1,720,961 research outputs found
Common-reflection-surface imaging of shallow and ultrashallow reflectors
Full text linkWe analyzed the feasibility of the common-reflection-surface
(CRS) stack for near-surface surveys as an alternative to the conventional
common midpoint (CMP) stacking procedure. The
data-driven, less user-interactive CRS method could be more
cost efficient for shallow surveys, where the high sensitivity
to velocity analysis makes data processing a critical step. We
compared the results for two field data sets collected to image
shallow and ultrashallow reflectors: an example of shallow Pwave
reflection for targets in the first few hundred meters,
and an example of SH-wave reflection for targets in the first
10 m. By processing the shallow P-wave records using the
CMP method, we imaged several nearly horizontal reflectors
with onsets from 60 to about 250 ms. The CRS stack produced
a stacked section more suited for a subsurface interpretation,
without any preliminary formal and time-consuming velocity analysis, because the imaged reflectors possessed greater coherency
and lateral continuity. With CMP processing of the SHwave
records, we imaged a dipping bedrock interface below
four horizontal reflectors in unconsolidated, very low velocity
sediments. The vertical and lateral resolution was very high, despite
the very shallow depth: the image showed the pinchout of
two layers at less than 10 m depth. The numerous traces used by
the CRS stack improved the continuity of the shallowest reflector,
but the deepest overburden reflectors appear unresolved,
with not well-imaged pinchouts. Using the kinematic wavefield
attributes determined for each stacking operation, we retrieved
velocity fields fitting the stacking velocities we had estimated in
the CMP processing. The use of CRS stack could be a significant
step ahead to increase the acceptance of the seismic reflection
method as a routine investigation method in shallow and
ultrashallow seismics
Real-time or Full-precision CRS Imaging Using a Cloud Computing Portal: Multi-offset GPR and Shear-wave Reflection Data
No full textThe presented cloud-computing portal allows automated imaging of near-surface structures either in full-precision using a global simultaneous search for the best fitting spatial CRS stacking operator or in real-time using a pragmatic sequence of line searches. The simultaneous search even though computationally very expensive becomes necessary when the pragmatic search fails due to strongly varying velocities, low CMP fold and high noise levels. In our first case study we apply real-time imaging by reprocessing multi-offset Ground Penetrating-Radar data with a good signal/noise ratio. Generating stacked and prestack time migrated sections, consistent with conventional results, required less than 10 minutes. In our second case study we analyze a challenging SH-wave seismic reflection data, recorded in an urban environment, where a high level of ambient noise and landfills below the sealed surface down to 2 m depth resulting from recent reconstruction work hampered seismic data quality severely. Here, we successfully applied our newly developed full-precision CRS imaging approach in addition to conventional CMP stack aided by VSP. In this urban environment, the relevance of applying both, high resolution SH-wave seismic and VSP surveys was demonstrated. Two recently drilled wells confirm the aquiclude to be found as predicted by the seismic forecast
Real-time imaging and data analysis for shallow seismic data using a cloud-computing portal
No full textIn-field real-time data processing is still today a crucial factor that could definitively boost the success
and spread of shallow seismic reflection and multichannel ground-penetrating radar (GPR)
methods in the near-surface geoscience community, as it allows efficient data acquisition and cost-effective
results, especially for modern surveys generating large volumes of data in a short time. To
help fulfil this need, we present a cloud-computing solution combining the powerful computational
capabilities of a cloud infrastructure with a subsurface imaging workflow based on a parallelized
grid version of the Common-Reflection-Surface (CRS) stack, a macro-velocity model independent
imaging method that is very suitable for real-time imaging, as its data-driven implementation avoids
time-consuming human interaction in prestack velocity analysis. Our portal is accessible from the
field by any mobile computer having a wireless data connection. The user-friendly web-browser
interface allows, already during acquisition, uploading of the recorded data to remote computing
facilities, where a quality control (QC) data analysis report is automatically produced. When the
number of uploaded shot records is large enough to produce a subsurface image, stacking, velocity
model determination and prestack time migration can be performed fast and highly automated,
optionally after applying some preprocessing (e.g., gain, trace balancing and filtering) using the
preprocessing toolbox of the portal. To demonstrate the use of the presented system, we simulate
in-field data processing for an already published shallow seismic data set and compare our results
with the original ones. The data set, consisting in seismic P-wave data, was collected to image
Palaeozoic bedrock at the Flumendosa River Delta, Sardinia (Italy), with a data acquisition set-up
driven by the expected bedrock depth, which, turning out wrong, prevented a detailed velocity
analysis and a good time migration. Using the QC data analysis reports to run the imaging routines
of our cloud portal we produced within less than an half an hour stacked and migrated sections very
close to those published in the original work. These results, if obtained in the field, would have
allowed an immediate update of the experimental set-up. Therefore, we are optimistic that the proposed
cloud-computing solution (or similar systems) can boost the spread of shallow seismic reflection
and multi-offset GPR surveys in near-surface investigations, similar to what happened in the
field of electrical and electromagnetic surveying after suitable real-time imaging and data analysis
systems emerged
CRS stack using global simultaneous multiparameter optimization - A spatial velocity analysis for near-surface data
No full textCRS stacking employs a spatial three-parameter stacking operator that extents in offset and midpoint directions. Since this operator covers several CMP gathers, a large number of traces contribute to every single stacking process, thus generating a large improvement in signal-to-noise ratio. However, the standard implementations use the spatial operator only for stacking and not for the global stacking parameter search. Instead, they rely on a sequence of three one-dimensional searches which decreases the computational effort compared to the simultaneous three-parameter search by two orders of magnitude, but does not always deliver optimal results. Particularly for near-surface data, characterized by low signalto- noise ratio and modest CMP fold, the efficiency gain is not crucial. Considering the huge computing power available today, we propose a new pragmatic search strategy using a spatial two-parameter diffraction operator for the global search, followed by a local optimization using the full CRS operator with the diffraction parameters as initial guess. For shallow shear-wave data, we show that while the computational cost of this hybrid approach reduces by an order of magnitude the results are still very close to those obtained by the full global three-parameter search and far superior to those obtained by three cascaded one-parameter searches
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
