# Input file commands¶

An input file has to be supplied to gprMax which should contain all the necessary information to run a GPR model. The input file is an ASCII text file which can be prepared with any text editor or word-processing program. In the input file the hash character (`#`

) is reserved and is used to denote the beginning of a command which will be passed to gprMax. The general syntax of commands is:

```
#command_name: parameter1 parameter2 parameter3 ...
```

A command and associated parameters should occupy a single line of the input file, and only one command per line is allowed. Hence, the first character of a line containing a command **must** be the hash character (`#`

). If the line starts with **any other character** it is ignored by the program. Therefore, user comments or descriptions can be included in the input file. If a line starts with a hash character (`#`

) the program will expect a valid command. If the name of the command is not correct the program will abandon execution and issue an error message. When a command requires more than one parameter then these should be separated using a white space character.

The order of commands in the input file is not important with the exception of object construction commands.

To describe the commands that can be used in the input file and their parameters the following conventions are used:

`f`

means a real number which can be entered using either a`[.]`

separating the integral from the decimal part, e.g. 1.5, or in scientific notation, e.g. 15e-1 or 0.15e1.`i`

means an integer number.`c`

means a single character, e.g.`y`

.`str`

means a string of characters with**no**white spaces in between, e.g`sand`

.`file`

means a filename.`[ ]`

square brackets are used to indicate optional parameters.

Unless otherwise specified, the SI system of units is used throughout gprMax:

- All parameters associated with simulated space (i.e. size of model, spatial increments, etc…) should be specified in
**metres**. - All parameters associated with time (i.e. total simulation time, time instants, etc…) should be specified in
**seconds**. - All parameters denoting frequency should be specified in
**Hertz**. - All parameters associated with spatial coordinates in the model should be specified in
**metres**. The origin of the coordinate system**(0,0)**is at the lower left corner of the model.

It is important to note that gprMax converts spatial and temporal parameters given in **metres** and **seconds** to integer values corresponding to **FDTD cell coordinates** and **iteration number** respectively. Therefore, rounding to the nearest integer number of the user defined values is performed.

The fundamental spatial and temporal discretization steps are denoted as \(\Delta x\) , \(\Delta y\), \(\Delta z\) and \(\Delta t\) respectively.

The commands have been grouped into six categories:

**Essential**- required to run any model, such as the domain size and spatial discretization**General**- provide further control over the model**Material**- used to introduce different materials into the model**Object construction**- used to build geometric shapes with different constitutive parameters**Source and output**- used to place source and output points in the model**PML**- provide advanced customisation and optimisation of the absorbing boundary conditions

## Essential commands¶

Most of the commands are optional but there are some essential commands which are necessary in order to construct any model. For example, none of the media and object commands are necessary to run a model. However, without specifying any objects in the model gprMax will simulate free space (air), which on its own, is not particularly useful for GPR modelling. If you have not specified a command which is essential in order to run a model, for example the size of the model, gprMax will terminate execution and issue an appropriate error message.

The essential commands are:

### #domain:¶

Allows you to specify the size of the model. The syntax of the command is:

```
#domain: f1 f2 f3
```

where `f1 f2 f3`

are the size of the model in the x, y, and z directions respectively. For example to specify a 500 x 500 x 1000mm model use: `#domain: 0.5 0.5 1.0`

### #dx_dy_dz:¶

Allows you to specify the discretization of space in the x , y and z directions respectively (i.e. \(\Delta x\) , \(\Delta y\), \(\Delta z\)). The syntax of the command is:

```
#dx_dy_dz: f1 f2 f3
```

where `f1`

is the spatial step in the x direction (\(\Delta x\)), `f2`

is the spatial step in the y direction (\(\Delta y\)) and `f3`

is the spatial step in the z direction (\(\Delta z\)). The spatial discretization controls the maximum permissible time step \(\Delta t\) with which the solution advances in time in order to reach the required simulated time window. The relation between \(\Delta t\) and \(\Delta x\) , \(\Delta y\), \(\Delta z\) is:

where \(c\) is the speed of light. In gprMax the equality is used to determine \(\Delta t\) from \(\Delta x\) , \(\Delta y\), and \(\Delta z\). Small values of \(\Delta x\) , \(\Delta y\), and \(\Delta z\) result in small values for \(\Delta t\) which means more iterations in order to reach a given simulated time. However, it is important to note that the smaller the values of \(\Delta x\) , \(\Delta y\), \(\Delta z\) and \(\Delta t\) are the more accurate your model will be. See the Guidance on GPR modelling section for tips on choosing a spatial discretisation.

### #time_window:¶

Allows you to specify the total required simulated time. The syntax of the command is:

```
#time_window: f1
```

or

```
#time_window: i1
```

In the first case the `f1`

parameter determines the required simulated time in seconds. For example, if you want to simulate a GPR trace of 20 nanoseconds then `#time_window: 20e-9`

can be used. gprMax will perform the necessary number of iterations in order to reach the required simulated time. Alternatively, if the command is specified with an `i1`

gprMax will interpret this value as a total number of iterations. Hence the command `#time_window: 100`

means that 100 iterations will be performed. The number of iterations and the total simulated time window are related by:

where \(t_w\) is the time window in seconds, \(\Delta t\) the time step, and \(N_{it}\) the number of iterations. gprMax converts the specified time window in seconds to a number of iterations internally using the aforementioned equation. The result of the division is rounded to the nearest integer.

## General commands¶

### #python: and #end_python:¶

Allows you to write blocks of Python code between `#python`

and `#end_python`

in the input file. The code is executed when the input file is read by gprMax. For further details see the Python section.

### #include_file:¶

Allows you to include commands from a file. It will insert the commands from the specified file at the location where the `#include_file`

command is placed. The syntax of the command is:

```
#include_file: file1
```

`file1`

can be the name of the file containing the commands in the same directory as the input file, or `file`

can be the full path to the file containing the commands (allowing you to specify any location).

### #time_step_stability_factor:¶

Allows you to alter the value of the time step \(\Delta t\) used by gprMax. gprMax uses the equality in the CFL condition, hence the maximum permissible time step. If a smaller time step is required then the syntax of the command is:

```
#time_step_stability_factor: f1
```

where `f1`

can take values \(0 < \textrm{f1} \leq 1\). Then the actual time step used will be \(\textrm{f1} \times \Delta t\), where \(\Delta t\) is calculated using the equality from the CFL condition.

### #title:¶

Allows you to include a title for your model. This title is saved in the output file(s). The syntax of the command is:

```
#title: str1
```

where `str1`

can contain white space characters to separate individual words. The title has to be contained in a single line.

### #messages:¶

Allows you to control the amount of information displayed on screen when gprMax is run. The syntax of the command is:

```
#messages: c1
```

where `c1`

can be either y (yes) or n (no) which turns on or off the messages on the screen. The default value is y. When messages are on, gprMax will display on the screen information the translation of space and time values to cell coordinates, iteration number, material parameters etc… This information can be useful for error checking.

