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Solve the following python programming problem given the following code:
# Configure model presets
from examples.seismic import demo_model

# Enable model presets here:
preset = 'layers-isotropic'  # A simple but cheap model (recommended)
# preset = 'marmousi2d-isotropic'  # A larger more realistic model

# Standard preset with a simple two-layer model
if preset == 'layers-isotropic':
    def create_model(grid=None):
        return demo_model('layers-isotropic', origin=(0., 0.), shape=(101, 101),
                          spacing=(10., 10.), nbl=20, grid=grid, nlayers=2)
    filter_sigma = (1, 1)
    nshots = 21
    nreceivers = 101
    t0 = 0.
    tn = 1000.  # Simulation last 1 second (1000 ms)
    f0 = 0.010  # Source peak frequency is 10Hz (0.010 kHz)

# A more computationally demanding preset based on the 2D Marmousi model
if preset == 'marmousi2d-isotropic':
    def create_model(grid=None):
        return demo_model('marmousi2d-isotropic', data_path='../../../../data/',
                          grid=grid, nbl=20)
    filter_sigma = (6, 6)
    nshots = 301  # Need good covergae in shots, one every two grid points
    nreceivers = 601  # One recevier every grid point
    t0 = 0.
    tn = 3500.  # Simulation last 3.5 second (3500 ms)
    f0 = 0.025  # Source peak frequency is 25Hz (0.025 kHz)
Given the above code, generate some markdown instructions for it.
assistant
#Here is the Python code you requested.

Computational considerations
Seismic inversion algorithms are generally very computationally demanding and require a large amount of memory to store the forward wavefield. In order to keep this tutorial as lightweight as possible we are using a very simple
velocity model that requires low temporal and spatial resolution. For a more realistic model, a second set of preset parameters for a reduced version of the 2D Marmousi data set [1] is provided below in comments. This can be run to create some more realistic subsurface images. However, this second preset is more computationally demanding and requires a slightly more powerful workstation.