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已同步 2025-08-06 12:36:51 +08:00
Added wire dipole model file.
这个提交包含在：
Craig Warren
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***********
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2D Examples
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***********
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This section provides some general example models in 2D that demonstrate how to use certain features of gprMax. Each example comes with an input file which you can download and run.
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.. _example-2D-Ascan:
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A-scan from a metal cylinder
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============================
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:download:`cylinder_Ascan_2D.in <models/cylinder_Ascan_2D.in>`
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This example is the gprMax equivalent of 'Hello World'! It demonstrates how to simulate a single trace (A-scan) from a metal cylinder buried in a dielectric half-space.
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.. literalinclude:: models/cylinder_Ascan_2D.in
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:language: none
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:linenos:
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The geometry of the scenario is straightforward and is shown in :numref:`cylinder_half_space_geo`.
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.. _cylinder_half_space_geo:
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.. figure:: images/cylinder_half_space_geo.png
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:width: 600 px
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Geometry of a 2D model of a metal cylinder buried in a dielectric half-space.
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For this initial example a detailed description of what each command in the input file does and why each command was used is given. The following steps explain the steps taken to build the input file:
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Determine the constitutive parameters for the materials
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-------------------------------------------------------
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There will be three different materials in the model representing air, the dielectric half-space, and the metal cylinder. Air (free space) already exists as a built-in material in gprMax which can be accessed using the ``free_space`` identifier. The metal cylinder will be modelled as a Perfect Electric Conductor, which again exists as a built-in material in gprMax and can be accessed using the ``pec`` identifier. So the only material which has to be defined is for the dielectric half-space. It is a non-magnetic material, i.e. :math:`\mu_r=1` and :math:`\sigma_*=0` and with a relative permittivity of six, :math:`\epsilon_r=6`, and zero conductivity, :math:`\sigma=0`. The identifier ``half_space`` will be used.
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.. code-block:: none
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#material: 6 0 1 0 half_space
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Determine the source type and excitation frequency
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--------------------------------------------------
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These should generally be known, often based on the GPR system or scenario being modelled. Low frequencies are used where significant penetration depth is important, whereas high frequencies are used where less penetration and better resolution are required. In this case a theoretical Hertzian dipole source fed with a Ricker waveform with a centre frequency of :math:`f_c=1.5~\textrm{GHz}` will be used to simulate the GPR antenna (later examples will demonstrate how to include a model of the actual GPR antenna in the simulation).
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.. code-block:: none
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#waveform: ricker 1 1.5e9 my_ricker
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#hertzian_dipole: z 0.100 0.170 0 my_ricker
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The Ricker waveform is created with the ``#waveform`` command, specifying an amplitude of one, centre frequency of 1.5 GHz and picking an arbitrary identifier of ``my_ricker``. The Hertzian dipole source is created using the ``#hertzian_dipole`` command, specifying a z direction polarisation (the survey direction if a B-scan were being created), location on the surface of the slab, and using the Ricker waveform already created.
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Calculate a spatial resolution and domain size
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----------------------------------------------
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In the :ref:`guidance` section it was stated that a good *rule-of-thumb* was that the spatial resolution should be one tenth of the smallest wavelength present in the model. To determine the smallest wavelength, the highest frequency present in the model is required. This is not the centre frequency of the Ricker waveform! By examining the spectrum of the Ricker waveform it is evident much higher frequencies are present, 2-3 times as high as the centre frequency. So the highest frequency present in the model is likely to be around 4 GHz. The wavelength at 4 GHz in the half-space would be:
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.. math:: \lambda = \frac{c}{f \sqrt{\epsilon_r}} = \frac{299792458}{4\times 10^9 \sqrt{6}} \approx 31~\textrm{mm}
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This would give a minimum spatial resolution of 3 mm. However, the diameter of the cylinder is 20 mm so would be resolved to 7 cells. Therefore a better choice would be 2 mm which resolves the diameter of the rebar to 10 cells.