### #output_dir:¶

Allows you to control the directory where output file(s) will be stored. The syntax of the command is:

```
#output_dir: str1
```

where `str1`

can be either the absolute path to the directory for the output file(s) or a path relative to the directory of the input files. The default value is the same as the directory of the input files.

### #num_threads:¶

Allows you to control how many OpenMP threads (usually the number of physical CPU cores available) are used when running the model. The most computationally intensive parts of gprMax, which are the FDTD solver loops, have been parallelised using OpenMP which supports multi-platform shared memory multiprocessing. The syntax of the command is:

```
#num_threads: i1
```

where `i1`

is the number of OpenMP threads to use. If `#num_threads`

is not specified gprMax will firstly look to see if the environment variable `OMP_NUM_THREADS`

exists, and if not will detect and use all available physical CPU cores on the machine.

## Material commands¶

### Built-in materials¶

gprMax has two builtin materials which can be used by specifying the identifiers `pec`

and `free_space`

. These simulate a perfect electric conductor and air, i.e. a non-magnetic material with \(\epsilon_r = 1\), \(\sigma = 0\), respectively. Additionally the identifiers `grass`

and `water`

are currently reserved for internal use and should not be used unless you intentionally want to change their properties.

### #material:¶

Allows you to introduce a material into the model described by a set of constitutive parameters. The syntax of the command is:

```
#material: f1 f2 f3 f4 str1
```

`f1`

is the relative permittivity, \(\epsilon_r\)`f2`

is the conductivity (Siemens/metre), \(\sigma\)`f3`

is the relative permeability, \(\mu_r\)`f4`

is the magnetic loss (Ohms/metre), \(\sigma_*\)`str1`

is an identifier for the material.

For example `#material: 3 0.01 1 0 my_sand`

creates a material called `my_sand`

which has a relative permittivity (frequency independent) of \(\epsilon_r = 3\), a conductivity of \(\sigma = 0.01\) S/m, and is non-magnetic, i.e. \(\mu_r = 1\) and \(\sigma_* = 0\)

### #add_dispersion_debye:¶

Allows you to add dispersive properties to an already defined `#material`

based on a multiple pole Debye formulation (see Software Features section). For example, the susceptability function for a single-pole Debye material is given by:

where \(\Delta \epsilon_{rp} = \epsilon_{rsp} - \epsilon_{r \infty}\), \(\epsilon_{rsp}\) is the zero-frequency relative permittivity for the pole, \(\epsilon_{r \infty}\) is the relative permittivity at infinite frequency, and \(\tau_p\) is the pole relaxation time.

The syntax of the command is:

```
#add_dispersion_debye: i1 f1 f2 f3 f4 ... str1
```

`i1`

is the number of Debye poles.`f1`

is the difference between the zero-frequency relative permittivity and the relative permittivity at infinite frequency, i.e. \(\Delta \epsilon_{rp1} = \epsilon_{rsp1} - \epsilon_{r \infty}\) , for the first Debye pole.`f2`

is the relaxation time (seconds), \(\tau_{p1}\), for the first Debye pole.`f3`

is the difference between the zero-frequency relative permittivity and the relative permittivity at infinite frequency, i.e. \(\Delta \epsilon_{rp2} = \epsilon_{rsp2} - \epsilon_{r \infty}\) , for the second Debye pole.`f4`

is the relaxation time (seconds), \(\tau_{p2}\), for the second Debye pole.- …
`str1`

identifies the material to add the dispersive properties to.

For example to create a model of water with a single Debye pole, \(\epsilon_{rsp1} = 80.1\), \(\epsilon_{r \infty} = 4.9\) and \(\tau_{p1} = 9.231\times 10^{-12}\) seconds use: `#material: 4.9 0 1 0 my_water`

and `#add_dispersion_debye: 1 75.2 9.231e-12 my_water`

.

Note

- You can continue to add pairs of values for \(\Delta \epsilon_{rp}\) and \(\tau_p\) for as many Debye poles as you have specified with
`i1`

. - The relative permittivity in the
`#material`

command should be given as the relative permittivity at infinite frequency, i.e. \(\epsilon_{r \infty}\). - Temporal values associated with pole frequencies and relaxation times should always be greater than the time step \(\Delta t\) used in the model.

### #add_dispersion_lorentz:¶

Allows you to add dispersive properties to an already defined `#material`

based on a multiple pole Lorentz formulation (see Software Features section). For example, the susceptability function for a single-pole Lorentz material is given by:

where

where \(\Delta \epsilon_{rp} = \epsilon_{rsp} - \epsilon_{r \infty}\), \(\epsilon_{rsp}\) is the zero-frequency relative permittivity for the pole, \(\epsilon_{r \infty}\) is the relative permittivity at infinite frequency, \(\omega_p\) is the frequency (Hertz) of the pole pair, \(\delta_p\) is the damping coefficient (Hertz) , and \(j=\sqrt{-1}\).

The syntax of the command is:

```
#add_dispersion_lorentz: i1 f1 f2 f3 f4 f5 f6 ... str1
```

`i1`

is the number of Lorentz poles.`f1`

is the difference between the zero-frequency relative permittivity and the relative permittivity at infinite frequency, i.e. \(\Delta \epsilon_{rp1} = \epsilon_{rsp1} - \epsilon_{r \infty}\) , for the first Lorentz pole.`f2`

is the frequency (Hertz), \(\omega_{p1}\), for the first Lorentz pole.`f3`

is the damping coefficient (Hertz), \(\delta_{p1}\), for the first Lorentz pole.`f4`

is the difference between the zero-frequency relative permittivity and the relative permittivity at infinite frequency, i.e. \(\Delta \epsilon_{rp2} = \epsilon_{rsp2} - \epsilon_{r \infty}\) , for the second Lorentz pole.`f5`

is the frequency (Hertz), \(\omega_{p2}\), for the second Lorentz pole.`f6`

is the damping coefficient (Hertz), \(\delta_{p2}\), for the second Lorentz pole.- …
`str1`

identifies the material to add the dispersive properties to.

Note

- You can continue to add triplets of values for \(\Delta \epsilon_{rp}\), \(\omega_p\) and \(\delta_p\) for as many Lorentz poles as you have specified with
`i1`

. - The relative permittivity in the
`#material`

command should be given as the relative permittivity at infinite frequency, i.e. \(\epsilon_{r \infty}\). - Temporal values associated with pole frequencies and relaxation times should always be greater than the time step \(\Delta t\) used in the model.

### #add_dispersion_drude:¶

Allows you to add dispersive properties to an already defined `#material`

based on a multiple pole Drude formulation (see Software Features section). For example, the susceptability function for a single-pole Drude material is given by:

where \(\omega_p\) is the frequency (Hertz) of the pole, and \(\gamma_p\) is the inverse of the pole relaxation time (Hertz).

