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Realistic numerical modelling of ground penetrating radar for landmine detection
Get PDFGround-Penetrating Radar (GPR) is a popular non-destructive geophysical
technique with a wide range of diverse applications. Civil engineering, hydrogeophysics,
forensic, glacier geology, human detection and borehole geology are
some of the fields in which GPR has been applied with successful or promising
results. One of the most mainstream applications of GPR is landmine detection.
A lot of methods have been suggested over the years to assist the landmine
detection issue. Metal detectors, trained rats or dogs, chemical methods and
electrical resistivity tomography are –amongst others– some of the suggested
techniques. The non-destructive nature of GPR makes it an attractive choice for
a problem such as demining in which contact to the ground is not allowed. The
main advantage of GPR is its ability to detect both metallic and non-metallic
targets. Furthermore, GPR can provide an insight regarding the nature of the
target (e.g. size, burial depth, type). From the above, it is evident that GPR
can potentially reduce the false alarms emerging from small metallic objects
(e.g. bullets, wires, etc.) usually encountered in battle-fields and industrialised
areas. Combining the robustness of the metal detector with the resolution of
GPR results in a reliable and efficient detection framework which has been
successfully applied in Cambodia and Afghanistan.
Despite the promising, and in some cases impressive results, aspects of
GPR can be further improved in an effort to optimise GPR’s performance and
decrease its limitations. The validation of a GPR system is usually achieved
through the so called Receiver Operation Characteristics (ROC) which depicts
the probability of detection with respect to the false alarm rate. ROC is a
highly nonlinear function which is sensitive to the environment as well as to
the antenna unit.
Landmines are typically small objects, often less than 10 cm diameter, which
are shallow buried, usually in less than 10 cm depth, and sometimes almost
exposed. In order for the landmines to be resolved, high frequency antennas
are essential. The latter are sensitive to soil’s inhomogeneities, rough surface,
water puddles, vegetation and so on. Apart from that, the near field nature of
the problem makes the antenna unit part of the medium which contributes to
the unwanted clutter. The above, outlines the multi-parametric nature of the
problem for which no straightforward approach has yet to be proposed.
Numerical modelling is a practical and solid approach to understand the
physical behaviour of a system. In the case of GPR for landmine detection,
numerical modelling can be a practical tool for designing and optimising
antennas in synthetic but nonetheless realistic conditions. Apart from that,
evaluation of a processing method only to a specific environment is not a
robust approach and does not provide any evidence for its wider inclusivity and
limitations. However, evaluation in different conditions can become costly and
unpractical. Numerical modelling can tackle this problem by providing data for
a wide range of scenarios. An extensive database of simulated responses, apart
from being a practical testbed, can be also employed as a training set for machine
learning. A multi-variable problem like demining, in order to be addressed using
machine learning, requires a large amount of data. These must equally include
all possible different scenarios i.e. different landmines, in different media with
stochastically varied properties and topography. Additionally, different heights
of the antenna and different depths of the landmines must also be examined.
Numerical modelling seems to be a practical approach to achieve an equally
distributed and coherent dataset like the one briefly described above.
Numerical modelling of GPR for landmine detection has been applied in
the past using generic antennas in simplified and clinical scenarios. This
approach can be used in an educational context just to provide a rough
estimation of GPR’s performance. In the present thesis a realistic numerical
scheme is suggested in which, simplifications are kept to a minimum. The
numerical solver, employed in the suggested numerical scheme, is the Finite-
Difference Time-Domain (FDTD) method. Both the dispersive properties and
the Absorbing Boundary Condition (ABC) are implemented through novel
and accurate techniques. In particular, a novel method which implements an
inclusive susceptibility function is suggested and it is shown that surpasses the
performance of the previous approaches while retaining their computational
efficiency. Furthermore, Perfectly Matched Layer (PML) and more specifically
Convolutional Perfectly Matched Layer (CPML) is implemented through a novel
time-synchronised scheme which it is proven to be more accurate compared to
the traditional CPML with no additional computational requirements.