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.. code-block:: none
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#dx_dy_dz: 0.002 0.002 0.002
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The domain size should be enough to enclose the volume of interest, plus allow 10 cells (if using the default value) for the PML absorbing boundary conditions and approximately another 10 cells of between the PML and any objects of interest. In this case the plan is to take a B-scan of the scenario (in the next example) so the domain should be large enough to do that. Although this is a 2D model one cell must be specified in the infinite direction (in this case the z direction) of the domain.
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.. code-block:: none
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#domain: 0.240 0.190 0.002
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Since this is a 2D model the PML should be switched off for the two faces of the domain in the infinite direction (in this case the z direction). This is achieved using the command:
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.. code-block:: none
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#pml_cells: 10 10 0 10 10 0
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Choose a time window
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--------------------
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It is desired to see the reflection from the cylinder, therefore the time window must be long enough to allow the electromagnetic waves to propagate from the source through the half-space to the cylinder and be reflected back to the receiver, i.e.
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.. math:: t = \frac{0.180}{\frac{c}{\sqrt{6}}} \approx 1.5~\textrm{ns}
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This is the minimum time required, but the pulse wavelet has a width of 1.2 ns, to allow for the entire pulse wavelet to be reflected back to the receiver an initial time window of 3 ns will be tested.
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.. code-block:: none
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#time_window: 3e-9
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gprMax will calculate the time step required for the model using the CFL condition in 3D. However, since this is a 2D model the time step can be relaxed to the CFL condition in 2D. This is achieved using the command:
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.. code-block:: none
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#time_step_limit_type: 2D
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Create the objects
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------------------
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Now physical objects can created for the half-space and the cylinder. First the ``#box`` command will be used to create the half-space and then the ``#cylinder`` command will be given which will overwrite the properties of the half-space with those of the cylinder at the location of the cylinder.
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.. code-block:: none
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#box: 0 0 0 0.240 0.170 0.002 half_space
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#cylinder: 0.120 0.080 0 0.120 0.080 0.002 0.010 pec
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Run the model
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-------------
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You can now run the model:
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.. code-block:: none
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python -m gprMax cylinder_Ascan_2D.in
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View the results
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----------------
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You should have produced an output file ``cylinder_Ascan_2D.out``. You can view the results (see :ref:`output` section) using the command:
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.. code-block:: none
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python -m tools.plot_Ascan cylinder_Ascan_2D.out
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:numref:`cylinder_Ascan_results` shows the time history of the electric and magnetic field components at the receiver location. The :math:`E_z` field component can be converted to voltage which represents the A-scan (trace). The initial part of the signal (<1.5 ns) represents the direct wave from transmitter to receiver. Then comes the reflected wavelet from the metal cylinder.
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.. _cylinder_Ascan_results:
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.. figure:: images/cylinder_Ascan.png
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Field outputs from a model of a metal cylinder buried in a dielectric half-space.
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B-scan from a metal cylinder
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============================
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:download:`cylinder_Bscan_2D.in <models/cylinder_Bscan_2D.in>`
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This example using the same geometry as the previous example but this time a B-scan is created. A B-scan is composed of multiple traces (A-scans) recorded as the source and receiver are moved over the target, in this case the metal cylinder.
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.. literalinclude:: models/cylinder_Bscan_2D.in
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:language: none
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:linenos:
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The differences between this input file and the one from the A-scan are the x coordinates of the source and receiver (lines 11 adnd 12), and the commands needed to move the source and receiver (lines 13 and 14). The source and receiver are offset by 40mm from each other as before. They are now shifted to the starting position for the scan. The ``#src_steps`` command is used to tell gprMax to move every source in the model by specified steps each time the model is run. Similarly, the ``#rx_steps`` command is used to tell gprMax to move every receiver in the model by specified steps each time the model is run. Note, the same functionality could be achieved by using a block of Python code in the input file to move the source and receiver (for further details see the :ref:`Python section <python>`).