The syntax of the command is:

```
#add_dispersion_drude: i1 f1 f2 f3 f4 ... str1
```

`i1`

is the number of Drude poles.`f1`

is the frequency (Hertz), \(\omega_{p1}\), for the first Drude pole.`f2`

is the inverse of the relaxation time (Hertz), \(\gamma_{p1}\), for the first Drude pole.`f3`

is the frequency (Hertz), \(\omega_{p2}\), for the second Drude pole.`f4`

is the inverse of the relaxation time (Hertz), \(\gamma_{p2}\) for the second Drude pole.- …
`str1`

identifies the material to add the dispersive properties to.

Note

- You can continue to add pairs of values for \(\omega_p\) and \(\gamma_p\) for as many Drude poles as you have specified with
`i1`

. - Temporal values associated with pole frequencies and relaxation times should always be greater than the time step \(\Delta t\) used in the model.

### #soil_peplinski:¶

Allows you to use a mixing model for soils proposed by Peplinski (http://dx.doi.org/10.1109/36.387598), valid for frequencies in the range 0.3GHz to 1.3GHz. The command is designed to be used in conjunction with the `#fractal_box`

command for creating soils with realistic dielectric and geometric properties. The syntax of the command is:

```
#soil_peplinski: f1 f2 f3 f4 f5 f6 str1
```

`f1`

is the sand fraction of the soil.`f2`

is the clay fraction of the soil.`f3`

is the bulk density of the soil in grams per centimetre cubed.`f4`

is the density of the sand particles in the soil in grams per centimetre cubed.`f5`

and`f6`

define a range for the volumetric water fraction of the soil.`str1`

is an identifier for the soil.

For example for a soil with sand fraction 0.5, clay fraction 0.5, bulk density \(2~g/cm^3\), sand particle density of \(2.66~g/cm^3\), and a volumetric water fraction range of 0.001 - 0.25 use: `#soil_peplinski: 0.5 0.5 2.0 2.66 0.001 0.25 my_soil`

.

Note

Further information on the Peplinski soil model and our implementation can be found in ‘Giannakis, I. (2016). Realistic numerical modelling of Ground Penetrating Radar for landmine detection. The University of Edinburgh. (http://hdl.handle.net/1842/20449)’

## Object construction commands¶

Object construction commands are processed in the order they appear in the input file. Therefore space in the model allocated to a specific material using for example the `#box`

command can be reallocated to another material using the same or any other object construction command. Space in the model can be regarded as a canvas in which objects are introduced and one can be overlaid on top of the other overwriting its properties in order to produce the desired geometry. The object construction commands can therefore be used to create complex shapes and configurations.

### Anisotropy¶

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. The `#plate`

command, which defines a surface, can specify up to two material identifiers, and the `#edge`

command, which defines a line, continues to take one material identifier. For example to create a box with different material properties in each of the x, y, and z directions use:

```
#material: 41 10 1 0 matX
#material: 35 10 1 0 matY
#material: 33 1 1 0 matZ
#box: 0 0 0 0.1 0.1 0.1 matX matY matZ
```

As another example, to create a cylinder of radius 10 mm that has the same properties in the x and y directions but different properties in the z direction use:

```
#material: 41 10 1 0 matXY
#material: 33 1 1 0 matZ
#cylinder: 0.1 0.1 0.1 0.5 0.1 0.1 0.01 matXY matXY matZ
```

### Dielectric smoothing¶

At the boundaries between different materials in the model there is the question of which material properties to use. Should the last object to be defined at that location dictate the properties? Should an average set of 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]. 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. For example to specify a sphere of material `sand`

with dielectric smoothing turned off use: `#sphere: 0.5 0.5 0.5 0.1 sand n`

.

Note

- If a material has dispersive properties then dielectric smoothing is automatically turned off for that material.
- If an object is anistropic then dielectric smoothing is automatically turned off for that object.
- Non-volumetric object building commands,
`#edge`

,`#plate`

, and`#triangle`

(applies to triangular patch not triangular prism) cannot have dielectric smoothing.

### #geometry_view:¶

Allows you output to file(s) information about the geometry of model. The file(s) use the open source Visualization ToolKit (VTK) format which can be viewed in many free readers, such as Paraview. The command can be used to create several 3D views of the model which are useful for checking that it has been constructed as desired. The syntax of the command is:

```
#geometry_view: f1 f2 f3 f4 f5 f6 f7 f8 f9 file1 c1
```

`f1 f2 f3`

are the lower left (x,y,z) coordinates of the volume of the geometry view in metres.`f4 f5 f6`

are the upper right (x,y,z) coordinates of the volume of the geometry view in metres.`f7 f8 f9`

are the spatial discretisation of the geometry view in metres. Typically these will be the same as the spatial discretisation of the model but they can be courser if desired.`file1`

is the filename of the file where the geometry view will be stored in the same directory as the input file.`c1`

can be either n (normal) or f (fine) which specifies whether to output the geometry information on a per-cell basis (n) or a per-cell-edge basis (f). The fine mode should be reserved for viewing detailed parts of the geometry that occupy small volumes, as using this mode can generate geometry files with large file sizes.

Tip

When you want to just check the geometry of your model, run gprMax using the optional command line argument `--geometry-only`

. This will build the model and produce any geometry view files, but will not run the simulation.

### #edge:¶

Allows you to introduce a wire with specific properties into the model. A wire is an edge of a Yee cell and it can be useful to model resistors or thin wires. The syntax of the command is:

```
#edge: f1 f2 f3 f4 f5 f6 str1
```

`f1 f2 f3`

are the starting (x,y,z) coordinates of the edge, and`f4 f5 f6`

are the ending (x,y,z) coordinates of the edge. The coordinates should define a single line.`str1`

is a material identifier that must correspond to material that has already been defined in the input file, or is one of the builtin materials`pec`

or`free_space`

.

For example to specify a x-directed wire that is a perfect electric conductor, use: `#edge: 0.5 0.5 0.5 0.7 0.5 0.5 pec`

. Note that the y and z coordinates are identical.

### #plate:¶

Allows you to introduce a plate with specific properties into the model. A plate is a surface of a Yee cell and it can be useful to model objects thinner than a Yee cell. The syntax of the command is:

```
#plate: f1 f2 f3 f4 f5 f6 str1
```

`f1 f2 f3`

are the lower left (x,y,z) coordinates of the plate, and`f4 f5 f6`

are the upper right (x,y,z) coordinates of the plate. The coordinates should define a surface and not a 3D object like the`#box`

command.`str1`

is a material identifier that must correspond to material that has already been defined in the input file, or is one of the builtin materials`pec`

or`free_space`

.

