NADM, the INADM for the 2-D case can reduce significantly the storage space and
increase time accuracy, while the space accuracy remains the same as that of the
original one. Numerical experiments show that the error of the INADM for the porous
case is less than those of the NADM and the fourth-order LWC scheme. The three-component
seismic wave-fields in the 2-D porous isotropic medium are compared with those
obtained by using the NADM, the LWC method, and exact solutions. Several characteristics
of waves propagating in porous anisotropic media, computed by the INADM, are also
reported in this study. Promising numerical results illustrate that the INADM
provides a useful tool for large-scale porous problems and it can effectively
suppress numerical dispersions | |
Febrero de 2007
Well temperature
testing-an extension of Slider's method
Authors: I M Kutasov and L V Eppelbaum
Link: Click here Abstract
A new technique has been developed for determination of the formation thermal
conductivity, skin factor and contact thermal resistance for boreholes where the
temperature recovery process after drilling operations is not completed. Slider
suggested a technique for analysing transient pressure tests when conditions are
not constant. We extend Slider's method for transient temperature well tests.
It assumes that the volumetric heat capacity of formations is known, and the instantaneous
heater's wall temperature and time data are available for a cylindrical probe
with a constant heat flow rate placed in a borehole. A semi-analytical equation
is used to approximate the dimensionless wall temperature of the heater. A simulated
example is presented to demonstrate the data processing procedure. |
| Febrero
de 2007
A 2D numerical study of the effect of particle shape and orientation
on resistivity in shallow formations
Authors: Etienne Rey and Denis Jongmans
Link: Click here Abstract
Surficial heterogeneous soils such as till, alluvial fans, or slope deposits are
difficult to characterize by geotechnical tests because of the presence of decimeter-
to meter-sized pebbles or rocks. The effective resistivity of such two-component
media composed of a percentage of resistive particles embedded in a conductive
matrix is given by the Bussian's equation. The application of this equation allows
the concentration of resistive particles to be determined if the resistivity values
of each component and of the mixture, as well as the cementation exponent m, are
known. However, previous theoretical and experimental studies have shown that
the effective resistivity is affected by the shape of the particles. The objective
of this study is to numerically determine the 2D effects of particle shape and
orientation on the resistivity. Two configurations have been considered in the
finite element (FE) modeling: laboratory-like measurements and field layout. For
circular particles, the numerical results fit the Bussian's equation with an exponent
m of 2. Aligned elongated particles induce an anisotropy which can raise or diminish
the exponent m, depending on the particle orientation and the tortuosity of the
current paths. Field experiment simulations showed that m varies between 2.5 and
3.1 for an aspect ratio of 5 and that anisotropy resulting from the particle shape
has little effect (m close to 2) when this ratio is lower than 2.5. This increase
of m with the aspect ratio is in agreement with both theoretical models and experimental
studies. For laboratory measurement simulations, m values vary between 1.3 and
4 for a particle aspect ratio of 5, whatever the resistivity contrast between
the particles and the matrix. The difference in results between the two configurations
is explained by the paradox of anisotropy. |
| Febrero
de 2007
Physical modeling and analysis of P-wave attenuation anisotropy
in transversely isotropic media
Authors: Yaping Zhu, Ilya Tsvankin et
al
Link: Click here Abstract
Anisotropic attenuation can provide sensitive attributes for fracture detection
and lithology discrimination. This paper analyzes measurements of the P-wave attenuation
coefficient in a transversely isotropic sample made of phenolic material. Using
the spectral-ratio method, we estimate the group (effective) attenuation coefficient
of P-waves transmitted through the sample for a wide range of propagation angles
(from 0° to 90°) with the symmetry axis. Correction for the difference
between the group and phase angles and for the angular velocity variation help
us to obtain the normalized phase attenuation coefficient governed by the Thomsen-style
attenuation-anisotropy parameters Q and Q. Whereas the symmetry axis of the angle-dependent
coefficient practically coincides with that of the velocity function, the magnitude
of the attenuation anisotropy far exceeds that of the velocity anisotropy. The
quality factor Q increases more than tenfold from the symmetry axis (slow direction)
to the isotropy plane (fast direction). Inversion of the coefficient using the
Christoffel equation yields large negative values of the parameters Q and Q. The
robustness of our results critically depends on several factors, such as the availability
of an accurate anisotropic velocity model and adequacy of the homogeneous concept
of wave propagation, as well as the choice of the frequency band. The methodology
discussed here can be extended to field measurements of anisotropic attenuation
needed for AVO (amplitude-variation-with-offset) analysis, amplitude-preserving
migration, and seismic fracture detection. | |
Febrero de 2007
Amplitude-variation-with-offset,
elastic-impedence, and wave-equation synthetics - A modeling study
Author:
Subhashis Mallick
Link: Click here Abstract
Amplitude-variation-with-offset (AVO) and elastic-impedance (EI) analysis use
an approximate plane P-wave reflection coefficient as a function of angle of incidence.