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To run the model for a B-scan is slightly different than for a single A-scan. You must pass an optional argument to gprMax to specify the number of times the model should be run, which in this case is the number of A-scans (traces) that will comprise the B-scan. For a B-scan over a distance of 120mm with a step of 2mm that is 60 A-scans.
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.. code-block:: none
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python -m gprMax cylinder_Bscan_2D.in -n 60
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Results
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-------
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You should have produced 60 output files, one for each A-scan, with names ``cylinder_Bscan_2D1.out``, ``cylinder_Bscan_2D2.out`` etc... These can be combined into a single file using the command:
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.. code-block:: none
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python -m tools.outputfiles_merge cylinder_Bscan_2D 60
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You should see a combined output file ``cylinder_Bscan_2D_all.out``. The tool will ask you if you want to delete the original single A-scan output files or keep them.
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You can now view an image of the B-scan using the command:
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.. code-block:: none
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python -m tools.plot_Bscan cylinder_Bscan_2D_all.out --field Ez
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:numref:`cylinder_Bscan_results` shows the B-scan (image of the Ez field). As expected a hyperbolic response is present from the metal cylinder.
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.. _cylinder_Bscan_results:
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.. figure:: images/cylinder_Bscan_results.png
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:width: 800px
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B-scan of model of a metal cylinder buried in a dielectric half-space.
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@@ -1,111 +0,0 @@
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***********
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3D Examples
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***********
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This section provides some general example models in 3D that demonstrate how to use certain features of gprMax. Each example comes with an input file which you can download and run.
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Bowtie antenna model
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====================
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:download:`antenna_MALA_1200_fs.in <models/antenna_MALA_1200_fs.in>`
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This example demonstrates how to use one of the built-in antenna models in a simulation. Using a model of an antenna rather than a simple source, such as a Hertzian dipole, can improve the accuracy of the results of a simulation for many situations. It is especially important when the target is in the near-field of the antenna and there are complex interactions between the antenna and the environment. The simulation uses the model of an antenna similar to a MALA 1.2GHz antenna.
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.. literalinclude:: models/antenna_MALA_1200_fs.in
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:language: none
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:linenos:
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.. figure:: images/antenna_MALA_1200.png
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:width: 600 px
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FDTD geometry mesh showing an antenna model similar to a MALA 1.2GHz antenna (skid removed for illustrative purposes).
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The antenna model is loaded from a Python module and inserted into the input file just like another geometry command. The arguments for the ``antenna_like_MALA_1200`` function specify its (x, y, z) location as 0.132m, 0.095m, 0.100m using a 1mm spatial resolution. In this example the antenna is the only object in the model, i.e. the antenna is in free space. When the simulation is run two geometry files for the antenna are produced along with an output file which contains a single receiver (the antenna output). The antenna bowties are aligned with the y axis so the output is the y component of the electric field. More information can be found in the :ref:`Python section <python>`.
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Results
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-------
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:numref:`antenna_MALA_1200_fs_results` shows the time history of the electric and magnetic field components from the receiver bowtie of the antenna model. The antenna bowties are aligned with the y axis so the output will be the Ey component of the electric field.
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.. _antenna_MALA_1200_fs_results:
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.. figure:: images/antenna_MALA_1200_fs_results.png
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Field outputs from the receiver bowtie of a model of an antenna similar to a MALA 1.2GHz antenna.
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B-scan with an antenna model
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============================
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:download:`GSSI_1500_cylinder_Bscan.in <models/GSSI_1500_cylinder_Bscan.in>`
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This example demonstrates how to create a B-scan with an antenna model. The scenario is purposely simple to illustrate the method. A metal cylinder of diameter 20mm is buried in a dielectric half-space which has a relative permittivity of six. The simulation uses the model of an antenna similar to a GSSI 1.5GHz antenna.
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.. literalinclude:: models/GSSI_1500_cylinder_Bscan.in
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:language: none
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:linenos:
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.. figure:: images/GSSI_1500_cylinder.png
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:width: 600 px
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FDTD geometry mesh showing a metal cylinder buried in a half-space and an antenna model similar to a GSSI 1.5GHz antenna.