For example to specify a xy oriented plate that is a perfect electric conductor, use: `#plate: 0.5 0.5 0.5 0.7 0.8 0.5 pec`

. Note that the z coordinates are identical.

### #triangle:¶

Allows you to introduce a triangular patch or a triangular prism with specific properties into the model. The patch is just a triangular surface made as a collection of staircased Yee cells, and the triangular prism extends the triangular patch in the direction perpendicular to the plane. The syntax of the command is:

```
#triangle: f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 str1 [c1]
```

`f1 f2 f3`

are the coordinates (x,y,z) of the first apex of the triangle,`f4 f5 f6`

the coordinates (x,y,z) of the second apex, and`f7 f8 f9`

the coordinates (x,y,z) of the third apex.`f10`

is the thickness of the triangular prism. If the thickness is zero then a triangular patch is created.`str1`

is a material identifier that must correspond to material that has already been defined in the input file, or is one of the builtin materials`pec`

or`free_space`

.`c1`

is an optional parameter which can be`y`

or`n`

, used to switch on and off dielectric smoothing. For use only when creating a triangular prism, not a triangular patch.

For example, to specify a xy orientated triangular patch that is a perfect electric conductor, use: `#triangle: 0.5 0.5 0.5 0.6 0.4 0.5 0.7 0.9 0.5 0.0 pec`

. Note that the z coordinates are identical and the thickness is zero.

### #box:¶

Allows you to introduce an orthogonal parallelepiped with specific properties into the model. The syntax of the command is:

```
#box: f1 f2 f3 f4 f5 f6 str1 [c1]
```

`f1 f2 f3`

are the lower left (x,y,z) coordinates of the parallelepiped, and`f4 f5 f6`

are the upper right (x,y,z) coordinates of the parallelepiped.`str1`

is a material identifier that must correspond to material that has already been defined in the input file, or is one of the builtin materials`pec`

or`free_space`

.`c1`

is an optional parameter which can be`y`

or`n`

, used to switch on and off dielectric smoothing.

### #sphere:¶

Allows you to introduce a spherical object with specific parameters into the model. The syntax of the command is:

```
#sphere: f1 f2 f3 f4 str1 [c1]
```

`f1 f2 f3`

are the coordinates (x,y,z) of the centre of the sphere.`f4`

is its radius.`str1`

is a material identifier that must correspond to material that has already been defined in the input file, or is one of the builtin materials`pec`

or`free_space`

.`c1`

is an optional parameter which can be`y`

or`n`

, used to switch on and off dielectric smoothing.

For example, to specify a sphere with centre at (0.5, 0.5, 0.5), radius 100 mm, and with constitutive parameters of `my_sand`

, use: `#sphere: 0.5 0.5 0.5 0.1 my_sand`

.

Note

- Sphere objects are permitted to extend outwith the model domain if desired, however, only parts of object inside the domain will be created.

### #cylinder:¶

Allows you to introduce a circular cylinder into the model. The orientation of the cylinder axis can be arbitrary, i.e. it does not have align with one of the Cartesian axes of the model. The syntax of the command is:

```
#cylinder: f1 f2 f3 f4 f5 f6 f7 str1 [c1]
```

`f1 f2 f3`

are the coordinates (x,y,z) of the centre of one face of the cylinder, and`f4 f5 f6`

are the coordinates (x,y,z) of the centre of the other face.`f7`

is the radius of the cylinder.`str1`

is a material identifier that must correspond to material that has already been defined in the input file, or is one of the builtin materials`pec`

or`free_space`

.`c1`

is an optional parameter which can be`y`

or`n`

, used to switch on and off dielectric smoothing.

For example, to specify a cylinder with its axis in the y direction, a length of 0.7 m, a radius of 100 mm, and that is a perfect electric conductor, use: `#cylinder: 0.5 0.1 0.5 0.5 0.8 0.5 0.1 pec`

.

Note

- Cylinder objects are permitted to extend outwith the model domain if desired, however, only parts of object inside the domain will be created.

### #cylindrical_sector:¶

Allows you to introduce a cylindrical sector (shaped like a slice of pie) into the model. The syntax of the command is:

```
#cylindrical_sector: c1 f1 f2 f3 f4 f5 f6 f7 str1 [c1]
```

`c1`

is the direction of the axis of the cylinder from which the sector is defined and can be`x`

,`y`

, or`z`

.`f1 f2`

are the coordinates of the centre of the cylindrical sector.`f3 f4`

are the lower and higher coordinates of the axis of the cylinder from which the sector is defined (in effect they specify the thickness of the sector).`f5`

is the radius of the cylindrical sector.`f6`

is the starting angle (in degrees) for the cylindrical sector (with zero degrees defined on the positive first axis of the plane of the cylindrical sector).`f7`

is the angle (in degrees) swept by the cylindrical sector (the finishing angle of the sector is always anti-clockwise from the starting angle).`str1`

is a material identifier that must correspond to material that has already been defined in the input file, or is one of the builtin materials`pec`

or`free_space`

.`c1`

is an optional parameter which can be`y`

or`n`

, used to switch on and off dielectric smoothing.

For example, to specify a cylindrical sector with its axis in the z direction, radius of 0.25 m, thickness of 2 mm, a starting angle of 330 \(^\circ\), a sector angle of 60 \(^\circ\), and that is a perfect electric conductor, use: `#cylindrical_sector: z 0.34 0.24 0.500 0.502 0.25 330 60 pec`

.

Note

- Cylindrical sector objects are permitted to extend outwith the model domain if desired, however, only parts of object inside the domain will be created.

### #fractal_box:¶

Allows you to introduce an orthogonal parallelepiped with fractal distributed properties which are related to a mixing model or normal material into the model. The syntax of the command is:

```
#fractal_box: f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 i1 str1 str2 [i2] [c1]
```

`f1 f2 f3`

are the lower left (x,y,z) coordinates of the parallelepiped, and`f4 f5 f6`

are the upper right (x,y,z) coordinates of the parallelepiped.`f7`

is the fractal dimension which, for an orthogonal parallelepiped, should take values between zero and three.`f8`

is used to weight the fractal in the x direction.`f9`

is used to weight the fractal in the y direction.`f10`

is used to weight the fractal in the z direction.`i1`

is the number of materials to use for the fractal distribution (defined according to the associated mixing model). This should be set to one if using a normal material instead of a mixing model.`str1`

is an identifier for the associated mixing model or material.`str2`

is an identifier for the fractal box itself.`i2`

is an optional parameter which controls the seeding of the random number generator used to create the fractals. By default (if you don’t specify this parameter) the random number generator will be seeded by trying to read data from`/dev/urandom`

(or the Windows analogue) if available or from the clock otherwise.`c1`

is an optional parameter which can be`y`

or`n`

, used to switch on and off dielectric smoothing. If`c1`

is specified then a value for`i2`

must also be present.

