# Tutorial 08: Elastic Wave Propagation

```{note}
This tutorial is available as a Python script [`examples/08_elastic_forward.py`](https://github.com/GeoBrain-Project/geobrain/blob/main/examples/08_elastic_forward.py) and an interactive Jupyter notebook [`examples/notebooks/08_elastic_forward.ipynb`](https://github.com/GeoBrain-Project/geobrain/blob/main/examples/notebooks/08_elastic_forward.ipynb).
```

Run 2D elastic wave simulations on the Marmousi2 model using the
finite-difference time-domain (FDTD) solver with PML absorbing boundaries.

## What You Will Learn

- Build `IsotropicElasticModel` with Vp, Vs, and density
- Configure PML boundaries with free surface
- Run `ElasticPropagator` and extract multi-component seismograms (P, Vx, Vz)
- Compare acoustic vs. elastic simulations on the same model
- Visualize wavefield energy and snapshot animations

## Key Concepts

**Elastic modeling** captures both P-wave and S-wave propagation, including
mode conversions at interfaces. Compared to the acoustic approximation,
elastic modeling reveals S-wave arrivals and converted phases that carry
additional information about lithology and fluid content.

## Code

```python
from geobrain.physics.wave import (
    GridConfig, BoundaryConfig, IsotropicElasticModel,
    Source, Receiver, Survey, ElasticPropagator, RickerWavelet,
)

grid = GridConfig(nx=200, nz=88, dx=40.0, dz=40.0)
boundary = BoundaryConfig(type='pml', n_layers=30, free_surface=True)

model = IsotropicElasticModel(
    grid=grid, boundary=boundary,
    vp=vp, vs=vs, rho=rho,
    device='cuda',
)

propagator = ElasticPropagator(model, survey, device='cuda')
result = propagator.forward(checkpoint_segments=4)

# Multi-component output
p_data = result.p       # Pressure
vx_data = result.vx     # Horizontal velocity
vz_data = result.vz     # Vertical velocity
```

## Results

```{figure} ../../examples/figs/08_elastic_model.png
:width: 100%
:name: fig-08-elastic-model

Marmousi2 elastic model: Vp, Vs, density.
```

```{figure} ../../examples/figs/08_wavefield_snapshots.png
:width: 100%
:name: fig-08-wavefield-snapshots

Elastic wavefield snapshots showing P and S waves.
```

```{figure} ../../examples/figs/08_elastic_shot_0.png
:width: 100%
:name: fig-08-elastic-shot-0

Elastic shot gather: pressure, Vx, Vz components.
```

```{figure} ../../examples/figs/08_acoustic_vs_elastic.png
:width: 100%
:name: fig-08-acoustic-vs-elastic

Acoustic vs. elastic simulation comparison.
```

```{figure} ../../examples/figs/08_wavefield_energy.png
:width: 100%
:name: fig-08-wavefield-energy

Wavefield energy evolution over time.
```

## Full Example

See [`examples/08_elastic_forward.py`](https://github.com/GeoBrain-Project/geobrain/blob/main/examples/08_elastic_forward.py).