Using Shear Wave Velocity Model

[snoltcaraway]

I am using this package to try to trace ray paths from a shear wave velocity model, which has shear wave velocity reductions up to 30%, similar to the PyKonal_rays image. When I import the velocity model, I interpolate it based on Vs, and get a matrix that plots how it should look. However, when I use the EikonalSolver (cartesian), the travel times and resulting ray paths do not seem correct. I have tried different source and receiver locations. I am tracing the rays using solver.trace_ray().

Do you have source code or an example on how you got the pykonal_rays figure (figure on the welcome page).

[malcolmw]

Hi @snoltcaraway, try the example below.

import numpy as np
import matplotlib.pyplot as plt
import pykonal
import scipy.ndimage

solver = pykonal.solver.PointSourceSolver(coord_sys='cartesian')
solver.vv.min_coords = 0, 0, 0
solver.vv.node_intervals = 1, 1, 1
solver.vv.npts = 128, 32, 1
solver.vv.values = 4.5 * np.ones(solver.vv.npts)
solver.vv.values[20:44, 8:24] = 2
solver.vv.values[84:108, 8:24] = 9
solver.vv.values += 5*np.random.randn(*solver.vv.npts)
solver.vv.values = scipy.ndimage.gaussian_filter(solver.vv.values, 2)

solver.src_loc = 64, 24, 0
solver.solve()

plt.close('all')
fig, ax = plt.subplots()
qmesh = ax.pcolormesh(
    solver.vv.nodes[:, :, 0, 0], 
    solver.vv.nodes[:, :, 0, 1], 
    solver.vv.values[:, :, 0],
    shading='gouraud',
    cmap=plt.get_cmap('nipy_spectral_r')
)
for rx in np.arange(0, 129, 8):
    ray = solver.trace_ray(np.array([rx, 0.1, 0.]))
    ax.plot(ray[:, 0], ray[:, 1], linewidth=1, color='k')

ax.scatter(solver.src_loc[0], solver.src_loc[1], marker='*', edgecolor='k', facecolor='white', s=1024, zorder=100)

ax.set_aspect(1)
ax.invert_yaxis()
fig.colorbar(qmesh, ax=ax, orientation='horizontal')

[snoltcaraway]

That worked.

I see that you are using a the PointSourceSolver in a heterogenous medium instead of the EikonalSolver. Is there a benefit of choosing one or the other? Eikonal is supposed to computer the shortest travel time correct?

Thank you so much!

[malcolmw]

Great!

The PointSourceSolver class implements a pair of computational grids to improve accuracy, particularly in the near-source region where wavefront curvature is high (see. And yes, both the EikonalSolver and PointSourceSolver compute the first arrival. This is illustrated in White et al. (2020) Figure 10a,c.

[malcolmw]

@snoltcaraway I'm closing this issue, but feel free to reach out if you have any further questions.

Cheers, Malcolm