493 research outputs found
Constant Q and a fractal, stratifed Earth
Frequency dependent measurements of the quality factor Q typically show a
constant behaviour for low frequencies and a positive power law dependence for higher
frequencies. In particular, the constant Q pattern is usually explained using intrinsic
attenuation models due to anelasticity with either a single or multiple superposed
relaxation mechanisms - each with a particular resonance peak.
However, in this study, I show using wave localisation theory that a constant
Q may also be due to apparent attenuation due to scattering losses. Namely, this
phenomenon occurs if the Earth displays fractal characteristics. Moreover, if fractal
characteristics exist over a limited range of scales only, even an absorption band can
be created - in accordance with observations. This indicates that it may be very
difficult to distinguish between intrinsic and scattering attenuation on the basis of
frequency dependent measurements of the quality factor only
Estimating anisotropy parameters and traveltimes in the tau-p domain
The presence of anisotropy influences many aspects of
seismic wave propagation and has therefore implications
for conventional processing schemes. To estimate the
anisotropy, we need both forward modelling and inversion
tools. Exact forward modelling in anisotropic media
is generally done by raytracing. However, we present a
new and fast method, using the tau-p transform, to calculate
exact P and SV reflection moveout curves in stratified,
laterally homogeneous, anisotropic media which
requires no ray tracing. Results are exact even if the
SV-waves display cusps. In addition, we show how the
same method can be used for parameter estimation.
Since inversion for anisotropic parameters is very
nonunique, we develop expressions requiring only a reduced
number of parameters. Nevertheless, predictions
using these expressions are more accurate than Taylor
series expansions and are also able to handle cusps in
the SV traveltime curves. In addition, layer stripping is
a linear process. Therefore, both effective (average) and
local (interval) estimates can be obtained
Recognition and reconstruction of coherent energy with application to deep seismic reflection data
Reflections in deep seismic reflection data tend to be
visible on only a limited number of traces in a common
midpoint gather. To prevent stack degeneration,
any noncoherent reflection energy has to be removed.
In this paper, a standard classification technique in
remote sensing is presented to enhance data quality. It
consists of a recognition technique to detect and extract
coherent energy in both common shot gathers and fi-
nal stacks. This technique uses the statistics of a picked
seismic phase to obtain the likelihood distribution of its
presence. Multiplication of this likelihood distribution
with the original data results in a “cleaned up” section.
Application of the technique to data from a deep seismic
reflection experiment enhanced the visibility of all
reflectors considerably.
Because the recognition technique cannot produce an
estimate of “missing” data, it is extended with a reconstruction
method. Two methods are proposed: application
of semblance weighted local slant stacks after recognition,
and direct recognition in the linear tau-p domain.
In both cases, the power of the stacking process to increase the signal-to-noise ratio is combined with the direct selection of only specific seismic phases. The joint
application of recognition and reconstruction resulted in
data images which showed reflectors more clearly than
application of a single technique
Traveltime and conversion-point computations and parameter estimation in layered, anisotropic media by tau-p transform
Anisotropy influences many aspects of seismic wave
propagation and, therefore, has implications for conventional
processing schemes. It also holds information
about the nature of the medium. To estimate anisotropy,
we need both forward modeling and inversion tools. Forward
modeling in anisotropic media is generally done
by ray tracing. We present a new and fast method using
the tau-p transform to calculate exact reflection-moveout
curves in stratified, laterally homogeneous, anisotropic
media for all pure-mode and converted phases which requires
no conventional ray tracing. Moreover, we obtain
the common conversion points for both P-SV and P-SH
converted waves. Results are exact for arbitrary strength
of anisotropy in both HTI and VTI media (transverse
isotropy with a horizontal or vertical symmetry axis,
respectively).
Since inversion for anisotropic parameters is a highly
nonunique problem, we also develop expressions describing
the phase velocities that require only a reduced
number of parameters for both types of anisotropy. Nevertheless,
resulting predictions for traveltimes and conversion
points are generally more accurate than those
obtained using the conventional Taylor-series expansions.
In addition, the reduced-parameter expressions
are also able to handle kinks or cusps in the SV traveltime
curves for either VTI or HTI symmetry
Short-time homomorphic wavelet estimation
Successful wavelet estimation is an essential step for seismic methods like
impedance inversion, analysis of amplitude variations with offset and full
waveform inversion. Homomorphic deconvolution has long intrigued as a
potentially elegant solution to the wavelet estimation problem. Yet a
successful implementation has proven difficult. Associated disadvantages like
phase unwrapping and restrictions of sparsity in the reflectivity function
limit its application. We explore short-time homomorphic wavelet estimation as
a combination of the classical homomorphic analysis and log-spectral averaging.
