# Tutorial 07: Acoustic Wave Propagation

```{note}
This tutorial is available as a Python script [`examples/07_acoustic_forward.py`](https://github.com/GeoBrain-Project/geobrain/blob/main/examples/07_acoustic_forward.py) and an interactive Jupyter notebook [`examples/notebooks/07_acoustic_forward.ipynb`](https://github.com/GeoBrain-Project/geobrain/blob/main/examples/notebooks/07_acoustic_forward.ipynb).
```

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

## What You Will Learn

- Set up `GridConfig` and `BoundaryConfig`
- Build `AcousticModel` with velocity and density
- Configure `Source`, `Receiver`, and `Survey`
- Run `AcousticPropagator` with gradient checkpointing
- Visualize shot gathers and wavefield snapshots

## Code

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

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

# Survey setup
source = Source(nt=NT, dt=DT, f0=5.0)
source.add_source(x=100, z=1, wavelet=wavelet_np)
receiver = Receiver(nt=NT, dt=DT)
receiver.add_receivers(x=rcv_x, z=rcv_z, receiver_type='pr')
survey = Survey(source, receiver)

# Forward modeling
propagator = AcousticPropagator(model, survey, device='cuda')
result = propagator.forward(checkpoint_segments=4)
```

## Results

```{figure} ../../examples/figs/07_velocity_density.png
:width: 100%
:name: fig-07-velocity-density

Marmousi2 velocity and density model.
```

```{figure} ../../examples/figs/07_survey_geometry.png
:width: 100%
:name: fig-07-survey-geometry

Survey geometry: sources and receivers.
```

```{figure} ../../examples/figs/07_wavefield_snapshots.png
:width: 100%
:name: fig-07-wavefield-snapshots

Acoustic wavefield snapshots at different time steps.
```

```{figure} ../../examples/figs/07_shot_gather_0.png
:width: 100%
:name: fig-07-shot-gather-0

Shot gather at the center of the model.
```

## Full Example

See [`examples/07_acoustic_forward.py`](https://github.com/GeoBrain-Project/geobrain/blob/main/examples/07_acoustic_forward.py).