Numerical modelling of high-frequency Ground-Penetrating Radar antennas

247 Pages·2009·33.85 MB·English

NUMERICAL MODELLING OF HIGH-FREQUENCY
GROUND-PENETRATING RADAR ANTENNAS
craig warren
Doctor of Philosophy
The University of Edinburgh
2009
Craig Warren: Numerical modelling of high-frequency Ground-Penetrating
Radar antennas, Doctor of Philosophy, © 2009
A mind is like a parachute. It doesnt work if it’s not open.
— Frank Zappa

DECLARATION
I hereby declare that this thesis and the work reported herein was composed
and originated entirely by myself, under the supervision of Dr. Antonios
Giannopoulos in the School of Engineering at The University of Edinburgh.
The following conference and journal publications are as a result of the
research conducted for this thesis, which has not been submitted for any other
degree or professional qualification.
• C. Warren and A. Giannopoulos. Optimising models of commercial GPR
antennas. In Proceedings of the 5th International Workshop on Advanced
Ground Penetrating Radar, 2009.
• C. Warren and A. Giannopoulos. Simulating Commercial GPR antennas:
Howclosecanweget?InProceedings of the 12th International Conference
on Ground Penetrating Radar, 2008.
• M.Gordon,C.Warren,andA.Giannopoulos.ExperimentalandComputa-
tional Modelling of GPR Signals within Natural Stone Sett Construction.
In Proceedings of the 12th International Conference on Structural Faults
and Repair, 2008.
• C. Warren and A. Giannopoulos. Numerical modelling of commercial
GPR antennas. In Proceedings of the 21st SAGEEP Symposium on the
Application of Geophysics to Engineering and Environmental Problems,
2008. Invited paper.
• C.WarrenandA.Giannopoulos.NumericalmodellingofcommercialGPR
antennas. In Proceedings of the 13th European Meeting of Environmental
and Engineering Geophysics of the Near Surface Geoscience Division of
EAGE, 2007.
Edinburgh, 2009
Craig Warren

ABSTRACT
Ground-Penetrating Radar (GPR) is a non-destructive electromagnetic inves-
tigative tool used in many applications across the fields of engineering and
geophysics. The propagation of electromagnetic waves in lossy materials is
complex and over the past 20 years, the computational modelling of GPR has
developed to improve our understanding of this phenomenon.
This research focuses on the development of accurate numerical models of
widely-used, high-frequency commercial GPR antennas. High-frequency, high-
resolution GPR antennas are mainly used in civil engineering for the evaluation
of structural features in concrete i.e., the location of rebars, conduits, voids
and cracking. These types of target are typically located close to the surface
and their responses can be coupled with the direct wave of the antenna. Most
numericalsimulationsofGPRonlyincludeasimpleexcitationmodel,suchasan
infinitesimal dipole, which does not represent the actual antenna. By omitting
the real antenna from the model, simulations cannot accurately replicate the
amplitudes and waveshapes of real GPR responses.
Numerical models of a 1.5 GHz Geophysical Survey Systems, Inc. (GSSI) an-
tenna and a 1.2 GHz MALÅ GeoScience (MALÅ) antenna have been developed.
Thegeometryofantennasisoftencomplexwithmanyfinefeaturesthatmustbe
capturedinthenumericalmodels.Tovisualisethislevelofdetailin3d,software
wasdevelopedtolinkParaview—anopensourcevisualisationapplicationwhich
uses the Visualisation Toolkit (VTK)—with GprMax3D—electromagnetic sim-
ulation software based on the Finite-Difference Time-Domain (FDTD) method.
Certain component values from the real antennas that were required for the
models could not be readily determined due to commercial sensitivity. Values
for these unknown parameters were found by implementing an optimisation
technique known as Taguchi’s method. The metric used to initially assess
the accuracy of the antenna models was a cross-corellation of the crosstalk
responses from the models with the crosstalk responses measured from the real
antennas. A 98 % match between modelled and real crosstalk responses was
achieved.
Further validation of the antenna models was undertaken using a series of
laboratory experiments where oil-in-water (O/W) emulsions were created to
simulate the electrical properties of real materials. The emulsions provided
homogeneous liquids with controllable permittivity and conductivity and en-
abled different types of targets, typically encountered with GPR, to be tested.
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The laboratory setup was replicated in simulations which included the antenna
models and an excellent agreement was shown between the measured and
modelled data. The models reproduced both the amplitude and waveshape of
the real responses whilst B-scans showed that the models were also accurately
capturing effects, such as masking, present in the real data. It was shown
that to achieve this accuracy, the real permittivity and conductivity profiles of
materials must be correctly modelled.
The validated antenna models were then used to investigate the radiation
dynamics of GPR antennas. It was found that the shape and directivity of
theoretically predicted far-field radiation patterns differ significantly from real
antenna patterns. Being able to understand and visualise in 3d the antenna
patterns of real GPR antennas, over realistic materials containing typical
targets, is extremely important for antenna design and also from a practical
user perspective.
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ACKNOWLEDGMENTS
I would like to acknowledge my supervisor, Doctor Antonios Giannopoulos, for
hiscontinualsupportthroughoutthedurationofmystudies.Hisencouragement,
guidance, and passion for the subject were invaluable.
I would like to thank my second supervisor, Professor Michael C. Forde,
for his support and guidance. Quite often a five minute chat turned into an
hour-long discussion, but I always came away with much useful advice.
I would like to thank my friends and colleagues in The Institute for Infras-
tructure and Environment, of whom there are too many to mention specifically,
for making it such a pleasant and stimulating environment to work in. Spe-
cific mention to the 11 am coffee-goers and the irreverent chat (important
networking) that always took place.
I would also like to acknowledge my office-mates over the years Ria Diamanti,
Francis Drossaert, Robert De Bold, and Scott Rodgers for their companionship,
and their understanding answers to my many stupid questions.
My apologies to Clare Baird and Bryony Davidson who were continually
subjected to my important whinging.
I owe sincere thanks to Holly Smith for her love, support and encouragement,
without which I would not have found the motivation to complete this work.
Finally, and most importantly, I would like to thank my parents, John and
Sheila, for their understanding, love and support throughout my university
career. I would also like to thank my brother, Simon, for unwittingly ensuring
I never lost touch with the real world!
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