517 research outputs found
Discontinuities without discontinuity: The Weakly-enforced Slip Method
Tectonic faults are commonly modelled as Volterra or Somigliana dislocations
in an elastic medium. Various solution methods exist for this problem. However,
the methods used in practice are often limiting, motivated by reasons of
computational efficiency rather than geophysical accuracy. A typical
geophysical application involves inverse problems for which many different
fault configurations need to be examined, each adding to the computational
load. In practice, this precludes conventional finite-element methods, which
suffer a large computational overhead on account of geometric changes. This
paper presents a new non-conforming finite-element method based on weak
imposition of the displacement discontinuity. The weak imposition of the
discontinuity enables the application of approximation spaces that are
independent of the dislocation geometry, thus enabling optimal reuse of
computational components. Such reuse of computational components renders
finite-element modeling a viable option for inverse problems in geophysical
applications. A detailed analysis of the approximation properties of the new
formulation is provided. The analysis is supported by numerical experiments in
2D and 3D.Comment: Submitted for publication in CMAM
Inverting elastic dislocations using the Weakly-enforced Slip Method
Earthquakes cause lasting changes in static equilibrium, resulting in global
deformation fields that can be observed. Consequently, deformation measurements
such as those provided by satellite based InSAR monitoring can be used to infer
an earthquake's faulting mechanism. This inverse problem requires a numerical
forward model that is both accurate and fast, as typical inverse procedures
require many evaluations. The Weakly-enforced Slip Method (WSM) was developed
to meet these needs, but it was not before applied in an inverse problem
setting. Consequently, it was unknown what effect particular properties of the
WSM, notably its inherent continuity, have on the inversion process. Here we
show that the WSM is able to accurately recover slip distributions in a
Bayesian-inference setting, provided that data points in the vicinity of the
fault are removed. In a representative scenario, an element size of 2 km was
found to be sufficiently fine to generate a posterior probability distribution
that is close to the theoretical optimum. For rupturing faults a masking zone
of 20 km sufficed to avoid numerical disturbances that would otherwise be
induced by the discretization error. These results demonstrate that the WSM is
a viable forward method for earthquake inversion problems. While our
synthesized scenario is basic for reasons of validation, our results are
expected to generalize to the wider gamut of scenarios that finite element
methods are able to capture. This has the potential to bring modeling
flexibility to a field that if often forced to impose model restrictions in a
concession to computability.Comment: The associated software implementation is openly available in zenodo
at https://doi.org/10.5281/zenodo.507179
Engaging local communities in aquatic resources research and activities: a technical manual
This document is part of a series of 5 technical manuals produced by the Challenge Program Project CP34 “Improved fisheries productivity and management in tropical reservoirs”. The objective of this technical manual is to relay the field experience of a group of scientists who have worked extensively in small fisheries in sub-Sahara Africa and Asia and lay out a series of simple and pragmatic pointers on how to establish and run initiatives for community catch assessment. The manual relies in particular on practical experience gained implementing Project 34 of the Challenge Programme on Water and Food: Improved Fisheries Productivity and Management in Tropical Reservoirs. (PDF contains 26 pages
Engaging local communities in aquatic resources research and activities: a technical manual
This document is part of a series of 5 technical manuals produced by the Challenge Program Project CP34 ôImproved fisheries productivity and management in tropical reservoirsö. The objective of this technical manual is to relay the field experience of a group of scientists who have worked extensively in small fisheries in sub-Sahara Africa and Asia and lay out a series of simple and pragmatic pointers on how to establish and run initiatives for community catch assessment. The manual relies in particular on practical experience gained implementing Project 34 of the Challenge Programme on Water and Food: Improved Fisheries Productivity and Management in Tropical Reservoirs.Research, Fishery data
First LOFAR results on galaxy clusters
Deep radio observations of galaxy clusters have revealed the existence of
diffuse radio sources related to the presence of relativistic electrons and
weak magnetic fields in the intracluster volume. The role played by this
non-thermal intracluster component on the thermodynamical evolution of galaxy
clusters is debated, with important implications for cosmological and
astrophysical studies of the largest gravitationally bound structures of the
Universe. The low surface brightness and steep spectra of diffuse cluster radio
sources make them more easily detectable at low-frequencies. LOFAR is the first
instrument able to detect diffuse radio emission in hundreds of massive galaxy
clusters up to their formation epoch. We present the first observations of
clusters imaged by LOFAR and the huge perspectives opened by this instrument
for non-thermal cluster studies.Comment: Proceedings of the 2012 week of the French Society of Astronomy and
Astrophysics (SF2A) held in Nice, June 5th-8t
Clients’ psychosocial communication and midwives’ verbal and nonverbal communication during prenatal counseling for anomaly screening
Objectives: This study focuses on facilitation of clients’ psychosocial communication during prenatal
counseling for fetal anomaly screening. We assessed how psychosocial communication by clients is
related to midwives’ psychosocial and affective communication, client-directed gaze and counseling
duration.