The introduced method of log-spectral averaging using a short-term Fourier
transform increases the number of sample points, thus reducing estimation
variances. We apply the developed method on synthetic and real data examples
and demonstrate good performance.Comment: 13 pages, 5 figures. 2012 J. Geophys. Eng. 9 67
Modeling of the in situ state of stress in elastic layered rock subject to stress and strain-driven tectonic forces
In this study we describe and compare eight different
strategies to predict the depth variation of stress within a layered rock
formation. This reveals the inherent uncertainties in stress prediction from
elastic properties and stress measurements, as well as the geologic
implications of the different models. The predictive strategies are based on
well log data and in some cases on in situ stress measurements, combined
with the weight of the overburden rock, the pore pressure, the depth
variation in rock properties, and tectonic effects. We contrast and compare
stresses predicted purely using theoretical models with those constrained by
in situ measurements. We also explore the role of the applied boundary
conditions that mimic two fundamental models of tectonic effects, namely the
stress- or strain-driven models. In both models, layer-to-layer tectonic
stress variations are added to initial predictions due to vertical variation
in rock elasticity, consistent with natural observations, yet describe
very different controlling mechanisms. Layer-to-layer stress variations are
caused by either local elastic strain accommodation for the strain-driven
model, or stress transfers for the stress-driven model. As a consequence,
stress predictions can depend strongly on the implemented prediction
philosophy and the underlying implicit and explicit assumptions, even for
media with identical elastic parameters and stress measurements. This
implies that stress predictions have large uncertainties, even if local
measurements and boundary conditions are honored
Star Formation in Extreme Environments: The Effects of Cosmic Rays and Mechanical Heating
Context: Molecular data of extreme environments, such as Arp 220, but also
NGC 253, show evidence for extremely high cosmic ray (CR) rates (10^3-10^4 *
Milky Way) and mechanical heating from supernova driven turbulence.
Aims: The consequences of high CR rates and mechanical heating on the
chemistry in clouds are explored.
Methods: PDR model predictions are made for low, n=10^3, and high, n=10^5.5
cm^-3, density clouds using well-tested chemistry and radiation transfer codes.
Column densities of relevant species are discussed, and special attention is
given to water related species. Fluxes are shown for fine-structure lines of O,
C+, C, and N+, and molecular lines of CO, HCN, HNC, and HCO+. A comparison is
made to an X-ray dominated region model.
Results: Fine-structure lines of [CII], [CI], and [OI] are remarkably similar
for different mechanical heating and CR rates, when already exposed to large
amounts of UV. HCN and H2O abundances are boosted for very high mechanical
heating rates, while ionized species are relatively unaffected. OH+ and H2O+
are enhanced for very high CR rates zeta > 5 * 10^-14 s^-1. A combination of
OH+, OH, H2O+, H2O, and H3O+ trace the CR rates, and are able to distinguish
between enhanced cosmic rays and X-rays.Comment: 13 pages, 8 figures, A&A accepte
A psychosocial and lifestyle assessment for childhood obesity:A scoping literature review and focus groups with healthcare professionals
Varied sensitivity to therapy of HIV-1 strains in CD4+ lymphocyte sub-populations upon ART initiation
<p>Abstract</p> <p>Background</p> <p>Although antiretroviral therapy (ART) has proven its success against HIV-1, the long lifespan of infected cells and viral latency prevent eradication. In this study we analyzed the sensitivity to ART of HIV-1 strains in naĂŻve, central memory and effector memory CD4<sup>+ </sup>lymphocyte subsets.</p> <p>Methods</p> <p>From five patients cellular HIV-1 infection levels were quantified before and after initiation of therapy (2-5 weeks). Through sequencing the C2V3 region of the HIV-1 gp120 envelope, we studied the effect of short-term therapy on virus variants derived from naĂŻve, central memory and effector memory CD4<sup>+ </sup>lymphocyte subsets.</p> <p>Results</p> <p>During short-term ART, HIV-1 infection levels declined in all lymphocyte subsets but not as much as RNA levels in serum. Virus diversity in the naĂŻve and central memory lymphocyte populations remained unchanged, whilst diversity decreased in serum and the effector memory lymphocytes. ART differentially affected the virus populations co-circulating in one individual harboring a dual HIV-1 infection. Changes in V3 charge were found in all individuals after ART initiation with increases within the effector memory subset and decreases found in the naĂŻve cell population.</p> <p>Conclusions</p> <p>During early ART virus diversity is affected mainly in the serum and effector memory cell compartments. Differential alterations in V3 charge were observed between effector memory and naĂŻve populations. While certain cell populations can be targeted preferentially during early ART, some virus strains demonstrate varied sensitivity to therapy, as shown from studying two strains within a dual HIV-1 infected individual.</p
- …