Time-zero adjustment or the true ground surface for Ground Penetrating Radar (GPR) applications is a very important aspect and an essential
factor in order to carry out accurate shallow depth measurements. As
the transmitted and received signals from GPR antennas are affected
by the presence of different materials with various dielectric constants
and electromagnetic properties adjusting the time-zero appropriately is
important. This study uses a realistic Three Dimensional (3D) numerical
model of a GPR transducer in order to examine where is the best location
for time-zero on a GPR trace. It is shown that in order to establish a
robust and consistent time-zero position careful consideration is needed
also of the way the two-way travel time of the reflected GPR wavelet is
estimated as well. Starting with a simple homogeneous model with a set
of different targets a better process of time-zero adjustment and time
picking of the GPR wavelets is put forward that is verified using further
more complex and realistic heterogeneous models. Further verification
is obtained by using experimental data.
Estimating the permittivity of heterogeneous mixtures based on the
permittivity of their individual components is of high importance with
many applications in GPR and in electrodynamics-based sensing in
general. The Complex Refractive Index Model (CRIM) is the most
mainstream approach for estimating the bulk permittivity of heterogeneous materials and has widely been applied for GPR applications. The
popularity of CRIM is primarily based on its simplicity while its accuracy
has never been rigorously tested. In the current study, an optimized
shape factor is derived that is fine-tuned for modelling the dielectric
properties of concrete. The bulk permittivity of concrete is expressed
with respect to its components i.e, aggregate particles, cement particles,
air-void and volumetric water fraction. Different combinations of the
above materials are accurately modelled using the Finite-Difference
Time-Domain (FDTD) method. The numerically estimated bulk permittivity is then used to fine-tune the shape factor of the CRIM model.
Then, using laboratory measurements it is shown that the revised CRIM
model over-performs the default shape factor and provides with more
accurate estimations of the bulk permittivity of concrete.
Numerical modelling of a heterogeneous concrete model and a bowtie
antenna with a separate transmitter and receiver that are able to move
independently are also presented in this study. Both models are used for
the optimisation of the time-zero position and the CRIM model shape
factor
