dispersion_phase_shift(
patch: Patch ,
phase_velocities: collections.abc.Sequence[float] ,
approx_resolution: float | None[float, None] = None,
approx_freq: tuple[tuple[float, float], None] = None,
)-> ‘PatchType’
Compute dispersion images using the phase-shift method.
| Parameter | Description |
|---|---|
| patch |
Patch to transform. Has to have dimensions of time and distance. It also needs to be right-sided (see notes below). |
| phase_velocities |
NumPy array of positive velocities, monotonically increasing, for which the dispersion will be computed. |
| approx_resolution |
Approximated frequency (Hz) resolution for the output. If left empty, the frequency resolution is dictated by the number of samples. |
| approx_freq |
Minimum and maximum frequency to compute dispersion for, in Hz If left empty, minimum is 0 Hz, and maximum is Nyquist |
See also Park, Miller, and Xia (1998)
Inspired by https://geophydog.cool/post/masw_phase_shift/.
Dims/Units of the output are forced to be ‘frequency’ (‘Hz’) and ‘velocity’ (‘m/s’).
Each channel’s distance is read as its position along the wave’s path, so data are effectively mapped along a 2-D line; the coordinate need not be sorted.
The image depends only on distances relative to each other, so any origin works for a one-sided gather, but the wave must travel toward increasing distance. For a gather whose wave travels toward lower distance, negate the coordinate; for a two-sided gather, use the offset from the source (abs(distance - source_distance)). The new values carry no units, so convert to metres first: p = patch.convert_units(distance="m") then p.update_coords(distance=-p.get_array("distance")). Reversing the patch with flip changes nothing, since it reverses the data and the coordinate together.
import dascore as dc
import numpy as np
# Example 1 - Right-sided wavefield
patch = (
dc.get_example_patch('dispersion_event')
)
disp_patch = patch.dispersion_phase_shift(np.arange(100,1500,1),
approx_resolution=0.1,approx_freq=[5,70])
ax = disp_patch.viz.waterfall(show=False, cbar=False)
ax.set_xlim(5, 70)
ax.set_ylim(1500, 100)
disp_patch.viz.waterfall(show=True, ax=ax)
```