@article{Dziewonski1976,
  title={The effect of small, aspherical perturbations on travel times and a re-examination of the corrections for ellipticity},
  author={Dziewonski, Adam M. and Gilbert, Freeman},
  journal={Geophysical Journal of the Royal Astronomical Society},
  volume={44},
  pages={7--17},
  year={1976},
}