For example, to create an orthogonal parallelepiped with fractal distributed properties using a Peplinski mixing model for soil, with 50 different materials over a range of water volumetric fractions from 0.001 - 0.25, you should first define the mixing model using: `#soil_peplinski: 0.5 0.5 2.0 2.66 0.001 0.25 my_soil`

and then specify the fractal box using `#fractal_box: 0 0 0 0.1 0.1 0.1 1.5 1 1 1 50 my_soil my_fractal_box`

.

### #add_surface_roughness:¶

Allows you to add rough surfaces to a `#fractal_box`

in the model. A fractal distribution is used for the profile of the rough surface. The syntax of the command is:

```
#add_surface_roughness: f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 str1 [i1]
```

`f1 f2 f3`

are the lower left (x,y,z) coordinates of a surface on a`#fractal_box`

, and`f4 f5 f6`

are the upper right (x,y,z) coordinates of a surface on a`#fractal_box`

. The coordinates must locate one of the six surfaces of a`#fractal_box`

but do not have to extend over the entire surface.`f7`

is the fractal dimension which, for an orthogonal parallelepiped, should take values between zero and three.`f8`

is used to weight the fractal in the first direction of the surface.`f9`

is used to weight the fractal in the second direction of the surface.`f10 f11`

define lower and upper limits for a range over which the roughness can vary. These limits should be specified relative to the dimensions of the`#fractal_box`

that the rough surface is being applied.`str1`

is an identifier for the`#fractal_box`

that the rough surface should be applied to.`i1`

is an optional parameter which controls the seeding of the random number generator used to create the fractals. By default (if you don’t specify this parameter) the random number generator will be seeded by trying to read data from`/dev/urandom`

(or the Windows analogue) if available or from the clock otherwise.

Up to six `#add_rough_surface commands`

can be given for any `#fractal_box`

corresponding to the six surfaces.

For example, if a `#fractal_box`

has been specified using: `#fractal_box: 0 0 0 0.1 0.1 0.1 1.5 1 1 1 50 my_soil my_fractal_box`

then to apply a rough surface that varys between 85 mm and 110 mm (i.e. valleys that are up to 15 mm deep and peaks that are up to 10 mm tall) to the surface that is in the positive z direction, use `#add_surface_roughness: 0 0 0.1 0.1 0.1 0.1 1.5 1 1 0.085 0.110 my_fractal_box`

.

### #add_surface_water:¶

Allows you to add surface water to a `#fractal_box`

in the model that has had a rough surface applied. The syntax of the command is:

```
#add_surface_water: f1 f2 f3 f4 f5 f6 f7 str1
```

`f1 f2 f3`

are the lower left (x,y,z) coordinates of a surface on a`#fractal_box`

, and`f4 f5 f6`

are the upper right (x,y,z) coordinates of a surface on a`#fractal_box`

. The coordinates must locate one of the six surfaces of a`#fractal_box`

but do not have to extend over the entire surface.`f7`

defines the depth of the water, which should be specified relative to the dimensions of the`#fractal_box`

that the surface water is being applied.`str1`

is an identifier for the`#fractal_box`

that the surface water should be applied to.

For example, to add surface water that is 5 mm deep to an existing `#fractal_box`

that has been specified using `#fractal_box: 0 0 0 0.1 0.1 0.1 1.5 1 1 1 50 my_soil my_fractal_box`

and has had a rough surface applied using `#add_surface_roughness: 0 0 0.1 0.1 0.1 0.1 1.5 1 1 0.085 0.110 my_fractal_box`

, use `#add_surface_water: 0 0 0.1 0.1 0.1 0.1 0.105 my_fractal_box`

.

Note

- The water is modelled using a single-pole Debye formulation with properties \(\epsilon_{rs} = 80.1\), \(\epsilon_{\infty} = 4.9\), and a relaxation time of \(\tau = 9.231 \times 10^{-12}\) seconds (http://dx.doi.org/10.1109/TGRS.2006.873208). If you prefer, gprMax will use your own definition for water as long as it is named
`water`

.

### #add_grass:¶

Allows you to add grass with roots to a `#fractal_box`

in the model. The blades of grass are randomly distributed over the specified surface area and a fractal distribution is used to vary the height of the blades of grass and depth of the grass roots. The syntax of the command is:

```
#add_grass: f1 f2 f3 f4 f5 f6 f7 f8 f9 i1 str1 [i2]
```

`f1 f2 f3`

are the lower left (x,y,z) coordinates of a surface on a`#fractal_box`

, and`f4 f5 f6`

are the upper right (x,y,z) coordinates of a surface on a`#fractal_box`

. The coordinates must locate one of three surfaces (in the positive axis direction) of a`#fractal_box`

but do not have to extend over the entire surface.`f7`

is the fractal dimension which, for an orthogonal parallelepiped, should take values between zero and three.`f8 f9`

define lower and upper limits for a range over which the height of the blades of grass can vary. These limits should be specified relative to the dimensions of the`#fractal_box`

that the grass is being applied.`i1`

is the number of blades of grass that should be applied to the surface area.`str1`

is an identifier for the`#fractal_box`

that the grass should be applied to.`i2`

is an optional parameter which controls the seeding of the random number generator used to create the fractals. By default (if you don’t specify this parameter) the random number generator will be seeded by trying to read data from`/dev/urandom`

(or the Windows analogue) if available or from the clock otherwise.

For example, to apply 100 blades of grass that vary in height between 100 and 150 mm to the entire surface in the positive z direction of a `#fractal_box`

that had been specified using `#fractal_box: 0 0 0 0.1 0.1 0.1 1.5 1 1 50 my_soil my_fractal_box`

, use `#add_grass: 0 0 0.1 0.1 0.1 0.1 1.5 0.2 0.25 100 my_fractal_box`

.

Note

- The grass is modelled using a single-pole Debye formulation with properties \(\epsilon_{rs} = 18.5087\), \(\epsilon_{\infty} = 12.7174\), and a relaxation time of \(\tau = 1.0793 \times 10^{-11}\) seconds (http://dx.doi.org/10.1007/BF00902994). If you prefer, gprMax will use your own definition for grass if you use a material named
`grass`

. The geometry of the blades of grass are defined by the parametric equations: \(x = x_c +s_x {\left( \frac{t}{b_x} \right)}^2\), \(y = y_c +s_y {\left( \frac{t}{b_y} \right)}^2\), and \(z=t\), where \(s_x\) and \(s_y\) can be -1 or 1 which are randomly chosen, and where the constants \(b_x\) and \(b_y\) are random numbers based on a Gaussian distribution.

