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gprMax User Guide, Release 3.1.5
2.2.3 Realistic soils, heterogeneous objects and rough surfaces
The inclusion of improved models of soils is important for many GPR simulations. gprMax can now be used to
create soils with more realistic dielectric and geometrical properties. A semi-empirical model, initially suggested
by [DOB1985], is used to describe the dielectric properties of the soil. The model relates relative permittivity of
the soil to bulk density, sand particle density, sand fraction, clay fraction and water volumetric fraction. Using this
approach, a more realistic soil with a stochastic distribution of the aforementioned parameters can be modelled.
The real and imaginary parts of this semi-empirical model can be approximated using a multi-pole Debye function
plus a conductive term. This can now be achieved in gprMax using the new dispersive material functionality. For
further details see the material commands section.
Fractals are scale invariant functions which can express the topography of the earth for a wide range of scales
with sufficient detail [TUR1987]. For this reason fractals have been chosen to represent the topography of soils.
Fractals can be generated by the convolution of Gaussian noise with an inverse Fourier transform of
1
𝑘𝑏
, where
𝑘 is the wavenumber and 𝑏 is a constant related to the fractal dimension [TUR1997]. gprMax can now generate
heterogeneous volumes (boxes) with realistic soil properties that can have rough surfaces applied. For further
details see the fractal object building commands section.
Fractal correlated noise [TUR1997] is used to describe the stochastic distribution of the properties of soils. This
approach has been chosen because it has been shown that soil-related environmental properties frequently obey
fractal laws [BUR1981], [HILL1998]. For further details see the material commands section and the fractal object
building commands section.
2.2.4 Library of antenna models
gprMax now includes Python modules with pre-defined models of antennas that behave similarly to commercial
antennas [WAR2011] [STA2017]. Currently models of antennas similar to Geophysical Survey Systems, Inc.
(GSSI) (http://www.geophysical.com) 1.5 GHz (Model 5100) antenna, and 400 MHz antenna, as well as MALA
Geoscience (http://www.malags.com/) 1.2 GHz antenna are included. By taking advantage of Python scripting in
input files, using such complex structures in a model is straightforward without having to be built step-by-step by
the user. For further details see the Python section.
2.2.5 Anisotropic materials
It is possible to specify objects that have diagonal anisotropy which allows materials such as wood and fibre-
reinforced composites, often imaged with GPR, to be more accurately modelled. Standard isotropic objects specify
one material identifier that defines the same properties in x, y, and z directions. However, every volumetric object
building command can also be specified with three material identifiers, which allows properties for the x, y, and z
directions to be separately defined.
2.2.6 Dielectric smoothing
At the boundaries between different materials in a model there is the question of what electric and magnetic
material properties to use?
• Should the last object to be defined at that location dictate the electric and magnetic properties?
• Should an average set of electric and magnetic properties of the materials of the objects that share that
location be used?
This latter option is often referred to as dielectric smoothing and has been shown to result in more accurate
simulations [LUE1994] [BOU1996] [WHI2009]. To address this question gprMax includes an option to turn
dielectric smoothing on or off for volumetric object building commands. The default behaviour (if no option is
specified) is for dielectric smoothing to be on. The option can be specified with a single character y (on) or n (off)
given after the material identifier in each object command. When dielectric smoothing is on gprMax calculates
the arithmetic mean of the electric and magnetic properties of the surrounding Yee cells, to use for the single Yee
cell edge (boundary) of interest.
12 Chapter 2. Software Features