Methods: During 184 videotaped prenatal counseling consultations with 20 Dutch midwives, verbal
psychosocial and affective behavior was measured by the Roter Interaction Analysis System (RIAS). We
rated the duration of client-directed gaze. We performed multilevel analyses to assess the relation
between clients’ psychosocial communication and midwives’ psychosocial and affective communication,
client-directed gaze and counseling duration.
Results: Clients’ psychosocial communication was higher if midwives’ asked more psychosocial questions
and showed more affective behavior (b = 0.90; CI: 0.45–1.35; p < 0.00 and b = 1.32; CI: 0.18–2.47;
p = 0.025, respectively). Clients “psychosocial communication was not related to midwives” clientdirected
gaze. Additionally, psychosocial communication by clients was directly, positively related to the
counseling duration (b = 0.59; CI: 0.20–099; p = 0.004).
Conclusions: In contrast with our expectations, midwives’ client-directed gaze was not related with
psychosocial communication of clients.
Practice implications: In addition to asking psychosocial questions, our study shows that midwives’
affective behavior and counseling duration is likely to encourage client’s psychosocial communication,
known to be especially important for facilitating decision-making
First LOFAR observations at very low frequencies of cluster-scale non-thermal emission: the case of Abell 2256
Abell 2256 is one of the best known examples of a galaxy cluster hosting
large-scale diffuse radio emission that is unrelated to individual galaxies. It
contains both a giant radio halo and a relic, as well as a number of head-tail
sources and smaller diffuse steep-spectrum radio sources. The origin of radio
halos and relics is still being debated, but over the last years it has become
clear that the presence of these radio sources is closely related to galaxy
cluster merger events. Here we present the results from the first LOFAR Low
band antenna (LBA) observations of Abell 2256 between 18 and 67 MHz. To our
knowledge, the image presented in this paper at 63 MHz is the deepest ever
obtained at frequencies below 100 MHz in general. Both the radio halo and the
giant relic are detected in the image at 63 MHz, and the diffuse radio emission
remains visible at frequencies as low as 20 MHz. The observations confirm the
presence of a previously claimed ultra-steep spectrum source to the west of the
cluster center with a spectral index of -2.3 \pm 0.4 between 63 and 153 MHz.
The steep spectrum suggests that this source is an old part of a head-tail
radio source in the cluster. For the radio relic we find an integrated spectral
index of -0.81 \pm 0.03, after removing the flux contribution from the other
sources. This is relatively flat which could indicate that the efficiency of
particle acceleration at the shock substantially changed in the last \sim 0.1
Gyr due to an increase of the shock Mach number. In an alternative scenario,
particles are re-accelerated by some mechanism in the downstream region of the
shock, resulting in the relatively flat integrated radio spectrum. In the radio
halo region we find indications of low-frequency spectral steepening which may
suggest that relativistic particles are accelerated in a rather inhomogeneous
turbulent region.Comment: 13 pages, 13 figures, accepted for publication in A\&A on April 12,
201
Optimized Trigger for Ultra-High-Energy Cosmic-Ray and Neutrino Observations with the Low Frequency Radio Array
When an ultra-high energy neutrino or cosmic ray strikes the Lunar surface a
radio-frequency pulse is emitted. We plan to use the LOFAR radio telescope to
detect these pulses. In this work we propose an efficient trigger
implementation for LOFAR optimized for the observation of short radio pulses.Comment: Submitted to Nuclear Instruments and Methods in Physics Research
Section
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