### #geometry_objects_read:¶

Allows you to insert pre-defined geometry into a model. The geometry is specified using a 3D array of integer numbers stored in a HDF5 file. The integer numbers must correspond to the order of a list of `#material`

commands specified in a text file. The syntax of the command is:

```
#geometry_objects_read: f1 f2 f3 file1 file2
```

`f1 f2 f3`

are the lower left (x,y,z) coordinates in the domain where the lower left corner of the geometry array should be placed.`file1`

is the path to and filename of the HDF5 file that contains an integer array which defines the geometry.`file2`

is the path to and filename of the text file that contains`#material`

commands.`c1`

is an optional parameter which can be`y`

or`n`

, used to switch on and off dielectric smoothing. Dielectric smoothing can only be turned on if the geometry objects that are being read were originally generated by gprMax, i.e. via the`#geometry_objects_write`

command.

Note

- The integer numbers in the HDF5 file must be stored as a NumPy array at the root named
`data`

with type`np.int16`

. - The integer numbers in the HDF5 file correspond to the order of material commands in the materials text file, i.e. if
`#sand: 3 0 1 0`

is the first material in the materials file, it will be associated with any integers that are zero in the HDF5 file. - You can use an integer of -1 in the HDF5 file to indicate not to build any material at that location, i.e. whatever material is already in the model at that location.
- The spatial resolution of the geometry objects must match the spatial resolution defined in the model.
- The spatial resolution must be specified as a root attribute of the HDF5 file with the name
`dx_dy_dz`

equal to a tuple of floats, e.g. (0.002, 0.002, 0.002) - If the geometry objects being imported were originally generated using gprMax, i.e. exported using #geometry_objects_write, then you can use dielectric smoothing as you like when generating the original geometry objects. However, if the geometry objects being imported were generated by an external method then dielectric smoothing will not take place.

For example, to insert a 2x2x2mm^3 AustinMan model with the lower left corner 40mm from the origin of the domain, and using disperive material properties use `#geometry_objects_read: 0.04 0.04 0.04 ../user_libs/AustinManWoman/AustinMan_v2.3_2x2x2.h5 ../user_libs/AustinManWoman/AustinManWoman_materials_dispersive.txt`

### #geometry_objects_write:¶

Allows you to write geometry generated in a model to file. The file can be read back into gprMax using the `#geometry_objects_read`

command. This allows complex geometry that can take some time to generate to be saved to file and more quickly imported into subsequent models. The geometry information is saved as a 3D array of integer numbers stored in a HDF5 file, and corresponding material information is stored in a text file. The integer numbers correspond to the order of a list of `#material`

commands specified in the text file. The syntax of the command is:

```
#geometry_objects_write: f1 f2 f3 f4 f5 f6 file1
```

`f1 f2 f3`

are the lower left (x,y,z) coordinates of the parallelepiped, and`f4 f5 f6`

are the upper right (x,y,z) coordinates of the parallelepiped.`file1`

is the basename for the files where geometry and material information will be stored.

Note

- The structure of the HDF5 file is the same as that described for the
`#geometry_objects_read`

command. - Objects are stored using spatial resolution defined in the model.

## Source and output commands¶

### #waveform:¶

Allows you to specify waveforms to use with sources in the model. The syntax of the command is:

```
#waveform: str1 f1 f2 str2
```

`str1`

is the type of waveform which can be:`gaussian`

which is a Gaussian waveform.`gaussiandot`

which is the first derivative of a Gaussian waveform.`gaussiandotnorm`

which is the normalised first derivative of a Gaussian waveform.`gaussiandotdot`

which is the second derivative of a Gaussian waveform.`gaussiandotdotnorm`

which is the normalised second derivative of a Gaussian waveform.`ricker`

which is a Ricker (or Mexican hat) waveform, i.e. the negative, normalised second derivative of a Gaussian waveform.`gaussianprime`

which is the first derivative of a Gaussian waveform, directly derived from the aforementioned`gaussian`

(see notes below).`gaussiandoubleprime`

which is the second derivative of a Gaussian waveform, directly derived from the aforementioned`gaussian`

(see notes below).`sine`

which is a single cycle of a sine waveform.`contsine`

which is a continuous sine waveform. In order to avoid introducing noise into the calculation the amplitude of the waveform is modulated for the first cycle of the sine wave (ramp excitation).

`f1`

is the scaling of the maximum amplitude of the waveform (for a`#hertzian_dipole`

the units will be Amps, for a`#voltage_source`

or`#transmission_line`

the units will be Volts).`f2`

is the centre frequency of the waveform (Hertz). In the case of the Gaussian waveform it is related to the pulse width.`str2`

is an identifier for the waveform used to assign it to a source.

For example, to specify the normalised first derivate of a Gaussian waveform with an amplitude of one and a centre frequency of 1.2GHz, use: `#waveform: gaussiandotnorm 1 1.2e9 my_gauss_pulse`

.

Note

- The functions used to create the waveforms can be found in the tools section.
`gaussiandot`

,`gaussiandotnorm`

,`gaussiandotdot`

,`gaussiandotdotnorm`

,`ricker`

waveforms have their centre frequencies specified by the user, i.e. they are not derived to the ‘base’`gaussian`

`gaussianprime`

and`gaussiandoubleprime`

waveforms are the first derivative and second derivative of the ‘base’`gaussian`

waveform, i.e. the centre frequencies of the waveforms will rise for the first and second derivatives.

### #excitation_file:¶

Allows you to specify an ASCII file that contains columns of amplitude values that specify custom waveform shapes that can be used with sources in the model.

The first row of each column must begin with a identifier string that will be used as the name of each waveform. Optionally, the first column of the file may contain a time vector of values (which must use the identifier `time`

) to interpolate the amplitude values of the waveform. If a time vector is not given, a vector of time values corresponding to the simulation time step and number of iterations will be used.

If there are less amplitude values than the number of iterations that are going to be performed, the end of the sequence of amplitude values will be padded with zero values up to the number of iterations. If extra amplitude values are specified than needed then they are ignored. The syntax of the command is:

```
#excitation_file: file1 [str1 str2]
```

`file1`

can be the name of the file containing the specified waveform in the same directory as the input file, or`file`

can be the full path to the file containing the specified waveform (allowing you to specify any location).`str1`

and`str2`

are an optional parameter pair that allow values for`kind`

and`fill_value`

to be passed to the interpolation function (scipy.interpolate.interp1d). If they are not given the default values for the function will be used.

For example, to specify the file `my_waves.txt`

, which contains two custom waveform shapes, use: `#excitation_file: my_waves.txt`

. The contents of the file `my_waves.txt`

would take the form:

```
time my_pulse1 my_pulse2
0 0 0
1.926e-12 1.2e-6 0
3.852e-12 1.3e-6 1.0e-1
5.778e-12 5.0e-6 1.5e-1
... ... ...
... ... ...
... ... ...
```

Then to use `my_pulse1`

custom waveform shape with, for example, a z-polarised Hertzian dipole source:

```
#hertzian_dipole: z 0.5 0.5 0.5 my_pulse1
```

Note

- The
`#waveform`

command is not necessary when using a custom waveform excitation, only the`#excitation_file`

command and whatever source is going to be used with the custom waveform excitation.

