1,721,003 research outputs found
Focusing inversion techniques applied to electrical resistance tomography in an experimental tank
No full textWe present an algorithm for focusing inversion of electrical resistivity tomography (ERT) data. ERT is a typical example of ill-posed problem. Regularization is the most common way to face this kind of problems; it basically consists in using a priori information about targets to reduce the ambiguity and the instability of the solution. By using the minimum gradient support (MGS) stabilizing functional, we introduce the following geometrical prior information in the reconstruction process: anomalies have sharp boundaries. The presented work is embedded in a project (L.A.R.A.) which aims at the estimation of hydrogeological properties from geophysical investigations. L.A.R.A. facilities include a simulation tank (4 m x 8 m x 1.35 m); 160 electrodes are located all around the tank and used for 3-D ERT. Because of the large number of electrodes and their dimensions, it is important to model their effect in order to correctly evaluate the electrical system response. The forward modelling in the presented algorithm is based on the so-called complete electrode model that takes into account the presence of the electrodes and their contact impedances. In this paper, we compare the results obtained with different regularizing functionals applied on a synthetic model
Focusing inversion techniques applied to electrical resistance tomography in an experimental tank
No full textWe present an algorithm for focusing inversion of electrical resistivity
tomography (ERT) data. ERT is a typical example of ill-posed problem. Regularization is the
most common way to face this kind of problems; it basically consists in using a priori
information about targets to reduce the ambiguity and the instability of the solution. By using
the minimum gradient support (MGS) stabilizing functional, we introduce the following
geometrical prior information in the reconstruction process: anomalies have sharp boundaries.
The presented work is embedded in a project (L.A.R.A.) which aims at the estimation of
hydrogeological properties from geophysical investigations. L.A.R.A. facilities include a
simulation tank (4 m x 8 m x 1.35 m); 160 electrodes are located all around the tank and used
for 3-D ERT. Because of the large number of electrodes and their dimensions, it is important
to model their effect in order to correctly evaluate the electrical system response. The forward
modelling in the presented algorithm is based on the so-called complete electrode model that
takes into account the presence of the electrodes and their contact impedances.
In this paper, we compare the results obtained with different regularizing functionals applied
on a synthetic model.PublishedLiège,Belgiqueope
Focused inversion of Vertical Radar Profile (VRP) travel-time data
No full textThe reconstruction of the GPR velocity vertical profile from vertical radar profile (VRP) traveltime data is a problem with a finite number of measurements and imprecise data, analogous to similar seismic techniques, such as the shallow down-hole test used for S-wave velocity profiling or the vertical seismic profiling (VSP) commonly used in deeper exploration. The uncertainty in data accuracy and the error amplification inherent in deriving velocity estimates from gradients of arrival times make this an example of an ill-posed inverse problem. In the framework of Tikhonov regularization theory, ill-posedness can be tackled by introducing a regularizing functional (stabilizer). The role of this functional is to stabilize the numerical solution by incorporating the appropriate a priori assumptions about the geometrical and/or physical properties of the solution. One of these assumptions could be the existence of sharp boundaries separating rocks with different physical properties. We apply a method based on the minimum support stabilizer to the VRP traveltime inverse problem. This stabilizer makes it possible to produce more accurate profiles of geological targets with compact structure. We compare more traditional inversion results with our proposed compact reconstructions. Using synthetic examples, we demonstrate that the minimum support stabilizer allows an improved recovery of the profile shape and velocity values of blocky targets. We also study the stabilizer behavior with respect to different noise levels and different choices of the reference model. The proposed approach is then applied to real cases where VPRs have been used to derive moisture content profiles as a function of depth. In these real cases, the derived sharper profiles are consistent with other evidence, such as GPR zero-offset profiles, GPR reflections and known locations of the water table
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
Identification of lateral discontinuities via multi-offset phase analysis of surface wave data
No full textSurface wave methods are based on the inversion of observed Rayleigh wave phase-velocity dispersion curves. The goal is to estimate mainly the shear-wave velocity profile of the investigated site. The model used for the interpretation is 1D, hence results obtained wherever lateral variations are present cannot be considered reliable.
In this paper, we study four synthetic models, all with a lateral heterogeneity. When we process the entire corresponding seismograms with traditional f-k approach, the resulting 1D profiles are representative of the subsurface properties averaged over the whole length of the receivers lines. These results show that classical analysis disregards evidences of sharp lateral velocity changes even when they show up in the raw seismograms.
In our research, we implement and test over the same synthetic models, a novel robust automated method to check the appropriateness of 1D model assumption and locate the discontinuities. This new approach is a development of the recent multi-offset phase analysis with the following further advantages: it does not need previous noise evaluation and more than one shot.
Only once the discontinuities are clearly identified, we confidently perform classical f-k dispersion curve extraction and inversion separately on both sides of the discontinuity. Thus the final results, obtained by putting side by side the 1D profiles, are correct 2D reconstructions of the discontinuous S-wave distributions obtained without any additional ad-hoc hypotheses
Focusing inversion technique applied to radar tomographic data
No full textTraveltime tomography is a very effective tool to reconstruct acoustic, seismic or
electromagnetic wave speed distribution. To infer the velocity image of the medium from the
measurements of first arrivals is a typical example of ill-posed problem. In the framework of
Tikhonov regularization theory, in order to replace an ill-posed problem by a well-posed one
and to get a unique and stable solution, a stabilizing functional (stabilizer) has to be
introduced. The stabilizer selects the desired solution from a class of solutions with a specific
physical and/or geometrical property; e.g., the existence of sharp boundaries separating media
with different petrophysical parameters. Usually stabilizers based on maximum smoothness
criteria are used during the inversion process; in these cases the solutions provide smooth
images which, in many situations, do not describe the examined objects properly. Recently a
new algorithm of direct minimization of the Tikhonov parametric functional with minimum
support stabilizer has been introduced; it produces clear and focused images of targets with
sharp boundaries. In this research we apply this new technique to real radar tomographic data
and we compare the obtained result with the solution generated by the more traditional
minimum norm stabilizer.PublishedPalermo, Italyope
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