We use TauP (Crotwell et al., 1999; Crotwell, 2022) published under the lesser GPL3 license.
Crotwell, H. P. (2022). The TauP Toolkit (2.6.1). Zenodo. https://doi.org/10.5281/zenodo.10794858
Crotwell, H. P., T. J. Owens, and J. Ritsema (1999). The TauP Toolkit: Flexible seismic travel-time and ray-path utilities, Seismological Research Letters 70, 154–160.