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The antenna must be moved to a new position for every single A-scan (trace) in the B-scan. In this example the B-scan distance will be 270mm with a trace every 5mm, so 54 model runs will be required.
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.. code-block:: none
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python -m gprMax GSSI_1500_cylinder_Bscan.in -n 54
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The total number of runs for a model as well as the number of the current run of the model are stored and can be accessed in Python as ``number_model_runs`` and ``current_model_run``. The ``current_model_run`` can be used to move the position of the antenna for every run of the model as shown in Line 13. The antenna will be moved 5mm in the x direction for every new run of the model.
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Results
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-------
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:numref:`GSSI_1500_cylinder_Bscan_results` shows the B-scan (image of the Ey field). As expected a hyperbolic response is present from the metal cylinder.
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.. _GSSI_1500_cylinder_Bscan_results:
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.. figure:: images/GSSI_1500_cylinder_Bscan_results.png
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:width: 800px
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B-scan of model of a metal cylinder buried in a dielectric half-space with a model of an antenna similar to a GSSI 1.5GHz antenna.
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Building a heterogeneous soil
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=============================
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:download:`heterogeneous_soil.in <models/heterogeneous_soil.in>`
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This example demonstrates how to build a more realistic soil model using a stochastic distribution of dielectric properties. A mixing model for soils proposed by Peplinski (http://dx.doi.org/10.1109/36.387598) is used to define a series of dispersive material properties for the soil.
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.. literalinclude:: models/heterogeneous_soil.in
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:language: none
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:linenos:
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.. figure:: images/heterogeneous_soil.png
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:width: 600 px
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FDTD geometry mesh showing a heterogeneous soil model with a rough surface.
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Line 10 defines a series of dispersive materials to represent a soil with sand fraction 0.5, clay fraction 0.5, bulk density :math:`2~g/cm^3`, sand particle density of :math:`2.66~g/cm^3`, and a volumetric water fraction range of 0.001 - 0.25. The volumetric water fraction is given as a range which is what defines a series of dispersive materials.
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These materials can then be distributed stochastically over a volume using the ``#fractal_box`` command. Line 11 defines a volume, a fractal dimension, a number of materials, and a mixing model to use. The fractal dimension, 1.5, controls how the materials are stochastically distributed. The fractal weightings, 1, 1, 1, weight the fractal in the x, y, and z directions. The number of materials, 50, specifies how many dispersive materials to create using the mixing model (``my_soil``).
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Adding rough surfaces
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---------------------
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A rough surface can be added to any side of ``#fractal_box`` using,
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.. code-block:: none
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#add_surface_roughness: 0 0 0.070 0.15 0.15 0.070 1.5 1 1 0.065 0.080 my_soil_box
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which defines one of the surfaces of the ``#fractal_box``, a fractal dimension, and minimum and maximum values for the height of the roughness (relative to the original ``#fractal_box`` volume). In this example the roughness will be stochastically distributed with troughs up to 5mm deep, and peaks up to 10mm high.
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More information, including adding surface water and vegetation, can be found in the :ref:`section on using the fractal box command <fractals>`.
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#title: Wire antenna - half-wavelength dipole in free-space
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#domain: 0.050 0.050 0.200
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#dx_dy_dz: 0.001 0.001 0.001
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#time_window: 5e-9
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#waveform: gaussian 1 1.5e9 mypulse
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#transmission_line: z 0.025 0.025 0.100 73 mypulse
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## 136mm length
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#cylinder: 0.025 0.025 0.032 0.025 0.025 0.168 0.003 pec
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#cylinder: 0.025 0.025 0.100 0.025 0.025 0.101 0.003 free_space
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geometry_view: 0 0 0 0.050 0.050 0.200 0.001 0.001 0.001 antenna_wire_dipole_fs n
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python:
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for i in range(1, 61):
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print('#snapshot: 0 0 0 0.049 0.049 0.199 0.001 0.001 0.001 {} snapshot{}'.format((i/10)*1e-9, i))
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end_python:
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