An accurate numerical solver, although essential, is not the only requirement
for a realistic numerical framework. Accurate implementation of the geometry
and the dielectric properties of the simulated model is highly important,
especially when it comes to high-frequency near-field scenarios such as GPR
for landmine detection. In the suggested numerical scheme, both the soil’s
properties as well as the rough surface are simulated using fractal correlated
noise. It is shown, that fractals can sufficiently represent Earth’s topography
and give rise to semi-variograms often encountered in real soils. Regarding the
dielectric properties of the soils, a semi-analytic function is employed which
relates soil’s dielectric properties to its sand fraction, clay fraction, sand density,
bulk density and water volumetric fraction. Subsequently, the semi-analytic
function is approximated using a Debye function that can be easily implemented
to FDTD. Vegetation is also implemented to the model using a novel method
which simulates the geometry of vegetation through a stochastic process. The
experimentally-derived dielectric properties of vegetation are approximated
–similarly to soil’s dielectric properties– with a Debye expansion. The antenna
units tested in the numerical scheme are two bow-tie antennas based on
commercially available transducers. Regarding the targets, three landmines are
chosen, namely, PMN, PMA-1 and TS-50. Dummy landmines are used in order
to obtain their geometrical characteristics and comparison between measured
and numerically evaluated traces are used to tune the dielectric properties of
the modelled landmines. Lastly, water puddles are realistically implemented in
the model in an effort to realistically simulate high-saturated scenarios.
The proposed numerical scheme has been employed in order to test and
evaluate widely used post-processing methods. The results clearly illustrate
that post-processing methods are sensitive to the antenna unit as well as the
medium. This highlights the importance of an accurate numerical scheme as a
testbed for evaluating different GPR systems and post-processing approaches
in wide range of scenarios.
Using an equivalent 2D numerical scheme –restricted to 2D due to computational
constrains– preliminary results are given regarding the effectiveness of
Artificial Neural Network (ANN) subject to an adequate and equally distributed
database. The results are promising, showing that ANN can be successfully
employed for detection as well as classification using only a single trace as
input. A basic requirement to do so is a representative training set. This can
be synthetically generated using a realistic numerical framework. The above,
provide solid arguments for further expanding the proposed machine learning
scheme to the more computationally demanding 3D case
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Associating Borehole Radar Imaging with Petrophysical Properties for a Mud-Contaminated Reservoir
No full textIn the phase of oil drilling, mud filtrate penetrates into porous formations and alters the pore fluid properties. This complicates well logging exploration, and inevitably gives rise to shift in reservoir estimation. Logging engineers deem mud invasion a harm and attempt to eliminate its impact on logging data exploration. However, from our point of view, the mudcontaminated parts of the formation do also carry some valuable information, notably with regard to the key hydraulic properties. Therefore, if adequately characterized, mud invasion effects, in turn, could be utilized for reservoir estimation. Typically, the invasion depth critically depends on the formation porosity and permeability. To achieve this objective, we propose to use borehole radar to determine the mud invasion depth considering a high spatial resolution of ground-penetrating radar (GPR) compared with the conventional logging tools. We implement numerical investigations on the feasibility of this approach by coupling electromagnetic (EM) modelling with fluid flow modelling in an oil-bearing formation disturbed by mud invasion effects. The simulations imply that a time-lapse radar logging is able to extract EM reflection signals from mud invasion front, and the invasion depth and EM velocity can be obtained by a downhole antenna displacement of one source and two receivers. We find that there exists a positive correlation between the estimated invasion depth and permeability curves, and a negative correlation between the estimated velocity and porosity curves. We suggest that borehole radar has potential to estimate permeability and porosity of oil reservoirs, wherein the mud invasion effect is positively utilized. The study demonstrates a potential method of oil reservoir estimation and a novel application of GPR in oil fieldsGreen Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Applied Geophysics and Petrophysic
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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