5,877 research outputs found
Predicting the movements of permanently installed electrodes on an active landslide using time-lapse geoelectrical resistivity data only
If electrodes move during geoelectrical resistivity monitoring and their new positions are not incorporated in the inversion, then the resulting tomographic images exhibit artefacts that can obscure genuine time-lapse resistivity changes in the subsurface. The effects of electrode movements on time-lapse resistivity tomography are investigated using a simple analytical model and real data. The correspondence between the model and the data is sufficiently good to be able to predict the effects of electrode movements with reasonable accuracy. For the linear electrode arrays and 2D inversions under consideration, the data are much more sensitive to longitudinal than transverse or vertical movements. Consequently the model can be used to invert the longitudinal offsets of the electrodes from their known baseline positions using only the time-lapse ratios of the apparent resistivity data. The example datasets are taken from a permanently installed electrode array on an active lobe of a landslide. Using two sets with different levels of noise and subsurface resistivity changes, it is found that the electrode positions can be recovered to an accuracy of 4 % of the baseline electrode spacing. This is sufficient to correct the artefacts in the resistivity images, and provides for the possibility of monitoring the movement of the landslide and its internal hydraulic processes simultaneously using electrical resistivity tomography only
Cross-Correlation Studies between CMB Temperature Anisotropies and 21 cm Fluctuations
During the transition from a neutral to a fully reionized universe,
scattering of cosmic microwave background (CMB) photons via free-electrons
leads to a new anisotropy contribution to the temperature distribution. If the
reionization process is inhomogeneous and patchy, the era of reionization is
also visible via brightness temperature fluctuations in the redshifted 21 cm
line emission from neutral Hydrogen. Since regions containing electrons and
neutral Hydrogen are expected to trace the same underlying density field, the
two are (anti) correlated and this is expected to be reflected in the
anisotropy maps via a correlation between arcminute-scale CMB temperature and
the 21 cm background. In terms of the angular cross-power spectrum,
unfortunately, this correlation is insignificant due to a geometric
cancellation associated with second order CMB anisotropies. The same
cross-correlation between ionized and neutral regions, however, can be studied
using a bispectrum involving large scale velocity field of ionized regions from
the Doppler effect, arcminute scale CMB anisotropies during reionization, and
the 21 cm background. While the geometric cancellation is partly avoided, the
signal-to-noise ratio related to this bispectrum is reduced due to the large
cosmic variance related to velocity fluctuations traced by the Doppler effect.
Unless the velocity field during reionization can be independently established,
it is unlikely that the correlation information related to the relative
distribution of ionized electrons and regions containing neutral Hydrogen can
be obtained with a combined study involving CMB and 21 cm fluctuations.Comment: 10 pages, 3 figure
Issues for the Next Generation of Galaxy Surveys
I argue that the weight of the available evidence favours the conclusions
that galaxies are unbiased tracers of mass, the mean mass density (excluding a
cosmological constant or its equivalent) is less than the critical Einstein-de
Sitter value, and an isocurvature model for structure formation offers a viable
and arguably attractive model for the early assembly of galaxies. If valid
these conclusions complicate our work of adding structure formation to the
standard model for cosmology, but it seems sensible to pay attention to
evidence.Comment: 14 pages, 3 postscript figures, uses rspublic.st
The Effect of Crystallization on the Pulsations of White Dwarf Stars
We consider the pulsational properties of white dwarf star models with
temperatures appropriate for the ZZ Ceti instability strip and with masses
large enough that they should be substantially crystallized. Our work is
motivated by the existence of a potentially crystallized DAV, BPM 37093, and
the expectation that digital surveys in progress will yield many more such
massive pulsators.
A crystallized core makes possible a new class of oscillations, the torsional
modes, although we expect these modes to couple at most weakly to any motions
in the fluid and therefore to remain unobservable. The p-modes should be
affected at the level of a few percent in period, but are unlikely to be
present with observable amplitudes in crystallizing white dwarfs any more than
they are in the other ZZ Ceti's. Most relevant to the observed light variations
in white dwarfs are the g-modes. We find that the kinetic energy of these modes
is effectively excluded from the crystallized cores of our models. As
increasing crystallization pushes these modes farther out from the center, the
mean period spacing between radial overtones increases substantially with the
crystallized mass fraction. In addition, the degree and structure of mode
trapping is affected. The fact that some periods are strongly affected by
changes in the crystallized mass fraction while others are not suggests that we
may be able to disentangle the effects of crystallization from those due to
different surface layer masses.Comment: 18 pages, 5 figures, accepted on 1999 July 2 for publication in the
Astrophysical Journa
The Extended Water Footprint and illegal groundwater use in the Upper Guadiana Basin (Spain)
Tablas de Daimiel National Park is located in the Upper Guadiana Basin and represents one of the largest and most important wetlands in Europe. The long term ecological integrity of this wetland is inherently associated with the maintenance of a shallow groundwater table, namely the Western Mancha aquifer (WMA) or Aquifer 23. The intensive use of groundwater, mainly for irrigation, has led over the last decades to deep socio‐economic changes. Such intensive use has also lowered the water table of Aquifer 23, drastically reducing the flooded area of the wetland and threatening its ecological integrity. A number of plans and measures have been developed and implemented since the declaration of overexploitation of Aquifer 23 in the year 1987. The most recent one is the Special Plan for the Upper Guadiana (SPUG), approved in 2008. This Plan is the main measure to comply with achieving the objective of good quantitative and qualitative status required under the Water Framework Directive (2000). This paper offers a new type of integrated analysis which allows assessing under a common lens the physical, economic and social dimensions of groundwater use in the area. The first objective is to calculate the groundwater footprint of agricultural production in the Upper Guadiana basin and its evolution during 2000‐2008. For this purpose, we have applied the Extended Water Footprint (EWF) methodology ‐a novel approach based on the classical Water Footprint (WF) approach‐ that includes an assessment of the water productivity from an economic and social perspective. Compared to the classical WF, the EWF allows for a more complete overview of the sector, providing new insights for policy decisions (e.g. to define options and possibilities on water re‐allocation in order to achieve both better ecosystem conservation and social equity). The second objective is to use the EWF to compare the existing authorized and non‐authorized or illegal use of water. This allows us to discuss current initiatives by public authorities in relation to the existing frame of water right
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