### #hertzian_dipole:¶

Allows you to specify a current density term at an electric field location - the simplest excitation, often referred to as an additive or soft source.

where \(J_s\) is the current density, \(I\) is the current, \(\Delta l\) is the length of the infinitesimal electric dipole, and \(\Delta x\), \(\Delta y\), and \(\Delta z\) are the spatial resolution of the grid.

Note

- \(\Delta l\) is set equal to \(\Delta x\), \(\Delta y\), or \(\Delta z\) depending on the specified polarisation.

The syntax of the command is:

```
#hertzian_dipole: c1 f1 f2 f3 str1 [f4 f5]
```

`c1`

is the polarisation of the source and can be`x`

,`y`

, or`z`

.`f1 f2 f3`

are the coordinates (x,y,z) of the source in the model.`f4 f5`

are optional parameters.`f4`

is a time delay in starting the source.`f5`

is a time to remove the source. If the time window is longer than the source removal time then the source will stop after the source removal time. If the source removal time is longer than the time window then the source will be active for the entire time window. If`f4 f5`

are omitted the source will start at the beginning of time window and stop at the end of the time window.`str1`

is the identifier of the waveform that should be used with the source.

For example, to use a x-polarised Hertzian dipole with unit amplitude and a 600 MHz centre frequency Ricker waveform, use: `#waveform: ricker 1 600e6 my_ricker_pulse`

and `#hertzian_dipole: x 0.05 0.05 0.05 my_ricker_pulse`

.

Note

- When a
`#hertzian_dipole`

is used in a 2D simulation it acts as a line source of current in the invariant (geometry) direction of the simulation.

### #magnetic_dipole:¶

This will simulate an infinitesimal magnetic dipole. This is often referred to as an additive or soft source. The syntax of the command is:

```
#magnetic_dipole: c1 f1 f2 f3 str1 [f4 f5]
```

`c1`

is the polarisation of the source and can be`x`

,`y`

, or`z`

.`f1 f2 f3`

are the coordinates (x,y,z) of the source in the model.`f4 f5`

are optional parameters.`f4`

is a time delay in starting the source.`f5`

is a time to remove the source. If the time window is longer than the source removal time then the source will stop after the source removal time. If the source removal time is longer than the time window then the source will be active for the entire time window. If`f4 f5`

are omitted the source will start at the beginning of time window and stop at the end of the time window.`str1`

is the identifier of the waveform that should be used with the source.

### #voltage_source:¶

Allows you to introduce a voltage source at an electric field location. It can be a hard source if it’s resistance is zero, i.e. the time variation of the specified electric field component is prescribed, or if it’s resistance is non-zero it behaves as a resistive voltage source. It is useful for exciting antennas when the physical properties of the antenna are included in the model. The syntax of the command is:

```
#voltage_source: c1 f1 f2 f3 f4 str1 [f5 f6]
```

`c1`

is the polarisation of the source and can be`x`

,`y`

, or`z`

.`f1 f2 f3`

are the coordinates (x,y,z) of the source in the model.`f4`

is the internal resistance of the voltage source in Ohms. If`f4`

is set to zero then the voltage source is a hard source. That means it prescribes the value of the electric field component. If the waveform becomes zero then the source is perfectly reflecting.`f5 f6`

are optional parameters.`f5`

is a time delay in starting the source.`f6`

is a time to remove the source. If the time window is longer than the source removal time then the source will stop after the source removal time. If the source removal time is longer than the time window then the source will be active for the entire time window. If`f5 f6`

are omitted the source will start at the beginning of time window and stop at the end of the time window.`str1`

is the identifier of the waveform that should be used with the source.

For example, to specify a y directed voltage source with an internal resistance of 50 Ohms, an amplitude of five, and a 1.2 GHz centre frequency Gaussian waveform use: `#waveform: gaussian 5 1.2e9 my_gauss_pulse`

and `#voltage_source: y 0.05 0.05 0.05 50 my_gauss_pulse`

.

### #transmission_line:¶

Allows you to introduce a one-dimensional transmission line model [MAL1994] at an electric field location. The transmission line can have a specified resistance greater than zero and less than the impedance of free space (376.73 Ohms). It is useful for exciting antennas when the physical properties of the antenna are included in the model. The syntax of the command is:

```
#transmission_line: c1 f1 f2 f3 f4 str1 [f5 f6]
```

`c1`

is the polarisation of the transmission line and can be`x`

,`y`

, or`z`

.`f1 f2 f3`

are the coordinates (x,y,z) of the transmission line in the model.`f4`

is the characteristic resistance of the transmission line source in Ohms. It can be any value greater than zero and less than the impedance of free space (376.73 Ohms).`f5 f6`

are optional parameters.`f5`

is a time delay in starting the excitation of the transmission line.`f6`

is a time to remove the excitation of the transmission line. If the time window is longer than the excitation of the transmission line removal time then the excitation of the transmission line will stop after the excitation of the transmission line removal time. If the excitation of the transmission line removal time is longer than the time window then the excitation of the transmission line will be active for the entire time window. If`f5 f6`

are omitted the excitation of the transmission line will start at the beginning of time window and stop at the end of the time window.`str1`

is the identifier of the waveform that should be used with the source.

Time histories of voltage and current values in the transmission line are saved to the output file. These are documented in the output file section. These parameters are useful for calculating characteristics of an antenna such as the input impedance or S-parameters. gprMax includes a Python module (in the `tools`

package) to help you view the input impedance and s11 parameter from an antenna model fed using a transmission line. Details of how to use this module is given in the tools section.

For example, to specify a z directed transmission line source with a resistance of 75 Ohms, an amplitude of five, and a 1.2 GHz centre frequency Gaussian waveform use: `#waveform: gaussian 5 1.2e9 my_gauss_pulse`

and `#transmission_line: z 0.05 0.05 0.05 75 my_gauss_pulse`

.

An example antenna model using a transmission line can be found in the examples section.

### #rx:¶

Allows you to introduce output points into the model. These are locations where the values of the electric and magnetic field components over the number of iterations of the model will be saved to file. The syntax of the command is:

```
#rx: f1 f2 f3 [str1 str2]
```

`f1 f2 f3`

are the coordinates (x,y,z) of the receiver in the model.`str1`

is the identifier of the receiver.`str2`

is a list of outputs with this receiver. It can be any selection from`Ex`

,`Ey`

,`Ez`

,`Hx`

,`Hy`

,`Hz`

,`Ix`

,`Iy`

, or`Iz`

.

Note

- When the optional parameters
`str1`

and`str2`

are not given all the electric and magnetic field components will be output with the receiver point.

