Updated geometry image and description for cylinder_Ascan_2D example.

docs/source/examples_simple_2D.rst

@@ -17,7 +17,7 @@ This example is the GPR modelling equivalent of 'Hello World'! It demonstrates h
     :language: none
     :linenos:
 
-The geometry of the scenario is straightforward and is shown in :numref:`cylinder_half_space_geo`.
+The geometry of the scenario is straightforward and is shown in :numref:`cylinder_half_space_geo`. The transparent area around the boundary of the domain represents the :ref:`PML region <pml>`. The red cell is the source and the blue cell is the receiver.
 
 .. _cylinder_half_space_geo:
 

docs/source/gprmodelling.rst

@@ -98,6 +98,8 @@ An other important factor which influences the spatial discretization is the err
 Note that in general low-loss media wavelengths are much smaller compared to free space.
 
 
+.. _pml:
+
 Absorbing boundary conditions
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