AVO and EI both can be used in a three-term or a two-term formulation. This study
uses synthetic data to demonstrate that the P-wave primary reflections at large
offsets can be contaminated by reflections from other wave modes that can affect
the quality of three-term AVO or EI results. The coupling of P-waves and S-waves
in seismic-wave propagation through finely layered media generates the interfering
wave modes. A methodology such as prestack-wave-equation modeling can properly
account for these coupling effects. Both AVO and EI also assume a convolutional
model whose accuracy decreases as incidence angles increase. On the other hand,
wave-equation modeling is based on the rigorous solution to the wave equation
and is valid for any incidence angle. Because wave interference is minimal at
small angles, a two-term AVO/EI analysis that restricts input from small angles
is likely to give more reliable parameter estimates than a three-term analysis.
A three-term AVO/EI analysis should be used with caution and should be calibrated
against well data and other data before being used for quantitative analysis. |
|
Febrero de 2007
On the relationship
between iron concentration and induced polarization in marsh soils
Authors:
Nasser Mansoor and Lee Slater
Link: Click here Abstract
Induced polarization (IP) measurements (0.1-1000 Hz) were conducted on seventeen
clay and peat marsh soils that were subsequently analyzed for heavy metal concentrations,
moisture content, organic matter, porosity, specific surface area, and pore fluid
conductivity. A Cole-Cole model was fit to each sample and model parameters analyzed
in terms of physicochemical properties. We found a linear relation between the
normalized chargeability (mn) and estimated surface area to pore volume (Sp) when
iron content (ranging from 0.25% to 1.63% by volume) is accounted for as a polarizable
element of the soil. In fact, the dependence of mn on volumetric Fe concentration
per unit volume of the bulk soil is described by a linear relationship with a
correlation coefficient (R2) of 0.94. As Fe concentration is a critical biogeochemical
parameter, our findings suggest that IP measurements may provide a hitherto unrecognized
approach to probing soil geochemistry, iron cycling and anaerobic microbial activity.
Furthermore, our results yield insights into physicochemical controls on IP in
natural soils. | |
Febrero de 2007
On the minimum of
vertical seismic noise near 3 mHz
Authors: W. Zürn and E. Wielandt
Link: Click here Abstract
Acceleration power spectral densities of vertical seismic noise at the best seismic
stations show a minimum near 3 mHz. We suggest that this minimum is caused by
a cancellation near this frequency of Newtonian attraction vs. free air and inertial
effects exerted by atmospheric phenomena on the sensor mass. Simplistic models
of atmospheric phenomena are used to quantify this effect and examples are shown
for special atmospheric events. | |
Febrero de 2007
Exploring the influence
of the non-dipole field on magnetic records for field reversals and excursions
Authors: Maxwell C. Brown, Richard Holme et al
Link: Click here Abstract
We have used the model CALS7K.2 to explore the possible influence of the time-varying
non-dipole components of the geomagnetic field during field reversals and excursions.
Our findings suggest that non-dipole components could add significant structure
to the field during the reversal and excursion processes. Globally, the main polarity
reversal is variable in duration and rapid reversals on subdecadal timescales
are seen for a small number of locations. The model generates variable reversal
paths; however, there is a longitudinal preference both spatially and, more weakly,
temporally. Directional reversal features are not globally synchronous: some polarity
changes finish before they start elsewhere. Global intensity variations, however,
appear more coherent. We also find support for the idea that field intensity changes
occur some time before and after the major directional changes of the reversal.
Large excursions appear naturally when the axial dipole has been reduced to 20
per cent for the whole time period; however, they are not globally synchronous
or uniform. | |
Enero de 2007
Shock-induced compaction,
melting, and entrapment of atmospheric gases in Martian meteorites
Authors:
P. Beck, T. Ferroir et al
Link: Click here Abstract
The strongest evidence for a Martian origin of the SNC meteorites is the strong
correlation between the rare gas abundances and isotopic compositions in shock-induced
melt pockets, and those measured for the Martian atmosphere. However, the formation
of melt pockets and the entrapment of atmospheric gases remain poorly | understood.
Here we report the presence in the melt pockets of three Martian meteorites of
the hollandite-structured high-pressure polymorph of feldspar. These occurrences
set constraints on the continuum pressure (21-25 GPa), the local temperature increase
(2000-2500 K) and the energy delivered during shock. We then test a mechanism
for melt pocket formation by compaction of pre-existing porous precursors. The
model explains the local temperature increase required for melting and the presence
of an atmospheric component in pores that were connected with the Martian atmosphere
prior to the shock event. | |
Enero de 2007
A climate change index:
Where climate change may be most prominent in the 21st century
Authors:
Michèle B. Baettig, Martin Wild
Link: Click here Abstract
A Climate Change Index (CCI) is developed that is composed of annual and seasonal
temperature and precipitation indicators. These indicators are aggregated to a
single index that is a measure for the strength of future climate change relative
to today's natural variability. The CCI does not | represent
climate impacts. Its aim is to comply with the increasing need of policy makers
to gain a quick overview of complex scientific findings by means of summarized
information. The index is calculated on the basis of three GCM simulations of
the 21st century under the IPCC emission scenarios A2 and B2. The results indicate
that the strongest climate changes by the end of the 21st century, relative to
today's natural variability, will occur in the tropics and in high latitudes (especially
in the northern hemisphere). The CCI is also calculated on a country basis, allowing
for comparison with social and economic country indicators. | |