### #rx_array:¶

Provides a simple method of defining multiple output points in the model. The syntax of the command is:

```
#rx_array: f1 f2 f3 f4 f5 f6 f7 f8 f9
```

`f1 f2 f3`

are the lower left (x,y,z) coordinates of the output line/rectangle/volume, and`f4 f5 f6`

are the upper right (x,y,z) coordinates of the output line/rectangle/volume.`f7 f8 f9`

are the increments (x,y,z) which define the number of output points in each direction.`f7`

,`f8`

, or`f9`

can be set to zero to prevent any output points in a particular direction. Otherwise, the minimum value of`f7`

is \(\Delta x\), the minimum value of`f8`

is \(\Delta y\), and the minimum value of`f9`

is \(\Delta z\).

### #src_steps: and #rx_steps:¶

Provides a simple method to allow you to move the location of all simple sources (`#src_steps`

) or all receivers (`#rx_steps`

) between runs of a model. The syntax of the commands is:

```
#src_steps: f1 f2 f3
#rx_steps: f1 f2 f3
```

`f1 f2 f3`

are increments (x,y,z) to move all simple sources (`#hertzian_dipole`

or `#magnetic_dipole`

) or all receivers (created using either `#rx`

or `#rx_array`

commands).

Note

`#src_steps`

and`#rx_steps`

are not suitable for moving sources which have associated geometry, e.g. antenna models.- Values for
`#src_steps`

and`#rx_steps`

should not be changed between model runs using Python scripting.

### #snapshot:¶

Allows you to obtain information about the electromagnetic fields within a volume of the model at a given time instant. The file(s) use the open source Visualization ToolKit (VTK) format which can be viewed in many free readers, such as Paraview. The syntax of this command is:

```
#snapshot: f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 file1
```

or

```
#snapshot: f1 f2 f3 f4 f5 f6 f7 f8 f9 i1 file1
```

`f1 f2 f3`

are the lower left (x,y,z) coordinates of the volume of the snapshot in metres.`f4 f5 f6`

are the upper right (x,y,z) coordinates of the volume of the snapshot in metres.`f7 f8 f9`

are the spatial discretisation of the snapshot in metres.`f10`

or`i1`

are the time in seconds (float) or the iteration number (integer) which denote the point in time at which the snapshot will be taken.`file1`

is the name of the file where the snapshot will be stored. Snapshot files are automatically stored in a directory with the name of the input file appended with ‘_snaps’. For multiple model runs each model run will have its own directory, i.e. ‘_snaps1’, ‘snaps2’ etc…

For example to save a snapshot of the electromagnetic fields in the model at a simulated time of 3 nanoseconds use: `#snapshot: 0 0 0 1 1 1 0.1 0.1 0.1 3e-9 snap1`

Tip

You can take advantage of Python scripting to easily create a series of snapshots. For example, to create 30 snapshots starting at time 0.1ns until 3ns in intervals of 0.1ns, use the following code snippet in your input file. Replace `x1 y1 z1 x2 y2 z2 dx dy dz`

accordingly.

```
#python:
for i in range(1, 31):
print('#snapshot: x1 y1 z1 x2 y2 z2 dx dy dz {} snapshot{}'.format((i/10)*1e-9, i))
#end_python:
```

## PML commands¶

The default behaviour is for gprMax to use a first order CFS PML that has a thickness of 10 cells on each of the six sides of the model domain. This can be altered by using the following commands.

### #pml_cells:¶

Allows you to control the number of cells of PML that are used on the six sides of the model domain. The PML is defined within the model domain, i.e. it is not added to the domain size. The syntax of the command is:

```
#pml_cells: i1 [i2 i3 i4 i5 i6]
```

`i1`

is the number of cells of PML to use on all sides of the model domain (can be set to zero to completely switch off the PML), or`i1`

is the number of cells of PML to use on the side of the model domain nearest the origin of the x-axis (x0).`i2`

is the number of cells of PML to use on the side of the model domain nearest the origin of the y-axis (y0).`i3`

is the number of cells of PML to use on the side of the model domain nearest the origin of the z-axis (z0).`i4`

is the number of cells of PML to use on the side of the model domain furthest from the origin of the x-axis (xmax).`i5`

is the number of cells of PML to use on the side of the model domain furthest from the origin of the y-axis (ymax).`i6`

is the number of cells of PML to use on the side of the model domain furthest from the origin of the z-axis (zmax).`i1 i2 i3 i4 i5 i6`

may be set to zero to turn off the PML on a specific side of the model domain.

For example to use a PML with 20 cells (thicker than the default 10 cells) on only the z-axis sides of the domain use:

```
#pml_cells: 10 10 20 10 10 20
```

### #pml_cfs:¶

Allows you (advanced) control of the parameters that are used to build each order of the PML. Up to a second order PML can currently be specified, i.e. by using two `#pml_cfs`

commands. The syntax of the command is:

```
#pml_cfs: str1 str2 f1 f2 str3 str4 f3 f4 str5 str6 f5 f6
```

`str1`

is the type of scaling to use for the CFS \(\alpha\) parameter. It can be`constant`

,`linear`

,`quadratic`

,`cubic`

,`quartic`

,`quintic`

and`sextic`

.`str2`

is the direction of the scaling to use for the CFS \(\alpha\) parameter. It can be`forward`

or`reverse`

.`f1 f2`

are the minimum and maximum values for the CFS \(\alpha\) parameter.`str3`

is the type of scaling to use for the CFS \(\kappa\) parameter. It can be`constant`

,`linear`

,`quadratic`

,`cubic`

,`quartic`

,`quintic`

and`sextic`

.`str4`

is the direction of the scaling to use for the CFS \(\kappa\) parameter. It can be`forward`

or`reverse`

.`f3 f4`

are the minimum and maximum values for the CFS \(\kappa\) parameter. The minimum value for the CFS \(\kappa\) parameter is one.`str5`

is the type of scaling to use for the CFS \(\sigma\) parameter. It can be`constant`

,`linear`

,`quadratic`

,`cubic`

,`quartic`

,`quintic`

and`sextic`

.`str6`

is the direction of the scaling to use for the CFS \(\sigma\) parameter. It can be`forward`

or`reverse`

.`f5 f6`

are the minimum and maximum values for the CFS \(\sigma\) parameter.

The CFS values (which are internally specified) used for the default standard first order PML are: `#pml_cfs: constant forward 0 0 constant forward 1 1 quartic forward 0 None`

. Specifying ‘None’ for the maximum value of \(\sigma\) forces gprMax to calculate it internally based on the relative permittivity and permeability of the underlying materials in the model.

The parameters will be applied to all slabs of the PML that are switched on.

Tip

`forward`

direction implies minimum parameter value at the inner boundary of the PML and maximum parameter value at the edge of computational domain, `reverse`

is the opposite.