How can I insert anomalies in a cylindrical mesh?

In this case, you need to find the indices of the cell-centered grid where you want the anomaly to be located and you can then change the model values at that location. This notebook shows an example where we input a sphere: https://github.com/simpeg/em-notebooks/blob/master/notebooks/TDEM_vmd_sounding_over_sphere.ipynb

```
# create a vector that has one entry for every cell center
sigma = sig_air*np.ones(mesh.nC) # start by defining the conductivity of the air everwhere
sigma[mesh.gridCC[:,2] < 0.] = sig_halfspace # assign halfspace cells below the earth
sigma_background = sigma.copy()
# indices of the sphere (where (x-x0)**2 + (z-z0)**2 <= R**2)
sphere_ind = (mesh.gridCC[:,0]**2 + (mesh.gridCC[:,2] - sphere_z)**2) <= sphere_radius**2
sigma[sphere_ind] = sig_sphere # assign the conductivity of the sphere
```