Kouli_etal_2008_Groundwater modelling_BOOK.pdf - Pantelis ...

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Nigel J. Cassidy
resolution minimum is linked to the signal wavelength, which is related to the permittivity of
the sub-surface materials (see later section on material properties). Typically, the resolution
limit is between ½ to ¼ of the signal wavelength and if the sub-surface features become
smaller than this, then the EM energy will become scattered and the recorded GPR signals
will be chaotic, incoherent and difficult to interpret. At feature sizes of less than 1/10 of the
wavelength, then the material property variations are too small to produce reflected or
scattered energy and the material effectively becomes macroscopically uniform in terms of
the propagating GPR wave. This target resolution limit, and its relationship to the macro-tomicroscopic
properties of the sub-surface materials, is very important for groundwater studies
as it establishes the physical size of variations that can be imaged by any given GPR system.
This could be a watertable interface, a high-permeability fluid pathway (e.g., a coarse sandy
layer or lens in a fine sand body), a contaminant plume or any other similar feature. However,
to actually produce a defined reflection in the GPR section, the interface between the two
materials must be sharp enough, and of significant contrast, to result in sufficient energy
being returned back to the receiving antenna. In practical terms, this means that materials
must have permittivity and/or conductivity contrasts of at least 5-10% and physical
boundaries at centimetre scales or less. Diffuse or gradational changes in material properties
(e.g., a large capillary zone or disseminated materials) do not produce defined GPR
reflections because the dimensions of the boundary are not sharp enough. This is a common
problem in groundwater mapping as users often expect to see a strong reflection from the
watertable interface in the GPR sections. This is true in coarse materials (see Figure 3), but in
finer sands and silts, the capillary zone thickness can be tens of centimetres or more and there
will be no distinct watertable reflector (Bano, 2006). That said, the form, velocity and
attenuation of the GPR wave will be altered as it propagates through the capillary zone (or
any diffuse boundary) and it is these properties that more sophisticated GPR studies attempt
to extract from the recorded data.
Table 1. Approximate imaging depth and target size (resolution) of a range of GPR
frequencies in an unsaturated, naturally damp, sandy soil. Values in bold represent the
most common frequencies used in groundwater and hydrological studies
Antenna centre
frequency
(MHz)
Approximate imaging
depth in a damp soil
(metres)
Approximate target size in a
damp soil
(metres)
1500 0.3 0.03
1000 1 0.05
500 2 0.1
250 3 0.2
100 5 0.5
50 10 1
20 20 2
Note that these are only ‘typical’ values for a hypothetical soil. In practice, penetration depths can different to
this and in most cases much shallower.