Kouli_etal_2008_Groundwater modelling_BOOK.pdf - Pantelis ...

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Nigel J. Cassidy
The influence of the two different saturating fluids can be seen in each case (i.e., either
water or LNAPL saturated). In the water-saturated sample, the real component of the
permittivity is high-valued (~26) and relatively constant over the whole frequency range. The
LNAPL saturated sample shows a similar form (i.e., limited variation) but is much lowervalued
(~5). This will result a velocity difference of over 40% between the two materials and
an interface reflection coefficient of –0.38, which represents a significant contrast between
the two materials. As such, very strong reflections would be expected from the LNAPL-water
table interface and a distinct, observable change in the GPR wave velocity (and therefore
travel time in the GPR sections). More interestingly, the groundwater sample shows higher,
more variable values of imaginary permittivity than the LNAPL sample, particularly at the
low frequency end of the spectrum. As the imaginary component of the permittivity adds to
the overall loss effect of the material, the groundwater saturated sands will exhibit higher
attenuation than the LNAPL saturated materials. Therefore, it would be reasonable to assume
that the presence of disseminated free-phase LNAPL in the subsurface would result is less
attenuation and potential bright-spots (or zones) of higher signal amplitudes in the GPR
section.
This simple example illustrates how mixing models can be used to accurately evaluate
and/or model the bulk permittivity of relatively simple sub-surface materials (sands, sandysoil,
rocks, etc) and aid in the interpretation of GPR sections. As a practical groundwaterrelated
mixing model, CRIM is an appropriate choice as it is easy to use, accurate, wellproven
and can cope with complex permittivities and conductivities. However, it is limited to
low-clay content materials with volumetric saturations above about 5%. The effects of lowfrequency
losses due to bonded water or interfacial polarisations are also not included. It has
been known to slightly underestimate the loss component of the permittivity below ~100MHz
but for most practical applications it is good analogy to the response of real materials.
6.0. GPR for Contaminant and Groundwater Assessment in the
Near-Surface: An Example from a LNAPL Contaminated Site
The CRIM-based mixing model example discussed in the precious section illustrates how a
deeper understanding of the macroscopic properties of materials can used to evaluate the
hydrological conditions at site. To demonstrate how this can be used in a practical context,
the results of a groundwater contamination study will be briefly discussed to show the linkage
between the GPR-related information (e.g., attenuation differences) and the physical
properties of the subsurface (i.e., contaminant saturation). The study site was a disused and
demolished coastal, hydrocarbon storage facility where LNAPL Benzene-Toluene-Styrene-
Ethylbenzene mixtures were present in free-phase, residual and dissolved forms (Figure 10).
It provided an ideal ‘test case’ scenario for the evaluation of advanced GPR data analysis,
processing and interpretation methods and is analogous to many other coastal hydrocarbon
processing facilities throughout the world. As part of a U.K. government research grant
funded by the Natural Environment Research Council (NERC - NER/2002/00100), the aim
of the work was to evaluate the degree of LNAPL contamination at the site and identify the
dominant fluid migration pathways in the vadose-to-saturated zone (full details on the study
can be found in Cassidy, 2007, 2004 & 2006).