Stima del contenuto idrico di suoli mediante tecniche di indagine elettromagnetica
Author(s)
Di Matteo, Andrea
Date Issued
April 22, 2008
Type
Doctoral Thesis
Abstract
Ground Penetrating Radar (GPR) is a fast and non destructive tool, widely used to obtain
information on electromagnetic wave propagating through the subsurface. Electromagnetic
propagation parameters are sensitive to the complex permittivity of soil, which is strongly
influenced by the water content. Several methodologies (e.g. reflection and ground wave methods),
using both single and multi offset configuration, have been tested and their accuracy has been
evaluated.
In this work, we want to test a new survey approach based on the analysis of early-time GPR
signals acquired in a surface-lunched single-offset configuration. In particular, my goal is to
demonstrate that the average envelope amplitude of the first half-cycle of the signal can be
conveniently used to obtain information about the soil surface permittivity (and therefore water
content). For this purpose, experimental measurements were conducted, both in a natural and in an
artificial site, and theoretical models, by numerical simulations, were developed.
The natural site selected for experimental measurements consists of a large gas vent with strong
lateral water content variations, which produce strong lateral gradients in the soil electrical
properties. This is an essential characteristic, as it allows us to better highlight and interpret the
relationship between GPR signal attributes and the spatial distribution of the electromagnetic
parameters. The GPR survey was conducted with a single-offset 250 MHz system and the average
envelope amplitude for the acquired traces was computed by using different time windows, with
time duration given by the inverse of the central frequency of the GPR emitted pulse. TDR (Time
Domain Reflectometry) permittivity measurements were also performed and a quantitative estimate
of the degree of linear correlation between the measured quantities was performed by the
correlation coefficient calculated using the pairs of measurements taken in the same locations.
Results show a good degree of correlation associated with the early time windows (0-4 ns),
indicating that the average envelope amplitude, of the first arrival time signal, can be used for
spatial variability mapping of the near-surface soil electrical permittivity.
Furthermore, in order to perform measurements in controlled water content conditions and to
eliminate the effects due to vegetation and surface roughness, a large test site has been constructed.
GPR, with single-offset 250 and 500 MHz antenna system, and TDR surveys were conducted. Also
in this case the correlation between TDR and GPR data was analyzed obtaining good results, better
than the natural site ones. A study was also performed to investigate the relationship between the
correlation goodness and the time duration of the window used for the average envelope amplitude
calculation. Results show a strong dependence on the correlation coefficient value from the chosen
time interval and, comparing the obtained results with the natural site ones, it is observed how the
best time windows, that gives the highest correlation values, is highly site specific. This can be a
problem for mapping soil water variation of heterogeneous soils and for this reason, the correlation
analyses were performed also using a variable time window, defined by the first half-cycle duration.
The obtained correlation values are good and indicate how this methodology of signal analysis can
be conveniently used for all kinds of soil.
To give a theoretical support to the new survey approach used in the experimental measurements
and to investigate its limitations, a numerical simulation of the GPR/soil system was implemented.
For this purpose, the effects of two antennas placed close together on the Earth’s surface are studied
and the antennas to ground coupling effects are numerically simulated, using a model for linear
electric dipoles. The model uses a thin-wire approximation for the transmitting and receiving
antennas and the interaction with the Earth is directly accounted for using appropriate Green’s
functions. The model consists of a homogenous upper half-space (air) and a lower half-space
representing the soil; several Earth configurations are assumed as input for the model:
i) The ground is modelled as an homogenous non-conductive half-space with a permittivity ranging
from 5 to 80. The linear correlation coefficient between the average envelope amplitude of the first
half-cycle of the GPR simulated signals and the input modelled permittivity values have been
evaluated. The simulations show that there is a linear correlation between the permittivity and the
average envelope amplitude values when the air and the ground wave are completely coupled; this
occurs when the wavelength is larger or equal to the antenna offset, as usually take place in singleoffset
field measurements (and as take place for the permittivity values measured in the
experimental part of this work).
ii) The ground is modelled as a homogenous conductive half-space. The simulation shows that the
conductivity affects the value of the signal amplitude. However, assuming for the soil a range of
electromagnetic values as the TDR measured ones (and similar to the ones usually present in a
natural site), the conductivity doesn’t seem to significantly influence the correlation between
permittivity and GPR signal attributes.
iii) The ground is modelled as a two layer half-space and the thickness of the upper layer is
progressively increased. The simulations show that if the top layer is sufficiently thick there is not
influence of the lower medium on the early time GPR signal. Nevertheless, if the thickness of the
upper layer is thinner than a critical length, the waveform is influenced both by the lower layer and
by the interference with the reflected signal coming from the interface between the two media. In
this case the average envelope amplitude could give wrong punctual information about the soil
permittivity. These phenomena, which in the field measurements could be due to the presence of
local dielectric discontinuities, could give wrong punctual information, however the medium scale
map of the permittivity will result regardless correct.
In Summary, the experimental measurements and the numerical simulations have shown that the
proposed GPR technique can be conveniently used to map, rapidly and with a good spatial
resolution, the soil water content variation in medium scale survey, without the limitation of other
GPR methodologies of acquisition and analyses (e.g. the presence of a shallow reflector in the
subsoil for the reflection method and the long time of acquisition of the ground wave method).
Additional information
Dottorato di ricerca in Scienze ambientali
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