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@PHDTHESIS{Mozaffari:843585,
author = {Mozaffari, Amirpasha},
othercontributors = {Reicherter, Klaus and Vereecken, Harry},
title = {{T}owards 3{D} crosshole {GPR} full-waveform inversion},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2022-03244},
pages = {1 Online-Ressource : Illustrationen},
year = {2022},
note = {Englische und deutsche Zusammenfassung. - Veröffentlicht
auf dem Publikationsserver der RWTH Aachen University;
Dissertation, Rheinisch-Westfälische Technische Hochschule
Aachen, 2022},
abstract = {High-resolution imaging of the subsurface improves our
understanding of the subsurface flow and solute
transportation that can directly help us protect groundwater
resources and remediate contaminated sites. The ground
penetrating radar (GPR) is a useful non/minimal invasive
method that consists of a transmitter (Tx) unit that emits
electromagnetic (EM) waves and a receiver (Rx) that measures
the arriving electromagnetic waves and can provide
high-resolution tomograms of the subsurface properties. In
specific, the crosshole GPR setup in which two-neighbouring
boreholes are placed in the earth can provide much more
in-depth access to the target area. However, the
interpretation of the GPR data remains challenging. The
simpler ray-based inversion (RBI) is computationally
attractive while fail to provide high-resolution tomograms
as the results always smoothed over the target area. The
full-waveform inversion (FWI) can provide detailed
subsurface tomograms that can carry up to more than an order
of the magnitude resolution compared to RBI from the same
data set. A sophisticated method such as FWI requires
detailed modelling tools and powerful inversion algorithm
that needs significant computational resources. In last
decades, by exponential increase in computing power and the
memory, alongside to wider usage of high performance
computing resources; FWI application in GPR data gain
popularity. All these computational advances such as FWI
method. could be very demanding to be modelled in 3D domain.
Thus, some fundamentals assumptions are made to reduce the
computational requirements, especially computational time
and required memory by using 2D modeling domain. Despite the
usefulness of these simplifications, these assumptions led
to introducing inaccuracy that compromises the performance
of the FWI in complex structures. We investigated the effect
of the assumption that enables us to use a 2D model instead
of a computationally expensive 3D modelling to simulate the
EM propagation. These assumptions are made for specific
state that not necessary is always valid, and therefore it
can introduce inaccuracies in transferred data. Study of
several synthetic cases revealed that the performance of the
3D to 2D transformation in complex structures such as high
contrast layer is much lower than what is anticipated.
Therefore, in the complex subsurface system; 2D transferred
data inherently carry inaccuracy that jeopardises the
accuracy of any further analysis such as FWI. Thus, we
introduced a FWI that utilise a native 3D forward model to
use the original measured 3D data. The novel method is
called 2.5D FWI, and it showed improvements compared to 2D
FWI for synthetic and measured data.A better modelling tool
such as the 3D forward model provides a useful platform for
simulating the subsurface and measuring devices involved to
a higher degree of accuracy. We used previously introduced
3D forward model to build a realistic model of the GPR Tx
and Rx antenna that called finite-length antennas and the
boreholes that these antenna are placed to carry out the
measurements. Our studies showed that realistic antenna and
borehole-fluid representation provides more realistic
travel-time and wave-form shape for GPR data. These more
accurate simulated data increases the accuracy of the FWI
results as reducing the uncertainty in the inversion
system.It is a known issue for GPR community that EM waves
that travelled with a high-angle between the Tx and Rx shows
inconsistency in their travel-time and therefore could
jeopardise travel-time inversion results. Even though this
effect is almost consistent, there was no concerts reason
for this issue except systematic erroneous measurements.
Thus, it is common pre-processing standard to discard these
high-angle data (usually above 50°). Our findings regarding
the contribution of the borehole-fluid to changes in
travel-time of the EM waves showed the high-angle
travel-time is not an error rather than consistent effect
the borehole-fluid in travel times. We laid the mathematical
explanation of this phenomena and introduced a correction
method that could predict this issue and compensate for it.
Lastly, we applied this correction method on the realistic
synthetic data and showed that RBI results improved when the
correction method is used.},
cin = {531320 / 530000},
ddc = {550},
cid = {$I:(DE-82)531320_20140620$ / $I:(DE-82)530000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2022-03244},
url = {https://publications.rwth-aachen.de/record/843585},
}
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