diff --git a/gprMax/subgrids/base.py b/gprMax/subgrids/base.py
index 9ba9ee82..0b1364cd 100644
--- a/gprMax/subgrids/base.py
+++ b/gprMax/subgrids/base.py
@@ -17,19 +17,9 @@
 # along with gprMax.  If not, see <http://www.gnu.org/licenses/>.
 from ..grid import FDTDGrid
 
-from ..materials import Material
-
-from scipy.constants import mu_0
-from scipy.constants import epsilon_0
 from scipy.constants import c
-
 import numpy as np
 
-from colorama import init
-from colorama import Fore
-from colorama import Style
-init()
-
 
 class SubGridBase(FDTDGrid):
 
@@ -64,6 +54,7 @@ class SubGridBase(FDTDGrid):
 
         # Number of sub cells to extend the sub grid beyond the IS boundary
         d_to_pml = self.s_is_os_sep + self.pml_separation
+        # index of the is
         self.n_boundary_cells = d_to_pml + self.pmlthickness['x0']
         self.n_boundary_cells_x = d_to_pml + self.pmlthickness['x0']
         self.n_boundary_cells_y = d_to_pml + self.pmlthickness['y0']
@@ -72,17 +63,17 @@ class SubGridBase(FDTDGrid):
         self.interpolation = kwargs['interpolation']
 
     def calculate_dt(self):
+        """Calculate dt at the CFL limit."""
         self.dt = (1 / (c * np.sqrt(
-                    (1 / self.dx) * (1 / self.dx) +
-                    (1 / self.dy) * (1 / self.dy) +
-                    (1 / self.dz) * (1 / self.dz))))
-
+                  (1 / self.dx) * (1 / self.dx) +
+                  (1 / self.dy) * (1 / self.dy) +
+                  (1 / self.dz) * (1 / self.dz))))
 
     def main_grid_index_to_subgrid_index(self, i, j, k):
+        """Calculate local subgrid index from global main grid index."""
         i_s = self.n_boundary_cells_x + (i - self.i0) * self.ratio
         j_s = self.n_boundary_cells_y + (j - self.j0) * self.ratio
         k_s = self.n_boundary_cells_z + (k - self.k0) * self.ratio
-
         return (i_s, j_s, k_s)
 
     def initialise_geometry_arrays(self):